Electrical training and Electrical safety training

 
Electrical training is a must for electrical fresh graduates.There are so many companies providing free electrical training.Electrical training and Electrical safety training is very essential for freshers to lay a rock solid career foundation.
Working with electricity is dangerous and you need to make sure you take all necessary safety precautions। The articles in this section provide information about checking insulation on a cord, making sure your light bulbs have proper wattage, moving appliances away from water and making sure your extension cords are used correctly.

Electrical Problems: 3 Common Solutions


For most people, solving electrical problems are a daunting task. Typically with electricity, the poblem is larger than it may seem, however, these minor problems can usually be fixed quite simply.

1. An Appliance Stops Working

If an electrical appliance stops working, check the power supply. If other appliances are working use a mains tester to check that there is power in the socket. No power in the socket means the fuse has blown or the circuit breaker has tripped. If there is power the appliance is at fault.

2. Smell of Hot Plastic

The smell of hot plastic should not be ignored. Find out where the smell is coming from. If it is from a plug or socket the most common cause is a wiring problem. Check that all terminals in the plug and socket are screwed tight. The heat is usually caused by electricity arcing across a gap.

3. No Power in the House

Check that the main fuse has not blown or the main circuit breaker has not tripped. Check to see if your neighbors have power. It could be a general outage.

If a fuse blows or a circuit breaker trips try to discover the cause before you repair it or it will blow again।


Common Circuit Breaker Problems


The most common circuit breaker problems occur when the circuit is left unprotected and the wire carrying the high voltage tends to heat up to a melting point causing damage and fire. Mentioned below are a few common circuit breaker problems:

Miswiring of the electrical system

Most often, the reason behind circuit breakdown is the miswiring of the electrical system. This problem can lead to improper shutting down of the electrical device. The electrical device may continue running even after the switch is shut down.

Another result of miswiring can be an electric shock. Generally this type of electrical shock is not critical, but it can have harmful effects on people using electrical home appliances. Miswiring can damage your appliances, switches and other electrical devices permanently or cause temporary operating problems.

You can surely cure a problem of this extent by reviewing the circuits that are affected and by testing the electrical system completely.

Tripped Circuit Breaker: Circuit Overload

You might have faced the problem of tripped circuit breakers. This problem happens frequently due to circuit overload. This happens when too many electronic devices are plugged or operated into a single electronic outlet. If you use too many electronic appliances plugged in one location, the single circuit will get overloaded and trip or switch off to lighten the load. The main idea behind tripping is to protect the circuit s from getting overloaded. The circuit breaker mechanism has been specially designed to protect the electrical system in households.

You can easily prevent circuit overload by following a few electrical safety tips. One method is by avoiding plugging much electronic equipment in one outlet. Turn off those devices which are not in use and also check for any loose connections in your outlets.

Short Circuit

One more common circuit breaker problems is short circuit. Short circuits can lead to tripped circuit breakers. Short circuits take place when a hot wire touches another neutral wire. This leads to the tripping of the circuit breaker because of the electrical current overload. Short circuits can be a serious problem. The main problem is in the electrical wiring hence; it is advisable to get it checked immediately to avoid further serious damage.

If you ever notice a short circuit in your device plugged into the outlet, do check the exterior of the cord. Check for a burning odor. Replace or repair the wires if you find them damaged. Exposed wires can pose a serious threat as they have electrical current flowing through them.

These are the common circuit breaker problems which need to be taken care of। You can easily avoid such problems by following certain procedures to ensure efficient working of the electrical system of your home.

Where You Need a Ground Fault Circuit Interrupter


There are certain places where a ground fault circuit interrupter is required by national and local building codes. They are intended to minimize the chance of electrocution, and add an additional layer of safety to your home electrical system.

Exterior Power Outlets

GFCI receptacles serve as a backup for your breaker panel. Anytime you install power outdoors, or in an area that is only partially protected from the weather, a GFCI is the correct type of connector to use.

Kitchen and Laundry

In the kitchen and laundry, where spills and splashes are a common problems, a gfci outlet could prevent severe shocks or electrocution, Similarly, the bathroom should be wired with GFCI receptacles.

Pool and Spa

Around a pool, spa, or hot tub, you should always use a GFCI circuit. If a leak should develop and the receptacle becomes soaked suddenly, the circuit will fail, protecting your breaker panel and reducing the hazard of fire.

GFCI is Required

You do not need an Electrician's license to install a GFCI, but your local building codes probably do require them in all of the areas mentioned above। They cost a little more than a traditional GFCI, but provide you with a measure of safety and security that makes the investment worthwhile.

Outdoor Electrical Outlet Safety Tips


Having an outdoor electrical outlet can be a huge convenience, but they can be a safety concern, too. Here are a few tips to keep you and your home safe.

Water Hazards

When considering outdoor electrical outlets, remember to be sure they are not placed near water sources. Some homes have a water hose connection near an outlet, and if this is the case at your home, one option is to cover the outlet with a weatherproof box. Also, keep plug covers in the outlet for added safety.

Using Extension Cords

If you need to use extension cords outdoors, make sure the cords you have are listed for outdoor use. They need to be able to weather harsh conditions and be made specifically for outdoor use. It’s also important to make sure you aren’t overloading the circuit being used, especially if you’re using high-powered tools.

Proximities of Fuel

It’s good safety practice to make sure you don't store any gas-powered tools or gas cans near the electrical outlet. Something as simple as turning on a power tool near a container of fuel can cause a spark, and sparks can ignite a larger gas source.

Tool Storage

Always be sure to store outdoor electrical tools in a clean, dry place। A secure, weatherproof storage shed is ideal. If this isn’t available it’s best to store them indoors. This will keep the electrical components in the tools intact and help avoid issues with the outlet in the future.

Cords

Every electrical appliance has a cord, and many homes use extension cords to increase the range of electrical outlets. These safety tips can help keep cords in good condition for safe operation.

  • Check cords regularly for frays, cracks or kinks, including power tool cords, holiday lights and extension cords.
  • Cords are not be jump ropes, clothes lines or leashes, and should never be used for anything other than their intended purpose.
  • Cords should be firmly plugged into outlets – if the cord is loose and can pull out easily, choose a different, more snug outlet.
  • Do not staple or nail cords in position at any time; if the cord does not remain where desired, use tape or twist ties to secure it.
  • Cords should not be placed beneath rugs where they can become a trip hazard or where frays will not be noticeable. Furthermore, covering a cord will prevent it from keeping as cool as possible.
  • Do not make modifications to a cord’s plug at any time – do not clip off the third prong or attempt to file down a wider prong to fit in a different outlet.
  • Extension cords are a temporary solution only and their use should be minimized whenever possible.
  • Use the proper weight and length of extension cord for the appropriate task, and be sure the cord is rated for indoor or outdoor use, whichever is required.
  • When unplugging a cord, pull on the cord at the outlet rather than tug on the cord itself.
 

Outlets

Every cord has to plug into an appropriate electrical outlet, but these tempting niches are inviting to unwelcome objects that can cause shorts and fires. Use these electrical safety tips at home to keep outlets safe.

  • Block unused outlets by changing to a solid cover plate or using childproof caps.
  • Do not overload outlets with multiple adaptors or power strips; relocate cords instead.
  • Never put any object other than the appropriate size plug into an outlet.
  • Install ground fault circuit interrupter outlets in potentially hazardous areas such as near pools, crawlspaces, kitchens, bathrooms and unfinished basements.
  • Keep all outlets properly covered with secure plates that cover all wiring.

Light Bulbs

Light bulbs are the single most common electrical fixture in homes, and proper light bulb safety can keep them from becoming a common electrical hazard.

