module no e04 lv switchgear

8/10/2019 Module No E04 LV Switchgear

1/53

Capability & Improvement Dept.2004

PETRO NA S

GA S

TRAINING MODULE

ELECTRICAL

TITLE : LOW VOLTAGE SWITCHGEAR/ MCC

MODULE NO : E04


2/53LV Switch ear & MCC Pa e 1 of 52

DUTY NO 07: LOW VOLTGAE SWITCHGEAR & MCC

OBJECTIVES

Upon completion of this module, the technician would be able to demonstrate

knowledge and understanding on the following:

1. low voltage electrical distribution system

2. Construction of LV switchgear

3. Functions of components LV switchgear

4. Construction & operation of ACB.

5. Functions of components of ACB

6. Construction & operation of MCC

7. Functions of components of MCC

8. Assembly drawings, schematic drawings.

9. Preventive maintenance and testing of LV switchgear

10. Preventive maintenance and testing of MCC.

11. Automatic transfer scheme



TABLE OF CONTENT

Description Page

1.0.0 Introduction 5

2.0.0 Low voltage power distribution. 6

2.1.0 The main components 7

2.2.0 LV switchgear and MCC 7

3.0.0 Metal enclosed low voltage switchgear 8

3.1.0 Construction 8

3.1.1. Structure 8

3.1.2. Bus bars 10

3.1.3. Circuit breakers 11

3.1.4. Protective devices 12

3.1.5. Metering 12

3.1.6. Wiring 13

3.2.0 Low voltage Metal enclosed SwitchgearSpecifications 14

4.0.0 Air Circuit Breaker (ACB) 15

4.1.0 Theory of arc extinction 15


4.2.1. Main circuit and quenching mechanism 17

4.2.2. Operating mechanism 17

4.2.3. Secondary circuit 17

4.2.4. Interlocks and indications 18

4.3.0 Air Circuit breaker ratings 19

4.3.1. Rated operational voltage 19

4.3.2. Rated insulation voltage 19

4.3.3. Rated thermal current 19

4.3.4. Rated uninterrupted current 20

4.3.5. Rated short circuit making capacity 204.3.6. Rated short circuit breaking capacity 20

4.3.7. Rated short time withstand capacity 20

4.3.8. Short circuit performance categories 20

5.0.0 Motor starter/ controller 21




Description Page

6.0.0 Molded Case Circuit Breaker (MCCB) 22


6.1.1. Molded case 23

6.1.2. Operating mechanism 23

6.1.3. Trip element 24

6.1.4. Line and load terminals 25

6.1.5. Accessories 25

6.2.0 Ratings 25

6.2.1. Voltage rating 25

6.2.2. Continuous current rating 25

6.2.3. Interrupting capacity rating 25

6.2.4. Ambient temperature rating 25

6.2.5. Frequency rating 26

7.0.0 Isolator and HRC fuses 27

7.1.0 Fuse rating 28

7.1.1. Voltage rating 28

7.1.2. Ampere rating 28

7.1.3. Interrupt rating 28

7.1.4. Current limiting rating 28

8.0.0 Contactors 298.1.0 Construction 29

8.2.0 Arc suppression 30

8.3.0 Contactor rating 30

8.3.1. Rated voltage 30

8.3.2. Rated current 30

8.3.3. Rated duty 31

8.3.4. Making capacity 31

8.3.5. Breaking capacity 31

8.3.6. Utilisation category 32

9.0.0 Thermal overload relay 33

10.0.0 Starter circuits and schematic drawing 34

10.1.0 Direct On Line (DOL) starter 34

10.2.0 Star Delta starter 34

10.3.0 Circuit description (Schematic diagram) 35



Description Page

11.0.0 Testing and maintenance of Air Circuit Breakers 37

11.1.0 Frequency of maintenance 37

11.2.0 Safety precautions 37

11.3.0 Maintenance procedure 38

11.4.0 Lubrication of circuit breakers 39

11.5.0 General guidelines for lubrication 39

11.6.0 Testing of Air circuit breaker 40

12.0.0 Testing and maintenance of Motor Control Centre (MCC) 41

12.1.0 Safety precautions 41

12.2.0 General guidelines 42

12.3.0 Visual inspection of MCC 42

12.3.1.Frame/ enclosure 42

12.3.2.Phase/ Neutral/ Ground bus 42

12.3.3.Bus supports 42

12.3.4.Panel indicators/ instrumentation/ control wiring 43

12.3.5.Cabinets/ cubicles/ interlocks 43

12.4.0 Cleaning of MCC/ starter unit 43

12.5.0 Mechanical checks 43

13.0.0 Testing and maintenance of motor starter/ controller 45

13.1.0 Preventive maintenance on MCCB 4513.2.0 Routine maintenance tests on MCCB 45

13.2.1.Insulation resistance test 45

13.2.2.Milivolt drop test 45

13.2.3.Connections test 45

13.2.4.Overload tripping test 45

13.2.5.Instantaneous tripping test 46

13.2.6.Mechanical operation 46

13.3.0 Isolator (Disconnect switch) 46

13.3.1.Fuse holders 46

13.4.0 Contactor 46

13.5.0 Protective relays 46

14.0.0 Automatic transfer scheme 48


6/53



2.0.0 Low voltage power distribution:Power requirement of the industry such as petrochemicals or fertilizers is very large.

Therefore the power intake form the supply company/ utility is at high voltage (11 kV or above), but

the majority of the load requires low voltage power supply. Hence every petrochemical or fertilizer

industry has the typical low voltage power distribution system as shown in fig.1. Utmost importanceis given to the reliability aspect of power supply while designing the power supply distribution

system. Keeping in mind other qualities of good distribution system such as flexibility in operation

and ease of maintenance a duplicate supply distribution system is adopted. Thus there are two

feeders, two transformers instead of one for incoming supply. Each feeder and transformer has

sufficient capacity to take entire load independently.

