Introduction of Grid Station Main and Auxiliary Equipment

7-1       TRANSFORMERS

Electrical transformer is a static device which transforms electrical energy from one circuit to another without any direct electrical connection and with the help of mutual induction between to windings. It transforms power from one circuit to another without changing its frequency but may be in different voltage level.

7-1-1    USE OF POWER TRANSFORMER

Generation of Electrical Power in low voltage level is very much cost effective. Hence Electrical Power is generated in low voltage level. Theoretically, this low voltage leveled power can be transmitted to the receiving end. But if the voltage level of a power is increased, the current of the power is reduced which causes reduction in ohmic or I²R losses in the system, reduction in cross sectional area of the conductor i.e. reduction in capital cost of the system and it also improves the voltage regulation of the system. Because of these, low leveled power must be stepped up for efficient. This is done by step up transformer at the sending side of the power system network. As this high voltage power may not be distributed to the consumers directly, this must be stepped down to the desired level at the receiving end with help of step down transformer. These are the use of electrical power transformer in the electrical power system.

7-1-2    TYPES OF TRANSFORMER

Transformers can be categorized in different ways, depending upon their purpose, use, construction etc. The types of transformer are as follows:

Step Up Transformer & Step Down Transformer - Generally used for stepping up and down the voltage level of power in transmission and distribution power network.

Three phase transformer  & Single Phase Transformer - Former is generally used in three phase power system as it is cost effective than later but when size matters it is preferable to use three phase transformer as it is easier to transport three single phase unit separately than one single three phase unit.

Electrical Power Transformer, Distribution Transformer & Instrument Transformer - Transformer generally used in transmission network is normally known as Power Transformer, Distribution Transformer is used in distribution network and this is lower rating transformer and Current Transformer & Potential Transformer, we use for relay and protection purpose in electrical power system and in different instruments in industries are called instrument transformer.

Two Winding Transformer & Auto-Transformer - Former is generally used where ratio between High Voltage and Low Voltage is greater than 2. It is cost effective to use later where the ratio between High Voltage and Low Voltage is less than 2.

Outdoor Transformer & Indoor Transformer - Transformers designed for installing at outdoor is Outdoor Transformer and Transformers designed for installing at indoor is Indoor Transformer.

7-2       CIRCUIT BREAKERS

Circuit Breaker is a switching device which can be operated manually as well as automatically for controlling and protection of electrical power system respectively. As the modern power system deals with huge currents, special attention should be given during designing of circuit breaker to safe interruption of arc produced during the operation of circuit breaker

The modern power system deals with huge power network and huge numbers of associated electrical equipment. During short circuit fault or any other types of electrical fault these equipment as well as the power network suffer a high stress of fault current in them which may damage the equipment and networks permanently. For saving these equipments and the power networks the fault current should be cleared from the system as quickly as possible. Again after the fault is cleared, the system must come to its normal working condition as soon as possible for supplying reliable quality power to the receiving ends. In addition to that for proper controlling of power system, different switching operations are required to be performed. So for timely disconnecting and reconnecting different parts of power system network for protection and control, there must be some special type of switching devices which can be operated safely under huge current carrying condition. During interruption of huge current, there would be large arcing in between switching contacts, so care should be taken to quench these arcs in safe manner.

Circuit breaker is the special device which does all the required switching operations during current carrying condition.

7.2.1    WORKING PRINCIPLE OF CIRCUIT BREAKER

Circuit breaker mainly consists of fixed contacts and moving contacts. In normal "ON" condition of circuit breaker, these two contacts are physically connected to each other due to applied mechanical pressure on the moving contacts. There is an arrangement stored potential energy in the operating mechanism of circuit breaker which is realized if switching signal given to the breaker. The potential energy can be stored in the circuit breaker by different ways like by deforming metal spring, by compressed air, or by hydraulic pressure. But whatever the source of potential energy, it must be released during operation. Release of potential energy makes sliding of the moving contact at extremely fast manner.

All circuit breaker have operating coils (tripping coils and close coil), whenever these coils are energized by switching pulse, and the plunger inside them displaced. This operating coil plunger is typically attached to the operating mechanism of circuit breaker, as a result the mechanically stored potential energy in the breaker mechanism is released in forms of kinetic energy, which makes the moving contact to move as these moving contacts mechanically attached through a gear lever arrangement with the operating mechanism. After a cycle of operation of circuit breaker the total stored energy is released and hence the potential energy again stored in the operating mechanism of circuit breaker by means of spring charging motor or air compressor or by any other means.