  • Use bulbs that have the correct wattage requirements for each fixture -- using a higher wattage bulb can cause the fixture to overheat.
  • Consider switching to more efficient compact fluorescent bulbs that provide the same level of light at a lower wattage level.
  • Always screw bulbs in tightly; a loose bulb can cause sparks or shorts.
  • Be sure to unplug or turn off a fixture completely before changing light bulbs.

INSTALLATION,OPERATION AND MAINTENANCE OF LOW VOLTAGE DISTRIBUTION BOARD

 
STORAGE

In the event of delay in installation, the panels should be stored in a covered space.

INSTALLATION

The Panels after reaching the site i.e. place of installation, the following procedure should be followed for installation: -

1.Ensure location of installation should be adequately ventilated and free from dipping water, water logging, sources of heat etc...

2.Remove the wooden packing case (if packed in wooded case) by removing top of cover first and then four sides. Due care should be take so that the packing case opening tools do not hit any of the instruments provided on the front of the panel. Remove the panel from wooden base frame.


3.Remove polythene and paper covering sheet from the panel. Remove corrugated paper packing (in case of paper packing only) from the panel.


4.Check the material as per the packing list provided for this purpose.



5. Ensure proper protection from mechanical damage which may be caused by activities in the surroundings is provided.

6.Components projecting out from the Panel Boards viz. metering glasses, instrument windows, rotary handles, switches, relays etc. shall be protected during installation activities.


7.Align various shipping sections of the panel (In case of panels with multi transport section) in one line on the place of installation. Check the straightness of panel. Check the vertical placement of panel with the help of plumb line and if required use thin packing to adjust vertical plumb of panel.


8.Tighten various panels with each other with the help of nuts and bolts provided for the purpose on one side of each transport section and ensure that rubber gasket is available between two transport sections which are also provided on one side of each transport section.


9.Join bus bar of various transport sections with bus bar jointing fishplates and nuts and bolts provided for the purpose. The required fish plates are fixed on one side of main horizontal bus bars of each transport section duly reversed and tightened with nuts and bolts. On the other side of horizontal bus bars required number and sizes of nuts and bolts are provided.


10.Fix panels with the base channel provided on the trench with the help of foundation bolts or by welding. The panels with top cable entry i.e., without trench can be fixed by grouting panel to pockets provided in the floor for this purpose with the help of foundation bolts.


11.Check all the bus bars jointing and supporting bolts for any looseness as they might have loosened in transport. If so found the same should be tightened properly preferably with a torque wrench.


12.Check for looseness of any wiring on auxiliary control equipment and tighten them properly as the same might have also loosened in transport.


13.Carry out power and control cabling for both incoming and outgoing feeders. Also connect control wires in the terminals for inter-panel wiring provided for this purposes on both sides of transport sections. (In case of electrical interlocking of 2 or more I/C feeders).


14.Un drilled gland plates are provided at the bottom to facilitate cable entries. Ensure to plug extra / vacant entries after completion of work to arrest entry of rodents, vermins and dust.

15.Restore insulated shrouding if removed while cabling is being done

16.Check for any foreign material or tools left over in the panel at the time of installation and cabling.



17.Clean the panel with the help of vacuum cleaner.


18.Station earthing should be properly connected to the earth studs / earth bar provided on both sides of Panel.


19.Current Transformers are provided with shorting links. Ensure links are open before commissioning of the Panel Board. In case a device is connected to CTs needs to be disconnected or not supplied, close the links to those CTs ensuring safe grounding.

20.Carry out Megger test (insulation) for the panel, check for satisfactory results.


21.After all above points are completed satisfactory, the Panel Board is ready for energisation.




OPERATION

1)The Panels should be put in to operation only after completing erection of pre-commissioning checks including the safety precautions i.e. all insulating guards/shrouds to be in position.


2)First, the incomer circuit breaker shall be made ON by pressing Knob of the ACB (or) by operating the handle of MCCB/ISOLATOR, to ON position and check the voltage between phases and neutral.

3)Switch ON the outgoing feeders step by step so as to feed the connected load.


4)Check the load condition on each outgoing feeder by means of a tong meter . The reading should be less than the rating of the MCCB.


5)Check total load on the panel by measuring the current in incoming feeder to ensure that connected load is not exceeded.


6)During exigencies, if any, switch off the incomer feeder.


7)Once the switching devices have been operated as required, ensure that all doors, covers etc. are properly closed.







MAINTENANCE


The Panels should be maintained periodically. A reference chart for maintenance is attached for reference. However, if required, frequency may be increased by concerned engineer depending on place of installation, and frequency of operation of panel & the electrical components mounted therein. Maintenance works inside the Panel shall be done only after taking proper power shutdown.

A. Precautions before starting maintenance:


1. Switch off incoming power supply before removing outer covers.

2. Measure voltage on busbars to ensure that busbars are de-energized.

3. Switch off the control supply.

4. Isolate remote control voltage sources, if any.

5. Ensure that only qualified personnel are instructed and permitted to attend the maintenance.

6. Do not attempt to disconnect any terminations when the door interlock mechanism is defeated to open a compartment door.

7. Current transformer primaries must not be energized when secondaries are open circuited. Short all CT secondaries

For safety of personnel working on busbars, provide temporary earthing using a metallic chain/ strip near the workplace. Remove this earthing only after the job is completed


B. Periodic Maintenance:


1.All busbar joints and tap off points to be periodically checked by torque wrench for their tightness at least once in a year.

2.All dust and foreign material should be removed.

3.All cable connections to MCCB/ISOLATOR should be properly tightened to avoid any undue stress on the terminals on for better contact pressure to avoid localized heating. Periodical checking of the contact should be done. Check for damage or pitting of contact of ACBs, MCCB’s and Contactors. It is advised to follow the respective instruction manuals of manufacturers of these components.

4.Periodical tripping and closing/opening of circuit breaker should be checked qualified maintenance person, if found not in order the circuit breaker should be replaced.

5.All removable covers to be checked for proper fixation and damage of gasket to avoid ingress of dust and vermin.

6.Check for damage of any shroud and guard while operation.


7.Check for trouble free operation of electrical components by manual operation on no load.


8.Ensure that all shrouds and guards are replaced to their original position after maintenance.


9.Ventilation provided on the panel should not be blocked periodical checking of the air vents to be taken care for proper air circulation there by minimizing heating of the component.


10. The operation of system can be checked as per the drawings enclosed.


11.The maintenance and operation of panel should be carried out in the supervision of qualified engineer duly authorized by electricity authority.


12.For the requirement of spares / components please refer to the drawing / BOM provided with the panel / enclosed.


PROTECTIVE RELAYS

 
A relay is an electrically operated switch. Many relays use an electromagnet to operate a switching mechanism, but other operating principles are also used. Relays find applications where it is necessary to control a circuit by a low-power signal, or where several circuits must be controlled by one signal. The first relays were used in long distance telegraph circuits, repeating the signal coming in from one circuit and re-transmitting it to another. Relays found extensive use in telephone exchanges and early computers to perform logical operations. A type of relay that can handle the high power required to directly drive an electric motor is called a contactor. Solid-state relays control power circuits with no moving parts, instead using a semiconductor device triggered by light to perform switching. Relays with calibrated operating characteristics and sometimes multiple operating coils are used to protect electrical circuits from overload or faults; in modern electric power systems these functions are performed by digital instruments still called "protection relays".

A simple electromagnetic relay, such as the one taken from a car in the first picture, is an adaptation of an electromagnet. It consists of a coil of wire surrounding a soft iron core, an iron yoke, which provides a low reluctance path for magnetic flux, a movable iron armature, and a set, or sets, of contacts; two in the relay pictured. The armature is hinged to the yoke and mechanically linked to a moving contact or contacts. It is held in place by a spring so that when the relay is de-energised there is an air gap in the magnetic circuit. In this condition, one of the two sets of contacts in the relay pictured is closed, and the other set is open. Other relays may have more or fewer sets of contacts depending on their function. The relay in the picture also has a wire connecting the armature to the yoke. This ensures continuity of the circuit between the moving contacts on the armature, and the circuit track on the printed circuit board (PCB) via the yoke, which is soldered to the PCB.