MToLDB

HV BUSBUS TIE

VCB

TRANSFORMER

MCCB

CONTACTOR

O/L RELAY

VCB VCB

ACB ACB

ACB

BUS TIELV BUS

M MM ToLDB



2.1.0 The main componentsa) HV feeders : These feeders fed the power to the transformer at high voltage (11 kV , 6.6 kV or

3.3kV)

b) Transformers : It transform the power to low voltage (430/ 415V) and fed the load through LV

switchgear and cables

c) Switchgear/ MCC : It performs the basic function of switching and protection of the system

d) LV cables : These cables fed the power to the equipment located inside the plant

e) Electrical equipment (motors, lighting, power sockets, heaters, UPS, Battery chargers etc.) :

These equipment convert the electrical energy to the required form of energy (Mechanical, heat,

light etc.)

2.2.0 LV switchgear and MCC:Switchgear is the general term covering wide range of equipment concerned with switching

and protection. Switchboard is an assembly consists of circuit breakers, switches, protective relays,

instrumentation, and control. Switchgear equipment comes in various forms and rating depending on

particular functions it is to perform. Metal enclosed switchgear is most commonly used for low

voltage power distribution.

MCC consists of motor starters backed up by HRC fuses or Molded Case Circuit Breaker (MCCB).

As the low voltage motor starter circuits need small clearance, the several circuits can be

conveniently arranged in the individual switchboard compartments. MCC essentially consists of

contactor, HRC fuses or MCCB, protective relays, instrumentation, and control.



3.0.0 Metal-Enclosed Low Voltage SwitchgearIt is completely enclosed on all sides and top with sheet metal. The assembly contains low

voltage circuit breakers, switching or interrupting devices, with buses and connections. It may

contain control, measuring, or protective devices.

Metal-enclosed low-voltage switchgear includes the following equipment and features:1) Low-voltage power circuit breakers that are mounted stationary or removable and contained in

individual grounded metal compartments.

2) When the circuit breaker is removed, automatic shutters close off and prevent exposure of the

primary conductors.

3) Bare bus and connections.

4) Instrument transformers

5) Instruments, meters and protective relays

6) Control wiring and accessory devices.

7) Circuit breakers may be controlled at the switchgear or from a remote point.

8) When the circuit breakers are removable, mechanical interlocks are provided for proper

operating sequence

Some typical arrangement of metal enclosed low voltage switchgear is shown in fig.2.

3.1.0 ConstructionThere are two basic types of low voltage switchgears, those for use outdoors and those are

use indoors. Indoor switchgear is, of course, considerably less expensive. Indoor types of

switchgears are most commonly used for Oil & Gas or Petrochemical and Fertilizer industry. The

transformers are located outside the switchgear building and connected to the switchgear with bus

duct or cables. The following discussion is limited to indoor type of switchgear only.

3.1.1 Structures:

The indoor switchgear will consist of a front section, which will contain the circuit breakers,

meters, relays and controls; a bus section; and a cable entrance section. Each circuit breaker is

isolated from all other equipment. Vents are provided in the circuit breaker compartments, for

cooling and to allow escape of gases, which are formed when the circuit breaker opens to interrupt

fault currents. Other sections of the switchgear are also ventilated to allow circulation of air for

cooling.

Barriers between the bus compartment and the cable compartment are provided for safety

reasons, to permit connecting or disconnecting the cables without danger of contacting the live bus.



Rear cable

Compartment

Bus

Compartment

Switch ear side view

Base channel

Through

doorcircuit

breaker

Hinged

door

Door

latch

Switchgear front view Switchgear rear view

Auxiliar

instrumentcompartment

Circuit

breakercompartment



All cable terminations are insulated after installation, so that there will be no danger of

contacting these live terminals when making changes in the cable connection.

In double-ended substations the barrier between the two sections of the switchgear is

provided. A horizontal barrier is placed between the upper and lower terminals of the tie breaker. A

vertical barrier is provided to isolate the two buses. Vertical barrier is so as to prevent an arcing fault

on one end of the substations from travelling to the other end and taking out both power sources.

When drawout breakers are used, a hoist for removing the breaker is desirable. This may not

be required if the overhead crane or other lifting facilities are available in the building.

3.1.2 Bus Bars:

Copper is the better material for the bus bars than aluminum, except in few cases where

corrosive atmospheres may have an advert effect of copper. Copper has a higher conductivity than

aluminum, it is more easily plated and bolted joints can be made more easily. Also the melting point

of aluminum is lower than that of copper, so that more damage will be done to aluminum buses in

case of arcing fault. However, copper is more expensive than aluminum and most switchgear

eutral

bus

Horizontal

Crossbus

Ground

bus

Bus Compartment (Rear view)



manufacturers now use aluminum for bus bars unless copper is specified, in which case they may

charge a premium price. Copper joints are normally silver plated. Aluminum bolted joints are

normally tin plated.

Each vertical section of low voltage switchgear will contain from one to four circuit breakers.

This requires branches and tap off the main bus supported on insulators. The insulating materials

used should be flame retardant, track resistant and nonhygroscopic. It also must have high impact

strength to be able to withstand stresses caused by the magnetic forces when fault occurs. Glass

polyester is the best material available for this purpose.

The insulators must be assembled to the bus in such a way that there are not continuous

horizontal surfaces between bus bars. Such surface may collect dust that may form a high resistance

path between the bars, which could develop into a fault. Supports for the bus bars should be placed

at frequent intervals so that the bars will not be deformed when a fault occurs.