Till now we have discussed about mechanical working principle of circuit breaker. But there are electrical characteristics of a circuit breaker which also should be considered in this discussion of operation of circuit breaker.

The circuit breaker has to carry large rated or fault power. Due to this large power there is always dangerously high arcing between moving contacts and fixed contact during operation of circuit breaker.

Again as we discussed earlier the arc in circuit breaker can be quenched safely if the dielectric strength between the current carrying contacts of circuit breaker increases rapidly during every current zero crossing of the alternating current. The dielectric strength of the media in between contacts can be increased in numbers of ways, like by compressing the ionized arcing media since compressing accelerates the deionization process of the media, by cooling the arcing media since cooling increase the resistance of arcing path or by replacing the ionized arcing media by fresh gasses. Hence a numbers of arc quenching processes should be involved in operation of circuit breaker.

7.2.2    TYPES OF CIRCUIT BREAKER

According to different criteria there are different types of circuit breaker

Classification Based on Arc Quenching Media:

1.                  Oil Circuit Breaker

2.                  Air Circuit Breaker

3.                  SF₆ Circuit Breaker

4.                  Vacuum Circuit Breaker

Classification Based on Service:

1.                   Outdoor Circuit Breaker

2.                   Indoor Circuit Breaker

Classification Based on Operating Mechanism of circuit breaker:

1.                  Spring Operated Circuit Breaker

2.                  Pneumatic Circuit Breaker

3.                  Hydraulic Circuit Breaker

Classification Based on Voltage level of installation:

1.                  High Voltage Circuit Breaker

2.                  Medium Voltage Circuit Breaker

3.                  Low Voltage Circuit Breaker

7.3       DISCONNECT SWITCHES/ISOLATORS

In electrical engineering, a disconnector or isolator switch or disconnect switch is used to make sure that an electrical circuit can be completely de-energized for service or maintenance. Such switches are often found in electrical distribution and industrial applications where machinery must have its source of driving power removed for adjustment or repair. High-voltage isolation switches are used in electrical substations to allow isolation of apparatus such as circuit breakers and transformers, and transmission lines, for maintenance. Often the isolation switch is not intended for normal control of the circuit and is used only for isolation.

Isolator switches have provisions for a Padlock so that inadvertent operation is not possible. In high voltage or complex systems, these padlocks may be part of a trapped-key interlocked to ensure proper sequence of operation. In some designs the isolator switch has the additional ability to earth the isolated circuit thereby providing additional safety. Such an arrangement would apply to circuits which inter-connect power systems where both end of the circuit need to be isolated.

The major difference between an isolator and a circuit breaker is that an isolator is an off-load device intended to be opened only after current has been interrupted by some other control device. Safety regulations of the utility must prevent any attempt to open the disconnector while it supplies a circuit.

7-4       LIGHTNING ARRESTER

A lightning arrester (in Europe: surge arrester) is a device used on electrical power system and communications systems to protect the insulation and conductors of the system from the damaging effects lightning. The typical lightning arrester has a high voltage terminal and a ground terminal. When a lightning surge (or switching surge, which is very similar) travels along the power line to the arrester, the current from the surge is diverted through the arrestor, in most cases to earth.

If protection fails or is absent, lightning that strikes the electrical system introduces thousands of kilovolts that may damage the transmission lines, and can also cause severe damage to transformers and other electrical or electronic devices. Lightning-produced extreme voltage spikes in incoming power lines can damage electrical home appliances.

A lightning arrester may be a spark gap or may have a block of a semiconducting material such as Silicon Carbide or Zinc Oxide. Some spark gaps are open to the air, but most modern varieties are filled with a precision gas mixture, and have a small amount of radioactive material to encourage the gas to ionize when the voltage across the gap reaches a specified level. Other designs of lightning arresters use a glow-discharge tube (essentially like a neon glow lamp) connected between the protected conductor and ground, or voltage-activated solid-state switches called varistors or MOVs.

Lightning arresters built for power system consist of a porcelain tube several feet long and several inches in diameter, typically filled with disks of zinc oxide. A safety port on the side of the device vents the occasional internal explosion without shattering the porcelain cylinder.

Lightning arresters are rated by the peak current they can withstand the amount of energy they can absorb, and the break over voltage that they require to begin conduction. They are applied as part of a lightning protection system, in combination with air terminals and bonding.