When an electric current is passed through the coil, the resulting magnetic field attracts the armature, and the consequent movement of the movable contact or contacts either makes or breaks a connection with a fixed contact. If the set of contacts was closed when the relay was de-energised, then the movement opens the contacts and breaks the connection, and vice versa if the contacts were open. When the current to the coil is switched off, the armature is returned by a force, approximately half as strong as the magnetic force, to its relaxed position. Usually this force is provided by a spring, but gravity is also used commonly in industrial motor starters. Most relays are manufactured to operate quickly. In a low voltage application, this is to reduce noise. In a high voltage or high current application, this is to reduce arcing.

If the coil is energized with DC, a diode is frequently installed across the coil, to dissipate the energy from the collapsing magnetic field at deactivation, which would otherwise generate a voltage spike dangerous to circuit components. Some automotive relays already include that diode inside the relay case. Alternatively a contact protection network, consisting of a capacitor and resistor in series, may absorb the surge. If the coil is designed to be energized with AC, a small copper ring can be crimped to the end of the solenoid. This "shading ring" creates a small out-of-phase current, which increases the minimum pull on the armature during the AC cycle.

By analogy with the functions of the original electromagnetic device, a solid-state relay is made with a thyristor or other solid-state switching device. To achieve electrical isolation an optocoupler can be used which is a light-emitting diode (LED) coupled with a photo transistor.


Latching relay

Latching relay, dust cover removed, showing pawl and ratchet mechanism. The ratchet operates a cam, which raises and lowers the moving contact arm, seen edge-on just below it. The moving and fixed contacts are visible at the left side of the image.

A latching relay has two relaxed states (bistable). These are also called "impulse", "keep", or "stay" relays. When the current is switched off, the relay remains in its last state. This is achieved with a solenoid operating a ratchet and cam mechanism, or by having two opposing coils with an over-center spring or permanent magnet to hold the armature and contacts in position while the coil is relaxed, or with a remanent core. In the ratchet and cam example, the first pulse to the coil turns the relay on and the second pulse turns it off. In the two coil example, a pulse to one coil turns the relay on and a pulse to the opposite coil turns the relay off. This type of relay has the advantage that it consumes power only for an instant, while it is being switched, and it retains its last setting across a power outage. A remanent core latching relay requires a current pulse of opposite polarity to make it change state.

Reed relay

A reed relay has a set of contacts inside a vacuum or inert gas filled glass tube, which protects the contacts against atmospheric corrosion. The contacts are closed by a magnetic field generated when current passes through a coil around the glass tube. Reed relays are capable of faster switching speeds than larger types of relays, but have low switch current and voltage ratings. See also reed switch.

Mercury-wetted relay

A mercury-wetted reed relay is a form of reed relay in which the contacts are wetted with mercury. Such relays are used to switch low-voltage signals (one volt or less) because of their low contact resistance, or for high-speed counting and timing applications where the mercury eliminates contact bounce. Mercury wetted relays are position-sensitive and must be mounted vertically to work properly. Because of the toxicity and expense of liquid mercury, these relays are rarely specified for new equipment. See also mercury switch.

Polarized relay

A polarized relay placed the armature between the poles of a permanent magnet to increase sensitivity. Polarized relays were used in middle 20th Century telephone exchanges to detect faint pulses and correct telegraphic distortion. The poles were on screws, so a technician could first adjust them for maximum sensitivity and then apply a bias spring to set the critical current that would operate the relay.

Machine tool relay

A machine tool relay is a type standardized for industrial control of machine tools, transfer machines, and other sequential control. They are characterized by a large number of contacts (sometimes extendable in the field) which are easily converted from normally-open to normally-closed status, easily replaceable coils, and a form factor that allows compactly installing many relays in a control panel. Although such relays once were the backbone of automation in such industries as automobile assembly, the programmable logic controller (PLC) mostly displaced the machine tool relay from sequential control applications.

Contactor relay

A contactor is a very heavy-duty relay used for switching electric motors and lighting loads. Continuous current ratings for common contactors range from 10 amps to several hundred amps. High-current contacts are made with alloys containing silver. The unavoidable arcing causes the contacts to oxidize; however, silver oxide is still a good conductor. Such devices are often used for motor starters. A motor starter is a contactor with overload protection devices attached. The overload sensing devices are a form of heat operated relay where a coil heats a bi-metal strip, or where a solder pot melts, releasing a spring to operate auxiliary contacts. These auxiliary contacts are in series with the coil. If the overload senses excess current in the load, the coil is de-energized. Contactor relays can be extremely loud to operate, making them unfit for use where noise is a chief concern.

Solid-state relay

Solid state relay, which has no moving parts
25 A or 40 A solid state contactors

A solid state relay (SSR) is a solid state electronic component that provides a similar function to an electromechanical relay but does not have any moving components, increasing long-term reliability. With early SSR's, the tradeoff came from the fact that every transistor has a small voltage drop across it. This voltage drop limited the amount of current a given SSR could handle. As transistors improved, higher current SSR's, able to handle 100 to 1,200 Amperes, have become commercially available. Compared to electromagnetic relays, they may be falsely triggered by transients.

Solid state contactor relay

A solid state contactor is a very heavy-duty solid state relay, including the necessary heat sink, used for switching electric heaters, small electric motors and lighting loads; where frequent on/off cycles are required. There are no moving parts to wear out and there is no contact bounce due to vibration. They are activated by AC control signals or DC control signals from Programmable logic controller (PLCs), PCs, Transistor-transistor logic (TTL) sources, or other microprocessor and microcontroller controls.

Buchholz relay

A Buchholz relay is a safety device sensing the accumulation of gas in large oil-filled transformers, which will alarm on slow accumulation of gas or shut down the transformer if gas is produced rapidly in the transformer oil.

Forced-guided contacts relay

A forced-guided contacts relay has relay contacts that are mechanically linked together, so that when the relay coil is energized or de-energized, all of the linked contacts move together. If one set of contacts in the relay becomes immobilized, no other contact of the same relay will be able to move. The function of forced-guided contacts is to enable the safety circuit to check the status of the relay. Forced-guided contacts are also known as "positive-guided contacts", "captive contacts", "locked contacts", or "safety relays".

Overload protection relay

One type of electric motor overload protection relay is operated by a heating element in series with the electric motor . The heat generated by the motor current operates a bi-metal strip or melts solder, releasing a spring to operate contacts. Where the overload relay is exposed to the same environment as the motor, a useful though crude compensation for motor ambient temperature is provided.