3.1.3 Circuit Breakers :

Breaker with required frame size and the desired trip rating be installed. The interrupting

rating is standardized with the frame size of breakers but some make breakers in other than standard

frame sizes. For information on the

interrupting rating of these breakers the

concern manufacturer may be consulted.

Low Voltage Circuit Breakers may

be obtained with either fixed or drawouttype. Switchgear with fixed breakers is less

expensive. However, removal of a circuit

breaker of the fixed type should not be

attempted unless the main bus is

deenergised. Therefore, if fixed type is used

a shut down may be necessary in case of

trouble with any one of the circuit breakers,

and it will definitely be required for periodically for routine maintenance. For this reason the extra

cost of the drawout type circuit breaker is usually justified.

When drawout type circuit breakers are used a method is provided for moving the breaker

from the fully withdrawn position to a test position, and to fully connected position. For safety of the

operator, it should be possible to move the breaker from one position to another with the breaker

compartment door closed and only when the breaker is tripped. Also it should not be possible to

close the breaker when it is intermediate position between the test and the connected position.

Vertical

Secondar

y

Horizonta

Circuit breaker compartment



When the breaker is in fully connected position, it is connected to the source and to the load

side terminals, and the frame is connected to the ground bus. This is the position for normal

operation. When in the test position, the breaker is disconnected from the source and from the load,

but secondary circuits for control of electrically operated breakers, and for monitoring, are

connected. In this position all secondary circuits may be tested and the breaker may be closed and

tripped, either manually or electrically, without affecting the primary circuits. In the withdrawn

position, all circuits to the breakers are disconnected. The frame of the breaker should be connected

to ground in both the test and fully connected position. It may or may not be connected in the

withdrawn position.

Circuit breakers may be obtained either electrically or manually operated. Breakers must be

electrically operated if they are to be operated from a remote location or if they are to be used in any

automatic transfer scheme.

Circuit breaker may be equipped with various auxiliary devices. All electrically operatedbreakers are equipped with auxiliary switches, some of which are used in the breaker control

circuits. Undervoltage devices may be supplied on all the breakers. These devices will trip the

breaker when the voltage on the source to which they are connected falls below a certain value. They

are usually self- resetting so that the breaker may be reclosed as soon as voltage is restored, and may

be obtained to trip the breaker instantly on loss of voltage, or after a time delay.

3.1.4 Protective Devices:

Low-voltage circuit breakers may be obtained without overcurrent trip devices(nonautomatic), with magnetic trip devices, or with static trip devices. If supplied without

overcurrent trip devices they may be used as a switch, or they may be supplied with a shunt trip

device, which is operated through a relay to give desired protection. With this arrangement a source

of power must be available for the shunt trip device. Batteries are the most reliable source of power,

but control power may be taken from the source, which is feeding the switchgear, usually through a

control power transformer. However, it is necessary to make sure that such power is available when

needed.

The magnetic-type overcurrent device was the standard type used by all manufacturers for

many years, and it may still be available from some manufacturers. The device utilizes magnetic

forces created by the current through the breaker to trigger the release mechanism, which allows the

breaker trip. Various mechanical, hydraulic or pneumatic devices are used to delay the tripping,

when desired.

All manufacturers today offer static trip devices on their low-voltage large air circuit

breakers. These devices are more reliable, and they have characteristic curves with narrow bands and



with shapes designed to match well with curves of fuses, relays, and molded case breakers. These

static trip devices are fed by sensors, which are special current transformers, which monitor the

current in each phase. The devices receives power from these sensors to trip the breaker when the

current in the sensor is greater than the pickup current for which the device has been set. Timing in

accordance with the characteristic curves is also accomplished by the electronic circuitry.

Low-voltage distribution systems may be solidly grounded, grounded through a resister. The

solidly grounded system is used most frequently. When it is used, some form of ground-fault

protection should be provided.

3.1.5 Metering:

The metering to be specified with low-voltage switchgear depends entirely on the needs for

the information which metering can supply. Usually the minimum requirement is for voltmeter on

the incoming power source and an ammeter to indicate total current. These instruments are of the

single-phase type. They are usually supplied with switches to permit reading current and voltage of

all three phases. If the voltage is greater than 240V, potential transformers are required, since

standards do not permit voltages on the panels which exceed 250 V to ground. Current transformers

are required for all ammeters.

Ammeters and voltmeters may be supplied with either 1 or 2% accuracy. For most purposes

the 2% instrument is satisfactory. The 1% instrument may be obtained at a moderate increase in

price.

Test switches or test blocks may be specified to permit plugging in portable instruments. Thismay be desirable to check the accuracy of the panel instruments or to obtain more accurate readings.

Usually meters required for the incoming power will be mounted on a panel above the main

breaker. Potential transformers and control power transformers will be mounted inside this

compartment. Current transformers may be mounted in the rear, or if a main breaker is provided,

they may be in the breaker compartment. Ammeters, and ammeter switches for the feeders can often

be mounted on the breaker panel, if the meters are the panel type having 2% accuracy.



3.1.6 Wiring:

Wire used for metering and control circuits should not be smaller than No. 14 AWG. It should be

insulated with a material rated not less than 90oC. Crimp-type terminals should be used, and

terminals should be insulated. A detailed wiring diagram is required, showing the related location of

terminals on various devices and on terminal blocks. When troubleshooting or making changes,

wires can be identified by referring to this wiring diagram. The use of wire markers to identify each

wire can be specified.