7-5       BATTERIES AND BATTERY CHARGERS

Supply of power to protection and control circuits is provided from storage batteries due to reliability point of view.

The simplest operating unit to produce emf chemically is called a cell, whereas several cells constitute a battery. Electrochemical devices consist of two dissimilar electrodes immersed in a conducting solution, normally known as electrolyte that is capable of storing electrical energy.

The voltage of the cell depends upon the material of electrolyte, while the current and power capacity of a cell depends upon the plate area and weight of active material in the electrodes.

Main types of storage batteries are:

1. Lead Acid Batteries
2. Alkaline Batteries

Active Parts of Lead Acid Battery:

1. Grid (Lead Antimony)
2. Positive Plates (Lead Per Oxide- PbO₂)
3. Negative Plates (Lead- Pb)
4. Electrolyte (Sulphuric Acid-H₂SO₄)

Chemical Reactions

At Anode:       PbO₂ + H₂SO₄↔ PbSO₄ + H₂O + ½O₂

At Cathode:    Pb + H₂SO₄↔ PbSO4 + H2O

7-6       STATION GROUNDING SYSTEM

Earthing or grounding is the term used for electrical connection to general mass of earth in such a manner as to ensure, at all times, an immediate discharge of energy without danger. A grounding system to be totally effective must satisfy the following conditions:

1. Provide a low impedance path to ground for personnel and equipment protection and effective circuit relaying.
2. Withstand and dissipate repeated fault and surge currents.
3. Provide corrosion allowance or corrosion resistance to various soil chemicals to insure continuous performance during the life of the equipment being protected.

Types of Earthing:

1. Solid or Effective Earthing
2. Resistance Earthing/Reactance Earthing

Classification of Earthing

1. System or Neutral Earthing: The neutral point of generator, transformer, transmission and distribution system or circuit, rotating machines etc. is connected to earth either directly or through a resistance, or a reactance.

2. Equipment Earthing: Equipment Earthing means connecting the non current carrying metallic parts in the    neighborhood of electrical circuits to earth.

Resistance to current through an earth electrode system has the following three components:

1. Resistance of the ground rod itself and connections to it.
2. Contact resistance between the ground rod and earth adjacent to it.
3. Resistance of the surrounding earth.

7-7       AC & DC Supply System

In any substation AC and DC supply system plays a very important role for protection, control and for all auxiliary services.

AC Supply System

For AC supply, normally a dedicated panel is specified in a substation which is only for the substation and no external load is connected to it in order to avoid interruptions on it. On the LT side two transformers are provided exclusively for the substation auxiliary services. For reliability purposes, load is fed from one transformer; however in case load can be shifted to the other transformer either from HT or LT side. Then we have distribution panels, from where load is distributed throughout the substation through appropriate Circuit Breakers/ Miniature Circuit Breakers.

DC Supply System

For DC supply system, Rectifiers, Batteries and Distribution Panels are provided in the substation. In important substations, normally Two sets of Batteries along with Three Rectifiers (One as standby) are provided for reliability purposes.

110 Volts Batteries                             Two Sets

110 Volts Rectifiers                            Three Sets

220 Volts Batteries                             Two Sets

220 Volts Rectifiers                            Three Sets

In 500 kV substations, Four sets of Batteries and Six Rectifiers (One as standby for Two banks). Even, in case of emergency, loads of the same rating can be coupled with one Rectifier/Battery.

7.8       POWER CABLES

There are four main parts of cable:-

1.                  Conductor

2.                  Insulation

3.                  Shield or Sheath

4.                  Protective Covering

7.8.1    PURPOSES OF SHIELDING / SHIELD GROUNDING

The application of conducting (copper etc) and semiconducting (metabolized paper tap or containing carbon or silicon etc) materials over the conductor insulation is called shielding. The main purpose of shield is to keep even voltage gradient across the insulation in order to avoid damage to insulation by corona or ionization.

Now shield may have induced voltages in it, so shield must be grounded in order to discharge these induced voltages. When shield is grounded, it provides some more advantages as well, which are:

1.                  Provides earth return path in case of phase to ground fault

2.                  Human safety

3.                  Protects the cable from external high voltages, produced by lightening etc

Shield must be grounded at one place only (especially in single phase cable) in order to avoid flow of current in shield and hence damage to it due to overheating. Shielding idea was given by Martin Hochsadter in 1915. He gave the idea that put shield around the conductor of each phase and then ground all shields. Such cables are called H-cables. Such cable fails phase to ground. In these cables it is very rare that cable may fail phase to phase.