The role of protective relaying in electric-power-system design and operation is explained
by a brief examination of the over-all background. There are three aspects of a power
system that will serve the purposes of this examination. These aspects are as follows:
A. Normal operation
B. Prevention of electrical failure.
C. Mitigation of the effects of electrical failure.
The term Ònormal operationÓ assumes no failures of equipment, no mistakes of personnel,
nor Òacts of God.Ó It involves the minimum requirements for supplying the existing load
and a certain amount of anticipated future load. Some of the considerations are:
A. Choice between hydro, steam, or other sources of power.
B. Location of generating stations.
C. Transmission of power to the load.
D. Study of the load characteristics and planning for its future growth.
E. Metering
F. Voltage and frequency regulation.
G. System operation.
E. Normal maintenance.
The provisions for normal operation involve the major expense for equipment and
operation, but a system designed according to this aspect alone could not possibly meet
present-day requirements. Electrical equipment failures would cause intolerable outages.
There must be additional provisions to minimize damage to equipment and interruptions
to the service when failures occur.
Two recourses are open: (1) to incorporate features of design aimed at preventing failures,
and (2) to include provisions for mitigating the effects of failure when it occurs. Modernpower-system design employs varying degrees of both recourses, as dictated by the
economics of any particular situation. Notable advances continue to be made toward
greater reliability. But also, increasingly greater reliance is being placed on electric power.
Consequently, even though the probability of failure is decreased, the tolerance of the
possible harm to the service is also decreased. But it is futile-or at least not economically
justifiable-to try to prevent failures completely. Sooner or later the law of diminishing
returns makes itself felt. Where this occurs will vary between systems and between parts of
a system, but, when this point is reached, further expenditure for failure prevention is
discouraged. It is much more profitable, then, to let some failures occur and to provide for
mitigating their effects.
The type of electrical failure that causes greatest concern is the short circuit, or ÒfaultÓ as
it is usually called, but there are other abnormal operating conditions peculiar to certain
elements of the system that also require attention. Some of the features of design and
operation aimed at preventing electrical failure are:
A. Provision of adequate insulation.
B. Coordination of insulation strength with the capabilities of lightning arresters.
C. Use of overhead ground wires and low tower-footing resistance.
D. Design for mechanical strength to reduce exposure, and to minimize the likelihood of
failure causable by animals, birds, insects, dirt, sleet, etc.
E. Proper operation and maintenance practices.
Some of the features of design and operation for mitigating the effects of failure are:
1. Features that mitigate the immediate effects of an electrical failure.
A. Design to limit the magnitude of short-circuit current.1
. By avoiding too large concentrations of generating capacity.
B. By using current-limiting impedance.
C. Design to withstand mechanical stresses and heating owing to short-circuit currents.
D. Time-delay undervoltage devices on circuit breakers to prevent dropping loads
during momentary voltage dips.
E. Ground-fault neutralizers (Petersen coils).
2. Features for promptly disconnecting the faulty element.
A. Protective relaying.
B. Circuit breakers with sufficient interrupting capacity.
C. Fuses.
3. Features that mitigate the loss of the faulty element.
A. Alternate circuits.
B. Reserve generator and transformer capacity.
C. Automatic reclosing.
4 Features that operate throughout the period from the inception of the
fault until after its removal, to maintain voltage and stability.
A. Automatic voltage regulation.
B. Stability characteristics of generators.
5 Means for observing the electiveness of the foregoing features.
A. Efficient human observation and record keeping.
B.Automatic oscillographs.
6. Frequent surveys as system changes or additions are made, to be sure that the foregoing
features are still adequate.

FAILURE TO OPERATE
In terms of equipment damage, if not system damage, the failure to operate for a fault is of great
concern to the relay engineer. Local backup can limit damage and the spread of tripping, but loss
of service will certainly be greater than if the relay operated correctly. By the time remote backup
takes place, numerous lines must be cleared. Because fault current is divided between these lines,
the delay in clearing is significant.
The fault shown in Appendix 2 (which occurred on 5/5/96) lasted at least 82 cycles (the fault
recorder stopped recording at that point; more on this topic later). Six lines connecting to six
different stations cleared to remove the fault current. Murphy’s law was strongly evident, as it
took a dispatcher five tries to find the correct breaker to open in order to restore the system. With
minimal fault data available, problems compounded, and it took 35 minutes before lines tripped
on backup could be closed.
Using the same categories and rankings as listed for misoperations, we can group the failure to
trip events as follows:
 Setting or coordination failure: 1 instance (7.7%)
 Accessory component failure: 10 instances (76.9%)
 Human-Caused: 0 instances
 Relay design hole: 0 instances
 Induced Signal/Noise: 1 instance (7.7%)
 Force majoure: 0 instances
 Relay component failure: 1 instance (7.7%)
 Mystery: 0 instances


FALSE OPERATIONS
Relay Component Failure
1. relay with shorted diode, closed in three times, loss of air pressure in
circuit breaker caused trip times to increase until backup relay (on 230 kV bank)
cleared fault on 34.5 kV feeder.
2. Staged fault caused adjacent 500 kV line to trip by “finding” a faulty component
that removed restraint and caused operation on reverse fault. This sent a direct
transfer trip to the other end.
3. 230 kV line tripped due to leaking capacitor in electromechanical distance relay.
Relay Design Hole
4. Two electromechanical distance relays operated for remote bus fault: “the relay
contacts have a history of drifting closed when the line voltage goes dead.” They
did not cause outage. The line was already dead.
5. Solid-state phase comparison relay tripped for a fault on parallel line. Relays were
tested with no problems found.
6. Electromechanical distance relays tripped on PT failure; line did not trip.
7. Electromechanical transformer differential misoperated during inrush. Relay
tested OK.
8. E/M DCB scheme misoperated at one end of line due to fault detector operating
for external fault and forward looking distance relay “drifting” closed on low
voltage (two occurrences on separate lines for same fault).
9. Electromechanical transformer differential misoperated during inrush. Relay
tested OK.
10. Electromechanical transformer differential misoperated during inrush. Relay
tested OK.
Accessory Component Failure
11. Electromechanical pilot wire differential false trip on bad pilot.
12. Electromechanical pilot wire differential false trip on bad pilot.
13. E/M POTT scheme false tripped on external fault due to e/m aux failure causing
transmitter to stay keyed on.
14. Solid-state bus differential tripped on external fault due to a ground return wire
not installed during addition of new equipment to station.
15. Three transformer banks tripped due to false transfer trip during test of breaker
failure relays. Blocking switches were mislabeled on newly installed equipment.
16. Directional overcurrent relay opened while switching a capacitor, due to a control
wiring problem.
17. Fault on adjacent line damaged pilot wires, causing electromechanical pilot wire
differential relays to trip three lines.
18. Electromechanical pilot wire differential tripped on external fault. Apparently
shorted pilot.
19. Transformer false tripped on first load because CT wired backwards.
20. Same transformer tripped again due to one phase wired incorrectly.
Setting or Coordination Failure
21. Electromechanical pilot wire differential operated on fuse-cleared fault.
Electromechanical pilot wire differential cannot coordinate with fuse, cleared
faults.
22. kV staged fault caused an echo-tripping permissive echo that eventually
caused a false trip on that line. Line tripped again on second staged fault test on
adjacent line.
23. Overfrequency relay tripped on transient caused by line tripping. Relay operated
correctly, given its settings, but incorrectly, given its application.
24. Relay operated for a repeated fault on an adjacent 345 kV line. This was a
“correct” incorrect operation. Could be described as a coordination failure.
25. Transfer trip inadvertently sent during disconnect switching 230 kV line.
26. Electromechanical pilot wire differential tripped after fuse-cleared fault—lack of
coordination.
27. Electromechanical pilot wire differential tripped after fuse-cleared fault—lack of
coordination.
28. Electromechanical pilot wire differential false tripped due to circulating current
when transformers were paralleled.
29. 4.8 kV bus tripped on backup due to slow trip of downstream fault (coordination
failure).
30. Overcurrent relay on transformer tripped on back-up when a fault on a feeder did
not clear; coordination error.
31. Underfrequency relays tripped on the transient when a breaker tripped on low SF6
pressure. Settings error (in my opinion).
32. EM TOC relay tripped on circulating current when bus tie closed for routine
work.
33. Electromechanical pilot wire differential overtripped on fault cleared by fuse
tapped on line.
34. Electromechanical pilot wire differential tripped due to circulating current when
lines paralleled.
35. EM directional overcurrent tripped when line was paralleled.
36. Electromechanical pilot wire differential overtripped on fault cleared by fuse
tapped on line.
37. Transformer relay false tripped on new energization because new settings had not
been applied.
Induced Signal/Noise
38. Staged fault at a 500 kV line caused false trips due to noise induced into phase
comparison relay at same station, which sent a transfer trip to other end.
39. Breaker tripped due to a spike in the dc circuit during a dc ground search. No
relay targets were reported.
40. Electromechanical pilot wire differential relay misoperated due to external
230 kV fault sending “noise spike” into pilot wires, which tripped one end of
34.5 kV line.
41. Fault on nearby line created a voltage spike, causing a pilot wire relay to operate
(line did not have drainage reactor).
42. 500 kV false trip due to microwave noise, causing current differential relay to
operate.
Mystery
43. 230 kV line tripped for fault on reverse line. No targets found on any relay.
44. 230 kV bus tripped during transfer of station service. No targets, no cause found.
Human Caused
45. 500 kV line tripped on transfer trip accidentally sent during maintenance.
46. pilot wire differential false tripped when “a construction crew
was drilling on the adjacent relay panel when the relay was jarred closed.”
47. Transformer tripped when RTU was bumped, causing it to operate. No relay
targets (shows advantage of using relay trip contacts for operation).
48. False trip of transformer due to wiring being dropped into a pool of water during
work on transformer pressure relay.
49. Vandals broke into substation. Tripped 8 breakers. No relay targets. Another
reason to use relays to operate breakers. Break-in at 6:04 pm in May.
Force Majoure
50. Water leaked into Buchholz relay.
51. “Concussion from a large explosion at X caused the relay contact to close” EM
directional overcurrent relay (3 lines).
52. False trip due to rain water leaked into the pressure relay on a LTC.