3.2.0 Low voltage Metal enclosed Switchgear SpecificationsThe standards institutions publish the standard specifications on LV switchgear to cover

wider applications. These standards provide the guideline to the manufacturer for design andconstruction, whereas it is useful to the user for selection, erection and maintenance.

The followings are general specifications prescribed for the LV metal enclosed switchgear.

1) Service : The switchgear suitable for service conditions such as indoor/ outdoor/ temperature

2) Power system : The type of the system, voltage and frequency

3) Metal enclosed assembly : The description of the features required. Following is the example:

The assembly consists of breaker compartments with hinged front doors and sheet steel

Vertica

Horizont

Secondardisconnectin

Secondarterminal

Secondardisconnectin

Control

Shutter

Secondary wiring system



enclosure. The enclosure and welded components chemically treated and painted with light gray

paint. Provisions for racking the breaker to connected, test and disconnected position with

door closed. Mechanical interlocks to prevent the racking of breaker when it is closed.

Arrangement of space heaters. Provision for the future expansion.

4) Circuit breakers : Types and specifications of the circuit breakers for the followings:

1. Main (incoming feeder)

2. Bus tie

3. Load (outgoing feeder)

It includes the voltage, current ratings, interrupting capacity, type of the mechanism, arc quenching

medium, different indicators, auxiliary switch, interlocks, and built in protective devices.

5) Bus : Material, current rating and short time withstand current rating of the main bus and neutral

bus. Length of the ground bus.

6) Control power transformer : Type, capacity, voltage ratio, protection7) Instrument transformer (CT/VT) : Type, voltage/ current ratio, burden

8) Indicating/ recording meters : Type, range, scale reading

9) Wiring : Installation method (metal channel or conduit), material, size and markers on the

secondary wiring.

10) Accessories : Crank for manual operation of the breaker drawout mechanism, lifting yoke for

each type of breaker element, test plugs/ blocks, test cables.



4.0.0 Air Circuit Breaker (ACB)Air circuit breakers are most commonly used for low voltage industrial applications. The air

at atmospheric pressure is used as an arc extinguishing medium. These breakers are generally indoor

type and installed on vertical panels or draw out type switchgear.

Air circuit breakers have several advantages:- Less contact erosion from short circuit and load current operation, resulting in greater time

between maintenance.

No insulation handling

Fire-risk is minimum

The main disadvantages are:-

The need to install them indoors

High initial cost

An increase insulation hazard due to free movement of insulation contamination

Overall dimensions are larger owing to air insulation

Limited fault clearing ability.

4.1.0 Theory of arc extinction :The resistance of the current path is increased gradually resulting in the increased voltage

drop. The arc extinguished when the system voltage can no longer maintain the arc , due to high

value of the voltage drop. This principle is used in air break type a.c. circuit breakers. The air at

atmospheric pressure is used as an arc extinguishing medium in Air Circuit Breakers. These circuit

breaker employ the principle of high resistance interruption principle. In the air circuit breaker the

contact separationand arc extinction takes place in air at atmospheric pressure. As the contacts of

Splitter lates

Arc

runners

Arc

chute Arc

Arcing

contacts

Main

contacts

Arc extinctio



breaker are opened arc is drawn between them. The arc is basically consists of a plasma surrounded

by the ionized particles of air. The arc is rapidly lengthened by means of arc runners and arc chutes.

The resistance of arc is increased by cooling,

lengthening and splitting the arc. The arc resistance is increased to such an extent the system

voltage can not maintain the arc and the arc gets extinguished.

4.2.0 Construction :The circuit breaker main components are as follows:

Main circuit (poles) and insulators

Arc quenching mechanism

Operating mechanism.

Secondary circuit (closing, tripping and control)



Interlocks and indication

The frame or structure

4.2.1 Main circuit and arc quenching mechanism:

There are two sets of contacts: Main contacts and Arcing contacts. Main contact conduct the

current in closed position of the breaker. They have lower contact resistance and are usually silverplated. The arcing contact are hard, heat resistant and are usually of copper alloy. While opening the

contact the main contact dislodge first. Then the current is shifted to the arcing contacts. The arcing

contacts dislodge later and the arc is drawn between them. The arc is forced upwards by the

electromagnetic action. The arc moves towards the arc chute where it is extinguished by splitting,

lengthening and cooling.

4.2.2 Operating mechanism :

The operating mechanism for air-break circuit breakers are generally with operating spring.The closing force is obtained from one of the following means :

Solenoid : The solenoid mechanism drive power from battery supply or rectifiers. The

solenoid energised by the direct current gives necessary force for closing the circuit breaker.

Spring charged manually or by motor : The springs used for closing operation can be charged

either by manually or by motor driven gears. At the time of closing the operation the energy stored

in the spring is released by unlatching of the spring and is utilised in closing of the breaker. During

the opening operation, the operating signal is given to trip coil. The movable system is unlatched and

the energy of the opening spring is released to obtain the opening. The closing spring is

automatically charged after each closing operation. Hence energy is always available for reclosing of

breaker. Both opening and closing operations are initiated by high speed, electromagnetically

operated latches.

4.2.3 Secondary circuit :

This consists of the followings:

Electrical operating mechanism: This includes the gear motor, a closing release, undervoltage

release and spring charged limit switch. Some of the breakers are also equipped with the

operation counter. This facilitate to read and record the total no. of breaker operating cycles. The

manual mechanism is always available for emergency use.

Generally the breaker is fitted with the time delayed undervoltage release. This will not allowed

the breaker to close if the voltage is below (typically say, 85% of ) rated voltage. The closed

breaker will open if the supply voltage drops below a value typically 35% and 70% of its rated


20/53



Door interlock: This interlock prevents the cubicle door from being opened when the breaker

is in the service (connect) position.