In a very long cable, sectionalized are used. In sectionalized shield each section is insulated from each other and then each section is grounded at one place only.

7.9             BUS-BARS

There are two types of bus bars used in grid station, which are:

1.                  The Flexile or Stranded Bus Bar

2.                  The Rigid Bus Bar (may be tubular or solid)

1.                  Flexible or Stranded Bus Bar: It is used where:

A.    Longer spans are involved.

B.     Where sufficient clearances are needed to allow for conductor sways and.

C.     It is used as a long drop from horizontal bus to equipment bushing.

In the flexible bus bar sag must be enough to account for temperature variations without affecting the clearances between phases and phases to ground.

2.         Rigid Bus Bar: It is used where:

A.    Heavy currents are involved 

B.     Short or less Spacing is available

To account for thermal expansion /contraction of rigid bus provision must be made by means of expansion joints and clamps to permit bus to slide both ways in order to avoid damage to equipment bushing and isolators etc.

7.9.1     BUS BAR SCHEMES

SINGLE BUS SYSTEM

Single Bus System is simplest and cheapest one. In this scheme all the feeders and transformer bay are connected to only one single bus as shown.

                                     

Fig (1)

SINGLE BUS SYSTEM WITH BUS SECTIONALIZER

                                       

Fig) (2

DOUBLE BUS SYSTEM

                                             

Fig (3)

DOUBLE BREAKER BUS SYSTEM

                                    

 

Fig (4)

ONE AND A HALF BREAKER BUS SYSTEM

                                           

                                                                        Fig (5)

MAIN AND TRANSFER BUS SYSTEM

                                           

                                                                        Fig (6)

                                    

 

 

 

 

DOUBLE BUS SYSTEM WITH BYPASS ISOLATORS

           

Fig (7)

RING BUS SYSTEM

 

                                              

                                                                        Fig (8)

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Basic Control Circuits

5.1       AUXILIARY SWITCHES

The names of various automatic switches are:

1. Level switch
2. Flow switch
3. Position or limit switch
4. Pressure switch
5. Temperature switch
6. Speed switch (centrifugal switch)

IMPORTANT LETTERS

Letters ‘a’, ‘b’, ‘aa’ and ‘bb’ are used in diagrams to represent switches or auxiliary switches.

a-                  It is closed when main device (Circuit Breaker, Isolator or Contactor/Relay) is closed /energized and it is opened when main device is open or de-energized. Sometimes, ‘a’ is also called normally open contact of a device.

b-                  It is closed when main device is opened and vice versa. ‘b’ is also called normally closed contact of a device.

aa-       It is always open. It only closes for a very short time when the driving force; (air pressure, hydraulic pressure or spring) operating the main device is in action. It returns to its original position when driving force ceases.

bb-       It is opposite to ‘aa’ i.e it is always closed. It opens for a very short time when driving force operating the main device is in action. It re-closes or re-sets when driving force is ceased

5.2       DEVICE FUNCTION NUMBERS

Function No.	Type of Relay
2	Time delay relay
3	Interlocking relay
21	Distance relay
25	Check synchronizing relay
27	Under voltage relay
30	Enunciator relay
32	Directional power (Reverse power) relay
37	Low forward power relay
40	Field failure (loss of excitation) relay
46	Negative phase sequence relay
49	Machine or Transformer Thermal relay
50	Instantaneous Over current relay
51	A.C IDMT over current relay
52	Circuit breaker
52a	Circuit breaker Auxiliary switch “Normally open” (‘a’ contact)
52b	Circuit breaker Auxiliary switch “Normally closed” (‘b’ contact)
55	Power Factor relay
56	Field Application relay
59	Overvoltage relay
60	Voltage or current balance relay
64	Earth fault relay
67	Directional relay
68	Locking relay
74	Alarm relay
76	D.C Over current relay
78	Phase angle measuring or out of step relay
79	AC Auto reclose relay
81	Frequency relay
81U	under frequency relay
81O	over frequency relay
83	Automatic selective control or transfer relay
85	Carrier or pilot wire receive relay
86	Tripping Relay
87	Differential relay
87G	Generator differential relay
87GT	overall differential relay
87U	UAT differential relay
87NT	Restricted earth fault relay (provided on HV side of Generator transformer)
95	Trip circuit supervision relay
99	Over flux relay
186A	Auto reclose lockout relay
186B	Auto recluse lockout relay

5.3       BASIC REQUIRMENTS OF CONTROL CIRCUITS

For circuit breakers there are a number of basic requirements which are desirable in the control circuit. These features can be found in the motor, solenoid, spring (stored energy) and pneumatically-operated Circuit Breakers.