CIRCUIT BREAKER DETAILS

 

A circuit breaker is an automatically-operated electrical switch designed to protect an electrical circuit from damage caused by overload or short circuit. Its basic function is to detect a fault condition and, by interrupting continuity, to immediately discontinue electrical flow. Unlike a fuse, which operates once and then has to be replaced, a circuit breaker can be reset (either manually or automatically) to resume normal operation. Circuit breakers are made in varying sizes, from small devices that protect an individual household appliance up to large switchgear designed to protect high voltage circuits feeding an entire city.

Magnetic circuit breakers are implemented using a solenoid (electromagnet) whose pulling force increases exponentially as the current increases. The circuit breaker's contacts are held closed by a latch and, as the current in the solenoid increases beyond the rating of the circuit breaker, the solenoid's pull releases the latch which then allows the contacts to open by spring action. Some types of magnetic breakers incorporate a hydraulic time delay feature wherein the solenoid core is located in a tube containing a viscous fluid. The core is restrained by a spring until the current exceeds the breaker rating. During an overload, the solenoid pulls the core through the fluid to close the magnetic circuit, which then provides sufficient force to release the latch. The delay permits brief current surges beyond normal running current for motor starting, energizing equipment, etc. Short circuit currents provide sufficient solenoid force to release the latch regardless of core position thus bypassing the delay feature. Ambient temperature affects the time delay but does not affect the current rating of a magnetic breaker.

Thermal breakers use a bimetallic strip, which heats and bends with increased current, and is similarly arranged to release the latch. This type is commonly used with motor control circuits. Thermal breakers often have a compensation element to reduce the effect of ambient temperature on the device rating.


Thermomagnetic circuit breakers, which are the type found in most distribution boards, incorporate both techniques with the electromagnet responding instantaneously to large surges in current (short circuits) and the bimetallic strip responding to less extreme but longer-term overcurrent conditions.
MCB (Miniature Circuit Breaker)—rated current not more than 100 A. Trip characteristics normally not adjustable. Thermal or thermal-magnetic operation. Breakers illustrated above are in this category.

MCCB (Moulded Case Circuit Breaker)—rated current up to 1000 A. Thermal or thermal-magnetic operation. Trip current may be adjustable.


Contents

Origins

An early form of circuit breaker was described by Edison in an 1879 patent application, although his commercial power distribution system used fuses.[1] Its purpose was to protect lighting circuit wiring from accidental short-circuits and overloads.

Operation

All circuit breakers have common features in their operation, although details vary substantially depending on the voltage class, current rating and type of the circuit breaker.

The circuit breaker must detect a fault condition; in low-voltage circuit breakers this is usually done within the breaker enclosure. Circuit breakers for large currents or high voltages are usually arranged with pilot devices to sense a fault current and to operate the trip opening mechanism. The trip solenoid that releases the latch is usually energized by a separate battery, although some high-voltage circuit breakers are self-contained with current transformers, protection relays, and an internal control power source.

Once a fault is detected, contacts within the circuit breaker must open to interrupt the circuit; some mechanically-stored energy (using something such as springs or compressed air) contained within the breaker is used to separate the contacts, although some of the energy required may be obtained from the fault current itself. Small circuit breakers may be manually operated; larger units have solenoids to trip the mechanism, and electric motors to restore energy to the springs.

The circuit breaker contacts must carry the load current without excessive heating, and must also withstand the heat of the arc produced when interrupting the circuit. Contacts are made of copper or copper alloys, silver alloys, and other materials. Service life of the contacts is limited by the erosion due to interrupting the arc. Mechanical circuit breakers are usually discarded when the contacts are worn, but power circuit breakers and high-voltage circuit breakers have replaceable contacts.

When a current is interrupted, an arc is generated. This arc must be contained, cooled, and extinguished in a controlled way, so that the gap between the contacts can again withstand the voltage in the circuit. Different circuit breakers use vacuum, air, insulating gas, or oil as the medium in which the arc forms. Different techniques are used to extinguish the arc including:

  • Lengthening of the arc
  • Intensive cooling (in jet chambers)
  • Division into partial arcs
  • Zero point quenching
  • Connecting capacitors in parallel with contacts in DC circuits

Finally, once the fault condition has been cleared, the contacts must again be closed to restore power to the interrupted circuit.

Arc interruption

Mechanical low-voltage circuit breakers use air alone to extinguish the arc. Larger ratings will have metal plates or non-metallic arc chutes to divide and cool the arc. Magnetic blowout coils deflect the arc into the arc chute.

In larger ratings, oil circuit breakers rely upon vaporization of some of the oil to blast a jet of oil through the arc.

Gas (usually sulfur hexafluoride) circuit breakers sometimes stretch the arc using a magnetic field, and then rely upon the dielectric strength of the sulfur hexafluoride (SF6) to quench the stretched arc.

Vacuum circuit breakers have minimal arcing (as there is nothing to ionize other than the contact material), so the arc quenches when it is stretched a very small amount (<2–3>

Air circuit breakers may use compressed air to blow out the arc, or alternatively, the contacts are rapidly swung into a small sealed chamber, the escaping of the displaced air thus blowing out the arc.

Circuit breakers are usually able to terminate all current very quickly: typically the arc is extinguished between 30 ms and 150 ms after the mechanism has been tripped, depending upon age and construction of the device.

Short circuit current

A circuit breaker must incorporate various features to divide and extinguish the arc.

The maximum short-circuit current that a breaker can interrupt is determined by testing. Application of a breaker in a circuit with a prospective short-circuit current higher than the breaker's interrupting capacity rating may result in failure of the breaker to safely interrupt a fault. In a worst-case scenario the breaker may successfully interrupt the fault, only to explode when reset.

Miniature circuit breakers used to protect control circuits or small appliances may not have sufficient interrupting capacity to use at a panelboard; these circuit breakers are called "supplemental circuit protectors" to distinguish them from distribution-type circuit breakers.

Types of circuit breaker



Front panel of a 1250 A air circuit breaker manufactured by ABB. This low voltage power circuit breaker can be withdrawn from its housing for servicing. Trip characteristics are configurable via DIP switches on the front panel.