Racking interlock : This interlock prevents the insertion of the breaker when the breaker

cubicle door is open.

Withdrawal/ spring charged interlock : This interlock prevents the removal of the breakerwhen the breaker is closed or closing spring is charged.

Shutter lock: The shutter automatically block the access to the disconnecting contacts when

the breaker is isolated (disconnected) or in test position. The provision could be made to padlock the

shutters when breaker is withdrawn from its cubicle.

Electrical interlocks: The contacts of auxiliary switch or releases or protection relays can be

used in the control circuit to provide the required electrical interlocks.

The breaker are provided with the mechanical as well as electrical indicators.

Mechanical interlocks could be to indicate:

The breaker position indicators

Service (connected)

Test

Isolate (disconnected)

The stored energy mechanism is charged/ discharged

The breaker is opened

The breaker is closed

Fault trip indication

Electrical indications are provided by mounting the indication lamps on the front door

of the panel. The standard indications are as follows:

Colour of the lamp Indication

Red Breaker is closed

Green Breaker is opened

Amber Breaker is tripped

White Breaker trip circuit is healthy



4.3.0 Air Circuit breaker ratings:

4.3.1 Rated operational voltage:

It is the value of the voltage to which the making and breaking capacities and short circuit

performance categories refer.

4.3.2 Rated insulation voltage:It refers to the voltage to which the test voltages , clearances and creepage distances refer. It

is generally the maximum operational voltage.

(for three phase circuits, the rated voltage is the line voltage)

4.3.3 Rated thermal current:

It is the maximum value of the a.c.(rms)current or steady value of d.c. current which breaker

can carry in eight hour duty without abnormal temperature rise.

4.3.4 Rated uninterrupted current: :It is the maximum value of the a.c.(rms)current or steady value of d.c. current which breaker

can carry in an uninterrupted duty without abnormal temperature rise.

4.3.5 Rated short circuit making capacity(*):

It is the value of prospective peak current that the circuit breaker is capable of making.

4.3.6 Rated short circuit breaking capacity(*):

It is the maximum value of a.c (rms) current that the circuit breaker is capable of breaking.

4.3.7 Rated short time withstand current (*):It is the maximum short-circuit current (rms) that the circuit-breaker can withstand for a

short period of time (0.05 to 1 s) without its properties being affected.

[(*) These currents are defined for a specific operational voltage rating]

4.3.8 Short circuit performance categories:

Category Operating sequence for short circuit tests

P-1 O - t - CO

P-2 O - t - CO - t - CO

O - Breaking operation t - Specific time intervalCO - Making operation followed by breaking



5.0.0 Motor Starter/ ControllerThe motor starters/ controllers range from a simple toggle switch to a complex system using

contactor, relays, and timers. The basic functions of a starter is to control and protect the operation of

a motor. This includes starting and stopping the motor, and protecting the motor from overcurrent,

undervoltage, and overheating conditions that would cause damage to the motor. There are two basiccategories of motor starters: the manual and the magnetic. As the manual starter is seldom used in

the plants, is not discussed here. A large percentage of control applications require that the motor

starter to be operated from a remote location or operate automatically in response to control signals.

5.1.0 Construction:The basic parts of the motor starter are:

Molded case circuit breaker (MCCB) or isolator with HRC fuses

Magnetic Contactor

Protection relay (Thermal Overload relay)

Indicating instruments/ lamps

All these components are connected electrically to each other to perform the basic function of the

starter.

A simple Direct On Line (DOL)starter circuit is shown below:

This circuit can be divided functionally in two parts. 1) Power circuit and 2) Control circuit.

The Power circuit contains all of the components that carry the full voltage and current to operate the

motor. Besides the magnetic contactor, these commonly includes isolator-HRC fuses or MCCB and

heater elements of the thermal overload relay.

The control circuit is usually operated at lower voltage and contains all the components

necessary to switch and monitor power to run the motor or to stop the motor under the

predetermined condition and time. These commonly includes the devices like push buttons,

protection relays, indicating devices, contacts of process instrument relay.



Operating

Knob

Arc chute

Moving

Contact

Quick make

Quick break

mechanis

Common

tri bar

Load terminalThermal

trip

ad ustment

Magnetic

trip

ad ustment

Molded

housing

Line

terminal

6.0.0 Molded Case Circuit Breaker

Molded case circuit breaker is used for overcurrent protection at the entrance of the starter

circuit. The MCCB in the motor circuit provides the protection to that branch of the distribution

system only. Like any other breaker its function is to make, break and carry the current in normal

and abnormal or fault condition without any damage.

The main difference in operation of ACB and MCCB is that MCCB can be closed manually

and not automatically. But it can open automatically at a predetermined overcurrent. MCCB are

available with thermal-magnetic protection or magnetic protection only. As most of the motor starter

are equipped with thermal overload protection, for motor circuit MCCB with magnetic (overcurrent)

protection only is preferred.



6.1.0 Construction:Most common components of any MCCB are: the molded case, an operating mechanism, trip

element, line/ load terminals and optional accessories such as shunt trip, undervoltage trip, auxiliary

switch etc.

6.1.1 Molded case:

The function of the molded case is to provide an insulated housing to mount all of the circuit

breaker components. The case is molded from a phenolic material which combines high dielectric

strength with ruggedness. Following factors determines the strength of the casing: Maximum

current, voltage and interruption capacity.

Higher the rating, stronger the case.