A good understanding in these control circuit features will allow an intelligent approach to trouble-shooting. Examination of the circuit diagram of the modern breakers will relatively complicated network of switches, contactors and coils, the correct functioning of which is essential. Each individual component of a circuit has a definite function to perform, thus removing any one element will cause some type of faulty operation.

When the maintenance electrician knows the function of each component, he also knows what type of faulty operation to expect when that component is inoperative. Conversely when mal-operation of a breaker is found, the likely component or components at fault will be known.

5.3.1        CONTROL THE CLOSING

The closing circuit must do more than merely close the breaker, it must control this closing. To do this, the following features are necessary.

Initiate The Closing Stroke: Means must be provided in the circuit to energize the closing device, for example, the solenoid of the solenoid-operated breaker or the motor of a motor-operated breaker.

Cut-Off: The closing power must be cut-off or disconnected automatically at the end of the closing stroke. This is necessary to prevent overheating of the closing device. Solenoid coils used on circuit breakers have only a short time rating, thus if a closing coil is left energized for more than 15 seconds. It will overheat and suffer damage to the insulation. For this reason it cannot be left to the operator to decide when to cut off the closing power since it left on too long, damage will result. The alternative where the closing power could be left on for too short a time is covered in Seal-In.

Seal-In: It is desirable to have the control circuit ensure that the breaker will fully close each time that closing operation is initiated, if the breaker is closed by a simple switch. Simply speaking; it completes the operation automatically started by us manually so as not to hold the push button all the times.

5.3.2   CONTROL THE TRIPPING

Initiate Trip Stroke: Means must be provided to trip the breaker. This may involve energizing a solenoid coil to trip a latch or in case of air blast breakers, to admit air to the blast valves and contacts.

Cut-Off: For the reasons noted in above, means must be provided to automatically disconnect the trip coil.

5.3.3        TRIP FREE FEATURE

When closing a breaker, the closing device (for example, the solenoid in a solenoid-operated breaker) is energized and the plunger operates through the linkage to close the breaker contacts. At the end of the closing stroke, appreciable time is required to de-energize the solenoid coil. In the event that the breaker has been close on a faulted circuit, it must be reopened as quickly as possible. If a breaker can trip automatically upon receiving a trip signal before closing operation is complete, it is said to be “trip free”.

Various arrangements are provided to obtained trip-free action. Solenoid-operated breakers are sometimes provided with action. Solenoid-operated breakers are sometimes provided with a collapsible linkage. Pneumatically-operated breakers may be equipped with two latches, one of which is unlatched during a normal trip operation and the other is only unlatched for a trip signal while the breaker is closing. Other breakers use a large dump valve to quickly exhaust the air under the closing piston. Many motor-operated breakers obtain a fast trip-free action by use of a relay energized from the trip circuit to open the closing circuit. These methods would be known as mechanically trip free, pneumatically trip free and electrically trip free respectively.

5.3.4    ANTI-PUMPING FEATURE

When a breaker is closed and a trip-free operation results, the close and trip stroke will be completed in a very short time for a modern pneumatically-operated high voltage breaker, the complete operation will take less than one-half of one second, thus it is quite likely that the operator will still have the control switch in the closed position. Means must therefore be provided to prevent the breaker from closing a second time, even though the operator is holding the control switch in the closed position. This is usually accomplished by the use of a sealed in relay which can only be released which in the closed position. This is usually accomplished by the use of a sealed in relay which can only be released by opening the closing control switch. When this feature is incorporated in the control circuit, the breaker is said to be “pump free” or “anti-pumping”. Following a trip-free operation of the breaker, the operator must release the control switch before a second attempt to close the breaker can be made.

5.3.5        ANTI-SLAM FEATURE

This feature prevents the energization of closing coil or tripping coil of an already closed breaker or tripped breaker respectively.