Many different classifications of circuit breakers can be made, based on their features such as voltage class, construction type, interrupting type, and structural features.

Low voltage circuit breakers

Low voltage (less than 1000 VAC) types are common in domestic, commercial and industrial application, include:

  • MCB (Miniature Circuit Breaker)—rated current not more than 100 A. Trip characteristics normally not adjustable. Thermal or thermal-magnetic operation. Breakers illustrated above are in this category.
  • MCCB (Molded Case Circuit Breaker)—rated current up to 1000 A. Thermal or thermal-magnetic operation. Trip current may be adjustable in larger ratings.
  • Low voltage power circuit breakers can be mounted in multi-tiers in LV switchboards or switchgear cabinets.

The characteristics of LV circuit breakers are given by international standards such as IEC 947. These circuit breakers are often installed in draw-out enclosures that allow removal and interchange without dismantling the switchgear.

Large low-voltage molded case and power circuit breakers may have electrical motor operators, allowing them to be tripped (opened) and closed under remote control. These may form part of an automatic transfer switch system for standby power.

Low-voltage circuit breakers are also made for direct-current (DC) applications, for example DC supplied for subway lines. Special breakers are required for direct current because the arc does not have a natural tendency to go out on each half cycle as for alternating current. A direct current circuit breaker will have blow-out coils which generate a magnetic field that rapidly stretches the arc when interrupting direct current.

Small circuit breakers are either installed directly in equipment, or are arranged in a breaker panel.

Low voltage circuit breakers

Low voltage (less than 1000 VAC) types are common in domestic, commercial and industrial application, include:

  • MCB (Miniature Circuit Breaker)—rated current not more than 100 A. Trip characteristics normally not adjustable. Thermal or thermal-magnetic operation. Breakers illustrated above are in this category.
  • MCCB (Molded Case Circuit Breaker)—rated current up to 1000 A. Thermal or thermal-magnetic operation. Trip current may be adjustable in larger ratings.
  • Low voltage power circuit breakers can be mounted in multi-tiers in LV switchboards or switchgear cabinets.

The characteristics of LV circuit breakers are given by international standards such as IEC 947. These circuit breakers are often installed in draw-out enclosures that allow removal and interchange without dismantling the switchgear.

Large low-voltage molded case and power circuit breakers may have electrical motor operators, allowing them to be tripped (opened) and closed under remote control. These may form part of an automatic transfer switch system for standby power.

Low-voltage circuit breakers are also made for direct-current (DC) applications, for example DC supplied for subway lines. Special breakers are required for direct current because the arc does not have a natural tendency to go out on each half cycle as for alternating current. A direct current circuit breaker will have blow-out coils which generate a magnetic field that rapidly stretches the arc when interrupting direct current.

Small circuit breakers are either installed directly in equipment, or are arranged in a breaker panel.

Photo of inside of a circuit breaker

The 10 ampere DIN rail-mounted thermal-magnetic miniature circuit breaker is the most common style in modern domestic consumer units and commercial electrical distribution boards throughout Europe. The design includes the following components:

  1. Actuator lever - used to manually trip and reset the circuit breaker. Also indicates the status of the circuit breaker (On or Off/tripped). Most breakers are designed so they can still trip even if the lever is held or locked in the "on" position. This is sometimes referred to as "free trip" or "positive trip" operation.
  2. Actuator mechanism - forces the contacts together or apart.
  3. Contacts - Allow current when touching and break the current when moved apart.
  4. Terminals
  5. Bimetallic strip
  6. Calibration screw - allows the manufacturer to precisely adjust the trip current of the device after assembly.
  7. Solenoid
  8. Arc divider / extinguisher


 

The 10 ampere DIN rail-mounted thermal-magnetic miniature circuit breaker is the most common style in modern domestic consumer units and commercial electrical distribution boards throughout Europe. The design includes the following components:

  1. Actuator lever - used to manually trip and reset the circuit breaker. Also indicates the status of the circuit breaker (On or Off/tripped). Most breakers are designed so they can still trip even if the lever is held or locked in the "on" position. This is sometimes referred to as "free trip" or "positive trip" operation.
  2. Actuator mechanism - forces the contacts together or apart.
  3. Contacts - Allow current when touching and break the current when moved apart.
  4. Terminals
  5. Bimetallic strip
  6. Calibration screw - allows the manufacturer to precisely adjust the trip current of the device after assembly.
  7. Solenoid
  8. Arc divider / extinguisher

Magnetic circuit breaker

Magnetic circuit breakers use a solenoid (electromagnet) whose pulling force increases with the current. Certain designs utilize electromagnetic forces in addition to those of the solenoid. The circuit breaker contacts are held closed by a latch. As the current in the solenoid increases beyond the rating of the circuit breaker, the solenoid's pull releases the latch which then allows the contacts to open by spring action. Some types of magnetic breakers incorporate a hydraulic time delay feature using a viscous fluid. The core is restrained by a spring until the current exceeds the breaker rating. During an overload, the speed of the solenoid motion is restricted by the fluid. The delay permits brief current surges beyond normal running current for motor starting, energizing equipment, etc. Short circuit currents provide sufficient solenoid force to release the latch regardless of core position thus bypassing the delay feature. Ambient temperature affects the time delay but does not affect the current rating of a magnetic breaker.

Thermal magnetic circuit breaker

Thermal magnetic circuit breakers, which are the type found in most distribution boards, incorporate both techniques with the electromagnet responding instantaneously to large surges in current (short circuits) and the bimetallic strip responding to less extreme but longer-term over-current conditions.

Rated circuits

Circuit breakers are rated both by the normal current that are expected to carry, and the maximum short-circuit current that they can safely interrupt.

Under short-circuit conditions, a current many times greater than normal can exist (see prospective short circuit current). When electrical contacts open to interrupt a large current, there is a tendency for an arc to form between the opened contacts, which would allow the current to continue. Therefore, circuit breakers must incorporate various features to divide and extinguish the arc. In air-insulated and miniature breakers an arc chute structure consisting (often) of metal plates or ceramic ridges cools the arc, and magnetic blowout coils deflect the arc into the arc chute. Larger circuit breakers such as those used in electrical power distribution may use vacuum, an inert gas such as sulphur hexafluoride or have contacts immersed in oil to suppress the arc.

The maximum short-circuit current that a breaker can interrupt is determined by testing. Application of a breaker in a circuit with a prospective short-circuit current higher than the breaker's interrupting capacity rating may result in failure of the breaker to safely interrupt a fault. In a worst-case scenario the breaker may successfully interrupt the fault, only to explode when reset, injuring the technician.

International Standard IEC 60898-1 and European Standard EN 60898-1 define the rated current In of a circuit breaker for low voltage distribution applications as the current that the breaker is designed to carry continuously (at an ambient air temperature of 30 °C). The commonly-available preferred values for the rated current are 6 A, 10 A, 13 A, 16 A, 20 A, 25 A, 32 A, 40 A, 50 A, 63 A, 80 A and 100 A (Renard series, slightly modified to include current limit of British BS 1363 sockets). The circuit breaker is labeled with the rated current in ampere, but without the unit symbol "A". Instead, the ampere figure is preceded by a letter "B", "C" or "D" that indicates the instantaneous tripping current, that is the minimum value of current that causes the circuit-breaker to trip without intentional time delay (i.e., in less than 100 ms), expressed in terms of In:

Type

Instantaneous tripping current

B

above 3 In up to and including 5 In

C

above 5 In up to and including 10 In

D

above 10 In up to and including 20 In

K

above 8 In up to and including 12 In

For the protection of loads that cause frequent short duration (approximately 400 ms to 2 s) current peaks in normal operation.