6.1.2 Operating mechanism:

Most of the multi-pole breakers have a single operating handle. A circuit can be connected

or disconnected using a circuit breaker by manually moving the operating handle to the ON or OFF

position. All breakers, with the exception of very small ones, have a linkage between the operating

handle and contacts that allows a quick make (quick break contact action) regardless of how fast the

operating handle is moved. The handle is also designed so that it cannot be held shut on a short

circuit or overload condition. If the circuit breaker opens under one of these conditions, the handle

will go to the trip-free position. The trip-free position is midway between the ON and OFF positions

and cannot be re-shut until the handle is pushed to the OFF position and reset.

(Trip-free feature: If the breaker closing signal is present, the operating mechanism will start

closing the breaker. Meanwhile if a protection relay closes the trip circuit and energises the trip

coil. The trip-free mechanism will permit the tripping (opening) of the breaker, even if it is under the

process of closing.)

When the separable contacts of an air circuit breaker are opened, an arc develops between the

two contacts. Different manufacturers use many designs and arrangements of contacts and their

surrounding chambers. The most common design places the moving contacts inside of an arc chute.

The construction of this arc chute allows the arc formed as the contacts open to draw out into the arc

chute. When the arc is drawn into the arc chute, it is divided into small segments and quenched. This

action extinguishes the arc rapidly, which minimizes the chance of a fire and also minimizes damage

to the breaker contacts.

Push-To-Trip is a another feature available for the MCCB. This permits the operator to

manually trip the circuit without exposing the operator to live parts.

6.1.3 Trip element:

Molded case breakers may have a thermal and magnetic

trip units. MCCB used in motor starter circuits have a magnetic

trip element only as the thermal protection is provided by a

separate relay installed in starter circuit. Magnetic trip is an

instantaneous trip. It is the part of a trip unit which contains an

electromagnet assembly to trip the circuit breaker

instantaneously at or above a predetermined value of the current.

Usually each pole of the breaker has the magnetic trip element.

This element responds to a given value of overcurrent. The

magnetic unit utilizes the magnetic force that surrounds the

conductor to operate the circuit breaker tripping linkage as



shown in fig. MCCB with fixed and adjustable type magnetic trip elements are available. The

adjustment will set all the poles simultaneously for same value of tripping current. The adjustment is

from approximately 5-10 times the breaker continuous current rating.

6.1.4 Line and Load terminals :

All the circuit breakers have provisions for making line and load connections in the external

electrical circuit. For industrial application, compression type lugs or mechanical type lugs are

provided to facilitate these connections.

6.1.5 Accessories:

A wide range of accessories are available for MCCB to suit it for particular application.

Some of them are discussed below.

Shunt trip: This is the mechanism operated by a solenoid when energised from separate

source. It, in turn, will trip breaker. This solenoid circuit can be closed by an external relay or other

means. A continuous flow of current through shunt trip coil may damage it because it is not rates for

that continuous current. Therefore the coil clearing switch is included to break the solenoid circuit

when the circuit breaker opens.

Undervoltage trip: It is a device which trips the circuit breaker automatically when the main

circuit voltage falls below 35-70% of its specified value. The breaker could be closed only if the

85% of rated voltage is available. An undervoltage trip with adjustable time delay unit is available to

avoid nuisance tripping of breaker in the event of voltage dip or momentary fluctuations.

Auxiliary switch: It is the mechanically operated switch by the breaker itself. They have

normally open (NO) or normally closed (NC) contacts. It is used for signaling, interlocking and

indicating contact position.

Padlocking attachments: These attachment for the padlocking of breaker in OFF an/or ON

position for safety reason. They do not interfere with the tripping of the breaker.

6.2.0 Ratings:

6.2.1 Voltage rating:

A circuit breaker can be rated for either alternating current (AC) or direct current (DC)system application or both. It is the maximum system voltage on which they can be applied. While

selecting the MCCB voltage one must consider the maximum voltage that could exist on the system.

Some of the typical ratings are : AC- 120, 240, 480 and 600 Volts and DC- 125, 250 and 600 Volts

6.2.2 Continuous current rating:

The maximum direct current or rms current in amperes at rated frequency which breaker

should carry continuously without exceeding the specified temperature rise limit of any of its parts.


28/53



7.0.0 Isolator and HRC fusesMost of the low voltage industrial motor control systems use either isolator-fuse (switch-fuse

unit) or molded case circuit breakers to rapidly cut power to the system when overcurrent (short

circuit fault) occur.

An isolator (disconnect) is normally the first device in a motor branch circuit. A singleoperating handle mounted on the front of the enclosure makes and breaks power to all lines

supplying power to the motor controller when repair or maintenance must be performed. Isolators

have interlocks which prevent the enclosure door from being opened unless the isolator is in the off

position. Once opened, power cannot be turned on without defeating the interlock. DO NOT

DEFEAT THE INTERLOCK to re-energize the starter unless you are a qualified and authorised

person capable of working safely around energized circuits.

Fusesare especially effective at interrupting overcurrent faults, even up to 200,000 amps.

These faults must be interrupted within a small fraction of a second to prevent damage to the motor

control system. Isolators are installed upstream of fuses so the fuses can be replaced safely.

Fuses contain thin strips of metal which conduct the same current that runs the motor. These

fusible elements melt when current exceeds acceptable levels. The size, design and composition of

the fusible elements determine the current level at which they melt. The filler of the fuse, often

quartz sand, helps to suppress the arc, dissipate the heat and speed up the process of interrupting the

circuit.

While many fuses respond almost instantly, most fuses used in motor control are dual-

element time-delay fuses. They are designed to pass the high inrush currents drawn while a motor is

starting. Time-delay fuses prevent nuisance fuse-blowing while still maintaining close protection for

the system after the motor is running. They are often sized 15 percent above the full-load amperage

of the motor. Single-element or one-time fuses are also common in motor control, but they must

usually be sized at 300 percent of the FLA of the motor in order to pass inrush current. Protection for

the system is substantially reduced.