1.      In closing circuit this feature is mostly achieved through auxiliary switch b.

2.      In opening circuit, this feature is mostly achieved through auxiliary switch a.  

5.3.6        RELIABILITY

A circuit breaker is a protective device. It will be called upon to open faulty circuits infrequently, however while it may stand inoperative for long periods; it must be relied upon to operate correctly in time of trouble. Reliability for such a protective device is essential. For this reason a battery supply is always used to provide the tripping power and in most cases for closing.

The control circuits usually have a separate trip and close bus. This is to give extra reliability to the trip circuit. On 115 kV and above circuit breakers there are dual trip buses thus, if a closing control circuit fuse fails during a closing stroke, the trip circuit or circuits are not affected.

The above requirement of extra reliability during tripping is also seen in the size of fuses used in the trip and closed circuits, for example, on a solenoid-operated breaker the fuses in the closing circuit will be rated at slightly less than value of current obtained by dividing the voltage by resistance. The fuses must be so rated to provide a measure of protection for the short time rated closing coil. During a normal closing operation, the closing coil will be energized for less than one second. To have the closing fuses blow in approximately six seconds, it is necessary to use a size which is actually less than the maximum current that the closing coil will normally draw. Conversely, in the trip circuit the fuses will be rated at several times the current obtained by voltage to resistance ratio. If the trip coil is not automatically disconnected at the end of the trip stroke, the trip coil may carry current for a long period and being are short time rated coil, it will be damaged. The fuses in this circuit will open only due to some fault condition. In order to gain more reliable tripping, we do not protect the short time rated trip coil. The trip fuses are not put in the breaker but are located in the control building.

5.3.7    GENERAL MAINTENANCE OF BREAKER CONTROL RELAYS

Frequent reference is made in this reference material to relays. These are control relays located in the operating mechanism housing. They are concerned entirely with the sequence of the mechanism of the breaker. The control circuit relays are all located on the breaker side of the four-pole isolating switch. Such relays are a part of the breaker, being required for the breaker’s correct functioning as much as possible, for example, a trip coil or interrupter and as such are the responsibility of the maintenance electrician.

Other relays remote from the breaker determine under what system conditions the breaker will be tripped. These protective relays, together with the interposing relays where such are used, form the protective network and are the responsibility of the Meter and Relay Department. The dividing line between the breaker control circuit relays and protective relays is well defined and there should be not confusion in this regard.

5.3.8        ASA DEFINATIONS

RELAY: A relay is a device that is operative by a variation in the conditions of one electric circuit to effect the operations of other devices in the same or another electric circuit.

CONTROL RELAY: A control relay is a relay which functions to initiate or to permit the next desired operation in a control circuit or scheme.

PROTECTIVE RELAY: A protective relay is a relay, the principal function of which is to protect service from interruption by removing defective components or to prevent or limit damage to apparatus.

5.4      OVERLOAD PROTECTION

In order to avoid damage to motors etc. due to temperature rise because of overloading and defective bearings etc., overload protection features are incorporated in motor control circuits. It should be noted that over load relay or element is always incorporated in the power circuit but its contact is installed in the control circuit. Due to this, the life of contact increases as it breaks small current because in control circuit current is small. Over load relays are mostly operated thermally and may be of bimetallic strip type or solder pot type.

5.5       OVER CURRENT OR SHORT CIRCUIT PROTECTION

The function of over current protective devices is to protect motors and its circuit elements etc. from damage in case of phase-phase short circuits or phase-ground faults etc. The over-current device must be capable of carrying the starting current of motors. Mostly fuses and magnetic devices are used as over current protective devices. Rating of fuse should not exceed 300% of full load current.

5.6              CONTACTOR

Contractor is a device which is operated electrically and controlling the operation of other circuits magnetically. Contactor may also be called as an ON-OFF Switch. Contactor has two types of contacts:

1.      Main Contacts: These are used in power circuits and hence must be strong to carry the full load current of motor continuously without undue heating.

2.      Auxiliary Contacts: These are small and used in control circuits only. These may be NO or NC and are used as seal in contact (NO), interlocking contact (NC) and for indications etc.

5.7              MAINTENANCE OF CONTACTOR

Contactor maintenance mainly includes

1.      Removal of rust or deposits etc. from contacts with emery paper and dry cloth. Never file the contacts as it will remove the elkonite from the contacts.

2.      Checking of contacts alignment

3.      Free movement of moving contacts assembly with binding etc.

4.      Checking of connections at terminal points.

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