Z

above 2 In up to and including 3 In for periods in the order of tens of seconds.

For the protection of loads such as semiconductor devices or measuring circuits using current transformers.

Common trip breakers



Three pole common trip breaker for supplying a three-phase device. This breaker has a 2 A rating

When supplying a branch circuit with more than one live conductor, each live conductor must be protected by a breaker pole. To ensure that all live conductors are interrupted when any pole trips, a "common trip" breaker must be used. These may either contain two or three tripping mechanisms within one case, or for small breakers, may externally tie the poles together via their operating handles. Two pole common trip breakers are common on 120/240 volt systems where 240 volt loads (including major appliances or further distribution boards) span the two live wires. Three-pole common trip breakers are typically used to supply three-phase electric power to large motors or further distribution boards.

Two and four pole breakers are used when there is a need to disconnect the neutral wire, to be sure that no current can flow back through the neutral wire from other loads connected to the same network when people need to touch the wires for maintenance. Separate circuit breakers must never be used for disconnecting live and neutral, because if the neutral gets disconnected while the live conductor stays connected, a dangerous condition arises: the circuit will appear de-energized (appliances will not work), but wires will stay live and RCDs will not trip if someone touches the live wire (because RCDs need power to trip). This is why only common trip breakers must be used when switching of the neutral wire is needed.

Medium-voltage circuit breakers

Medium-voltage circuit breakers rated between 1 and 72 kV may be assembled into metal-enclosed switchgear line ups for indoor use, or may be individual components installed outdoors in a substation. Air-break circuit breakers replaced oil-filled units for indoor applications, but are now themselves being replaced by vacuum circuit breakers (up to about 35 kV). Like the high voltage circuit breakers described below, these are also operated by current sensing protective relays operated through current transformers. The characteristics of MV breakers are given by international standards such as IEC 62271. Medium-voltage circuit breakers nearly always use separate current sensors and protection relays, instead of relying on built-in thermal or magnetic overcurrent sensors.

Medium-voltage circuit breakers can be classified by the medium used to extinguish the arc:

  • Vacuum circuit breaker—

15 kV Medium Voltage Vacuum Circuit Breaker manufactured by ABB.

With rated current up to 3000 A, these breakers interrupt the current by creating and extinguishing the arc in a vacuum container. These are generally applied for voltages up to about 35,000 V,[4] which corresponds roughly to the medium-voltage range of power systems. Vacuum circuit breakers tend to have longer life expectancies between overhaul than do air circuit breakers.

  • Air circuit breaker—Rated current up to 10,000 A. Trip characteristics are often fully adjustable including configurable trip thresholds and delays. Usually electronically controlled, though some models are microprocessor controlled via an integral electronic trip unit. Often used for main power distribution in large industrial plant, where the breakers are arranged in draw-out enclosures for ease of maintenance.
  • SF6 circuit breakers extinguish the arc in a chamber filled with sulfur hexafluoride gas.

Medium-voltage circuit breakers may be connected into the circuit by bolted connections to bus bars or wires, especially in outdoor switchyards. Medium-voltage circuit breakers in switchgear line-ups are often built with draw-out construction, allowing the breaker to be removed without disturbing the power circuit connections, using a motor-operated or hand-cranked mechanism to separate the breaker from its enclosure.

High-voltage circuit breakers




 



115 kV bulk oil circuit breaker

Electrical power transmission networks are protected and controlled by high-voltage breakers. The definition of high voltage varies but in power transmission work is usually thought to be 72.5 kV or higher, according to a recent definition by the International Electrotechnical Commission (IEC). High-voltage breakers are nearly always solenoid-operated, with current sensing protective relays operated through current transformers. In substations the protection relay scheme can be complex, protecting equipment and busses from various types of overload or ground/earth fault.

High-voltage breakers are broadly classified by the medium used to extinguish the arc.

Some of the manufacturers are ABB, AREVA, Mitsubishi Electric, Pennsylvania Breaker, Siemens, Toshiba, Končar HVS, BHEL and others.

Due to environmental and cost concerns over insulating oil spills, most new breakers use SF6 gas to quench the arc.

Circuit breaker can be classified as "live tank", where the enclosure that contains the breaking mechanism is at line potential, or dead tank with the enclosure at earth potential. High-voltage AC circuit breakers are routinely available with ratings up to 765 kV.

High-voltage circuit breakers used on transmission systems may be arranged to allow a single pole of a three-phase line to trip, instead of tripping all three poles; for some classes of faults this improves the system stability and availability.

Sulfur hexafluoride (SF6) high-voltage circuit-breakers

High-voltage circuit-breakers have greatly changed since they were first introduced about 40 years ago, and several interrupting principles have been developed that have contributed successively to a large reduction of the operating energy. These breakers are available for indoor or outdoor applications, the latter being in the form of breaker poles housed in ceramic insulators mounted on a structure.

Current interruption in a high-voltage circuit-breaker is obtained by separating two contacts in a medium, such as SF6, having excellent dielectric and arc quenching properties. After contact separation, current is carried through an arc and is interrupted when this arc is cooled by a gas blast of sufficient intensity.

Gas blast applied on the arc must be able to cool it rapidly so that gas temperature between the contacts is reduced from 20,000 K to less than 2000 K in a few hundred microseconds, so that it is able to withstand the transient recovery voltage that is applied across the contacts after current interruption. Sulphur hexafluoride is generally used in present high-voltage circuit-breakers (of rated voltage higher than 52 kV).

In the 1980s and 1990s, the pressure necessary to blast the arc was generated mostly by gas heating using arc energy. It is now possible to use low energy spring-loaded mechanisms to drive high-voltage circuit-breakers up to 800 kV.

Contents

Brief history

The first patents on the use of SF6 as an interrupting medium were filed in Germany in 1938 by Vitaly Grosse (AEG) and independently later in the USA in July 1951 by H.J. Lingal, T.E. Browne and A.P. Storm (Westinghouse). The first industrial application of SF6 for current interruption dates back to 1953. High-voltage 15 kV to 161 kV load switches were developed with a breaking capacity of 600 A. The first high-voltage SF6 circuit-breaker built in 1956 by Westinghouse, could interrupt 5 kA under 115 kV, but it had 6 interrupting chambers in series per pole. In 1957, the puffer-type technique was introduced for SF6 circuit breakers where the relative movement of a piston and a cylinder linked to the moving part is used to generate the pressure rise necessary to blast the arc via a nozzle made of insulating material (figure 1). In this technique, the pressure rise is obtained mainly by gas compression. The first high-voltage SF6 circuit-breaker with a high short-circuit current capability was produced by Westinghouse in 1959. This dead tank circuit-breaker could interrupt 41.8 kA under 138 kV (10,000 MV·A) and 37.6 kA under 230 kV (15,000 MV·A). This performance was already significant, but the three chambers per pole and the high pressure source needed for the blast (1.35 MPa) was a constraint that had to be avoided in subsequent developments. The excellent properties of SF6 lead to the fast extension of this technique in the 1970s and to its use for the development of circuit breakers with high interrupting capability, up to 800 kV.



The achievement around 1983 of the first single-break 245 kV and the corresponding 420kV to 550 kV and 800 kV, with respectively 2, 3, and 4 chambers per pole, lead to the dominance of SF6 circuit breakers in the complete range of high voltages.

Several characteristics of SF6 circuit breakers can explain their success:

  • Simplicity of the interrupting chamber which does not need an auxiliary breaking chamber;
  • Autonomy provided by the puffer technique;
  • The possibility to obtain the highest performance, up to 63 kA, with a reduced number of interrupting chambers;
  • Short break time of 2 to 2.5 cycles;
  • High electrical endurance, allowing at least 25 years of operation without reconditioning;
  • Possible compact solutions when used for "gas insulated switchgear" (GIS) or hybrid switchgear;
  • Integrated closing resistors or synchronized operations to reduce switching over-voltages;
  • Reliability and availability;
  • Low noise levels.