The fuse possess inverse time-current characteristics. It means that fuses work faster for

high-level faults and more slowly for low-level faults. For example, the 100-amp fuse described by

this curve will blow in five minutes at 160 amps, one second at about 1100 amps but well under one-

hundredth of a second at 5,000 amps. Since it is current-limiting, this fuse will blow much faster

under very high faults, well under four thousandths (.004) of a second when subjected to a 200,000-

amp fault.

These characteristics make fuses ideal for interrupting high-level faults, so their use is very

common in circuits that have high available fault currents. However, their slower response at lower



fault currents means that the current that gets past the fuse before it blows - the let through current -

may be too great in some applications. That's one of the reasons MCCBs are often used in motor

control systems.

7.1.0 Fuse Ratings

There are four important ratings to consider when choosing replacement fuses: voltage,amperage, interrupt capacity and current-limiting ability.

7.1.1 Voltage ratings

Voltage ratings of motor control fuses are usually 250 or 600 volts. The rating must match or

exceed the voltage of the circuit where the fuse is used. Most 250-volt fuses are smaller than most

600-volt fuses, so fuses with different voltage ratings are usually not interchangeable. Some of them

are, however -especially control fuses - so you should never assume the voltage is correct just

because the fuse fits in the fuse clip.

7.1.2 Amperage rating

Current-carrying capacity of fuses varies from an eighth of an amp to 600 amps. Fuses of the

different amperage ratings can be installed in the same fuse base again, don't assume the amperage is

correct just because it fits in the fuse base. For example, one manufacturer's 600-volt fuses can all be

swapped with each other in the 35-amp to 60-amp range. While physical size does not prevent the

mis-application of fuses, it does make gross mis-application less likely. This is important because

undersized fuses will blow too easily while oversized fuses may not provide sufficient protection.

7.1.3 Interrupt capacity

Thisis the total current which the fuse can interrupt without being damaged. Many fuses

today have interrupt capacities as high as 200,000 amps because many industrial facilities have

available fault currents that high. The typical common interrupt ratings are 50,000 and 100,000

amps.

7.1.4 Current limiting ability

It is a measure of how much current is "let through" the system. Even a single cycle of a

200,000-amp current can severely damage motor control equipment. Current-limiting fuses must act

much faster when currents are so high. The current-limiting ability of the fuse is expressed as a

number, "K1" or "K5." K1 is more current limiting than K5.



8.0.0 CONTACTORSThe contactor is the central component of motor starter. During its useable life, it may switch

power to the motor on and off thousands of times, or even millions of times.

A contactor, by definition, is a device capable of making, carrying and breaking currents

under normal circuit conditions including operating overload conditions; the speed of make and

break being independent of the operator. In simple language a contactor can be described as a special

switch suitable for frequent remote operations. Contactors when used in motor circuit are usually

backed up by MCCB or HRC fuses for overcurrent protection.

The contactors can be classified into different categories according to its operating

mechanism such as electro-magnetic contactor, pneumatic contactor, electro-pneumatic contactor

and latched contactor and according to arc quenching medium, such as airbreak contactor and oil-

immersed contactor. In line with the present day tendency to eliminate the use of oil., modern

designs of contactors are of airbreak type. Most commonly used contactors are electromagnetic type.

Hence all further discussion refers the electromagnetic contactor only.

8.1.0 Construction:

Contactors have three basic parts: a set of stationary contacts, a set of movable contacts and

an electromagnet.

One side of the set of stationary contacts is wired directly to the power source while the other

side is wired directly to the motor. A gap separates the two sides, keeping power from reaching the

motor. The motor will not operate until the gap between the stationary contacts is bridged by the

movable contacts.



The movable contacts are mounted on aspring-loaded armature assembly. When the

electromagnet is energized, the armature shifts the movable contacts into position across the gap

between the stationary contacts, and power flows to the motor. When the electromagnet is

deenergised, the armature assembly is released, the movable contacts return to their normal position,

and the motor stops.

The electromagnet is energized by a coil, powered through a separate circuit, the control

circuit. When power flows through the control circuit, the coil is energized, the magnet attracts the

armature, the movable contacts bridge the gap between the stationary contacts, and the motor is

energized.

In addition to the main power contacts, most contactors have sets of auxiliary contacts that

are actuated as the armature closes. These contact sets are often used as seal-in contacts or as

contacts for powering indicating lights or other parts of the circuit. The contacts are usually easily

removed, so they can be changed from normally-open to norm ally-closed. Many contactors will alsoaccept contact blocks with multiple auxiliary contact sets that may be configured in various

combinations, such as normally-open, normally closed, on-delay and off-delay.

8.2.0 Arc SuppressionDamage to contacts is most often caused by electrical arcs. They can develop whenever the

circuit is made or broken. Arcs can develop at any voltage, usually when current is one ampere or

more. Each arc vaporizes a little of the metal surface of the contact and heats up the contact pad, the

mounting base, and the contactor. Suppressing arcs is an important part of making them work better.

Arcs are usually suppressed with conductive arc chambers which cool the arc environment; arc

chutes which divide the arc into smaller segments; arc horns which stretch the arc out

In AC contactors, the arc will be automatically extinguished as current drops through the

zero point if the arc environment has cooled enough to prevent re-ignition as the current rises again.

Arc chambers and chutes are often sufficient arc suppression mechanisms for AC contactors.

8.3.0 Contactor Ratings

8.3.1 Rated Voltages

-Rated operational voltage for three phase contactors. It is the rated voltage between phase.