The reduction in the number of interrupting chambers per pole has led to a considerable simplification of circuit breakers as well as the number of parts and seals required. As a direct consequence, the reliability of circuit breakers improved, as verified later on by CIGRE surveys.

Thermal blast chambers

New types of SF6 breaking chambers, which implement innovative interrupting principles, have been developed over the past 15 years, with the objective of reducing the operating energy of the circuit-breaker. One aim of this evolution was to further increase the reliability by reducing the dynamic forces in the pole. Developments since 1996 have seen the use of the self-blast technique of interruption for SF6 interrupting chambers.

These developments have been facilitated by the progress made in digital simulations that were widely used to optimize the geometry of the interrupting chamber and the linkage between the poles and the mechanism.

This technique has proved to be very efficient and has been widely applied for high voltage circuit breakers up to 550 kV. It has allowed the development of new ranges of circuit breakers operated by low energy spring-operated mechanisms.



The reduction of operating energy was mainly achieved by the lowering energy used for gas compression and by making increased use of arc energy to produce the pressure necessary to quench the arc and obtain current interruption. Low current interruption, up to about 30% of rated short-circuit current, is obtained by a puffer blast.

Self-blast chambers

Further development in the thermal blast technique was made by the introduction of a valve between the expansion and compression volumes. When interrupting low currents the valve opens under the effect of the overpressure generated in the compression volume. The blow-out of the arc is made as in a puffer circuit breaker thanks to the compression of the gas obtained by the piston action. In the case of high currents interruption, the arc energy produces a high overpressure in the expansion volume, which leads to the closure of the valve and thus isolating the expansion volume from the compression volume. The overpressure necessary for breaking is obtained by the optimal use of the thermal effect and of the nozzle clogging effect produced whenever the cross-section of the arc significantly reduces the exhaust of gas in the nozzle. In order to avoid excessive energy consumption by gas compression, a valve is fitted on the piston in order to limit the overpressure in the compression to a value necessary for the interruption of low short circuit currents.



Self-blast circuit breaker chamber (1) closed, (2) interrupting low current, (3) interrupting high current, and (4) open.

This technique, known as “self-blast” has now been used extensively since 1996 for the development of many types of interrupting chambers. The increased understanding of arc interruption obtained by digital simulations and validation through breaking tests, contribute to a higher reliability of these self-blast circuit breakers. In addition the reduction in operating energy, allowed by the self blast technique, leads to longer service life.

Double motion of contacts

An important decrease in operating energy can also be obtained by reducing the kinetic energy consumed during the tripping operation. One way is to displace the two arcing contacts in opposite directions so that the arc speed is half that of a conventional layout with a single mobile contact.



The thermal and self blast principles have enabled the use of low energy spring mechanisms for the operation of high voltage circuit breakers. They progressively replaced the puffer technique in the 1980s; first in 72.5 kV breakers, and then from 145 kV to 800 kV.

Comparison of single motion and double motion techniques

The double motion technique halves the tripping speed of the moving part. In principle, the kinetic energy could be quartered if the total moving mass was not increased. However, as the total moving mass is increased, the practical reduction in kinetic energy is closer to 60%.The total tripping energy also includes the compression energy, which is almost the same for both techniques. Thus, the reduction of the total tripping energy is lower, about 30%, although the exact value depends on the application and the operating mechanism.Depending on the specific case, either the double motion or the single motion technique can be cheaper. Other considerations, such as rationalization of the circuit-breaker range, can also influence the cost.

Thermal blast chamber with arc-assisted opening

In this interruption principle arc energy is used, on the one hand to generate the blast by thermal expansion and, on the other hand, to accelerate the moving part of the circuit breaker when interrupting high currents. The overpressure produced by the arc energy downstream of the interruption zone is applied on an auxiliary piston linked with the moving part. The resulting force accelerates the moving part, thus increasing the energy available for tripping.

With this interrupting principle it is possible, during high-current interruptions, to increase by about 30% the tripping energy delivered by the operating mechanism and to maintain the opening speed independently of the current. It is obviously better suited to circuit-breakers with high breaking currents such as Generator circuit-breakers.

Generator circuit-breakers

Generator circuit-breakers (GCB's) are connected between a generator and the step-up voltage transformer. They are generally used at the outlet of high power generators (100 MVA to 1800 MVA) in order to protect them in a reliable, fast and economic manner. Such circuit breakers must be able to allow the passage of high permanent currents under continuous service (6.3 kA to 40 kA), and have a high breaking capacity (63 kA to 275 kA).

They belong to the medium voltage range, but the TRV withstand capability required by ANSI/IEEE Standard C37.013 is such that the interrupting principles developed for the high-voltage range must be used. A particular embodiment of the thermal blast technique has been developed and applied to generator circuit-breakers. The self-blast technique described above is also widely used in SF6 generator circuit breakers, in which the contact system is driven by a low-energy, spring-operated mechanism. An example of such a device is shown in the figure below; this circuit breaker is rated for 17.5 kV and 63 kA.



Generator circuit breaker rated for 17.5 kV and 63 kA

Evolution of tripping energy

The operating energy has been reduced by 5 to 7 times during this period of 27 years. This illustrates well the great progress made in this field of interrupting techniques for high-voltage circuit-breakers.

Future perspectives

In the near future, present interrupting technologies can be applied to circuit-breakers with the higher rated breaking currents (63 kA to 80 kA) required in some networks with increasing power generation.

Self blast or thermal blast circuit breakers are now accepted world wide and they have been in service for high voltage applications for about 15 years, starting with the voltage level of 72.5 kVToday this technique is also available for the voltage levels 420/550/800 kV.

High Power testing

The short-circuit interrupting capability of high-voltage circuit breakers is such that it cannot be demonstrated with a single source able to generate the necessary power. A special scheme is used with a generator that provides the short-circuit current until current interruption and afterwards a voltage source applies the recovery voltage across the terminals of the circuit breaker. Tests are usually performed single-phase but can also be performed three-phase

Issues related to SF6 Circuit Breakers

The following issues are associated with SF6 circuit breakers

SF6 is the most potent greenhouse gas that the Intergovernmental Panel on Climate Change has evaluated. It has a global warming potential that is 23,900 times worse than CO2. SF6 has been classified as a restricted gas under the Kyoto Protocol.

Toxic lower order gases

When an arc is formed in SF6 gas small quantities of lower order gases are formed. Some of these byproducts are toxic and can cause irritation to eyes and respiratory system.

Oxygen displacement

SF6 is heavier than air so care must be taken when entering low confined spaces due to the risk of oxygen displacement.

Other breakers

The following types are described in separate articles.

  • Breakers for protections against earth faults too small to trip an over-current device:
    • Residual-current device (RCD, formerly known as a residual current circuit breaker) — detects current imbalance, but does not provide over-current protection.
    • Residual current breaker with over-current protection (RCBO) — combines the functions of an RCD and an MCB in one package. In the United States and Canada, panel-mounted devices that combine ground (earth) fault detection and over-current protection are called Ground Fault Circuit Interrupter (GFCI) breakers; a wall mounted outlet device providing ground fault detection only is called a GFI.
    • Earth leakage circuit breaker (ELCB) — This detects earth current directly rather than detecting imbalance. They are no longer seen in new installations for various reasons.
  • Autorecloser — A type of circuit breaker which closes again after a delay. These are used on overhead power distribution systems, to prevent short duration faults from causing sustained outages.
  • Polyswitch (polyfuse) — A small device commonly described as an automatically resetting fuse rather than a circuit breaker.


 

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