-Rated insulation voltage Itis the voltage to which the dielectric rests, creepage distance are

referred.

8.3.2 Rated Current

-Rated thermal current: It is the maximum current the contactor can carry on eight-hour duty

without the temperature rise exceeding the permissible limits.


33/53



8.3.6 Utilization Categories of Contactors

Category Applications

AC-1 Non-inductive or slightly inductive loads, resistance furnaces

AC-2 Slip ring induction motors : Starting, plugging*

AC-3 Squirrel-cage induction motor : Starting, switching off

AC-4 Squirrel-cage motor : Starting, plugging, inching**

DC-1 Non-inductive and slightly inductive loads.

DC-2 Shunt-motors Starting, switching off

DC-3 Shunt motors Starting, plugging, inching

DC-4 Series motors Starting, switching off

DC-5 Series motors Starting, plugging, inching.


35/53


36/53



The typical schematic diagram of a DOL starter is shown in fig.

10.3.0 Circuit description:The dark lines drawn indicate power circuit. Power circuit has MCCB, Electromagnetic

contactor and heater elements of bimetallic thermal overload relay. Power is drawn from the LV bus

and flows to the motor through MCCB and closed contacts of contactor, when the power circuit is

energised and closing signal to the contactor is given.

The control circuit has fuse, earth leakage relay, coil of contactor, auxiliary relay, hour-run meter,

indicating lamps, start-stop push buttons and shunt trip solenoid of MCCB. After the energisation of

power circuit, if the start pushbutton is momentarily pressed then the contactor coil is energisedthrough series of contacts. The contact C1 of the contactor seal-in the start push button and contactor

coil ( C ) remain energised even if start push button is released. The hour-run meter and red

indicating lamp will get power supply as they are connected in parallel with the coil. The motor can

be stopped by pressing the stop or emergency stop push button, thus breaking the circuit of coil C

at the same time green lamp will illuminate indicating motor has stopped.

If the overload relay senses the excess current in the main (power) circuit, it will open its

contact O/L after a prescribed time delay and the coil C will deenergise resulting opening of the

O/L

CF NL

ELR

ELR MCP

C

ST

RS

HR

R

AUTO STOPSTOP START

A

LCS

IR P

G

A

C-2 RS-1

RS-2

C-1

STOP P/B

AREMOTE LINK FOR

CONNECTION TO

REMOTE AMMETER

R Y B N

415V 3PH. 50HZ

MCP

Z CT

C

M

T1

T2

T3

ST

O/L

SCHEMATIC DIAGRAM FOR 415V MOTOR



contactor. Therefore the motor will be stopped automatically. The operation of the overload relay

will also energise the auxiliary relay RS and its contact RS-2 will close. This will illuminate the

amber lamp on the MCC panel. Thus indicating motor has tripped on overload.

Earth leakage relay (ELR) will sense the unbalance in current due to earth fault through zero

current transformer (ZCT) and will operate to close the contact in the branch where shunt trip

solenoid is connected. Shunt trip will energise and open the MCCB to isolate the motor. Opening of

MCCB will result in opening of its contact in series with shunt trip solenoid and thus the solenoid

will denergised and will not be damaged.



11.0.0 Testing and Maintenance of Air Circuit Breaker

The maintenance of circuit breakers deserves special consideration because of their

importance for routine switching and for protection of other equipment. Electric transmission system

breakups and equipment destruction can occur if a circuit breaker fails to operate because of a lack

of preventive maintenance. The need for maintenance of circuit breakers is often not obvious as

circuit breakers may remain idle, either open or closed, for long periods of time. Breakers that

remain idle for 6 months or more should be made to open and close several times in succession to

verify proper operation and remove any accumulation of dust or foreign material on moving parts

and contacts.

11.1.0 Frequency of maintenanceLow-voltage circuit breakers operating at 600 volts alternating current and below should be

inspected and maintained very 1 to 3 years, depending on their service and operating conditions.

Conditions that make frequency maintenance and inspection necessary are:

a. High humidity and high ambient temperature.

b. Dusty or dirty atmosphere.

c. Corrosive atmosphere.

d. Frequent switching operations.

e. Frequent fault operations.

f. Older equipment.

A breaker should be inspected and maintained if necessary whenever it has interrupted

current at or near its rated capacity.

11.2.0 Safety precautionsFollowing basic safety precautions should be taken before conducting any work on the circuit

breaker

NEVER work alone.

Obtain all necessary permits/ certificates required to perform the task/s.

Switch off all power supplying circuit breaker before working on it or inside its cubicle.

Always practice lock-out tag-out procedures.

Always use a properly rated voltage sensing device to confirm that all power is off.

Beware of potential hazards, wear personal protective equipment and take adequate safety

precautions.



Open circuit breakers and discharge all springs before performing maintenance work,

disconnecting, or removing.

If the circuit breaker is to be left with it is in withdrawn position, both the bus bar and the circuit

shutters must be padlocked, if they are not confirm dead.

If the circuit breaker is to be left in any position other than in service position, a warning notice

must be displayed prominently.

When switchgear is isolated make sure it is earthed accordingly via integral earthing mechanism

or earthing truck or by earthing/discharge stick with warning notice at both ends.

Verify that no tools or installation equipment are left inside the switchgear before turning on

power to this equipment. Conduct electrical testing to verify that no short circuits were created

during maintenance, or inspection.

Never insert a circuit breaker into a cubicle that is not complete and functional.

Replace all devices, doors, and covers before turning on power to the breaker. The purpose of the isolation/de-isolation must be recorded in the substation daily logbook.

( Do not attempt to defeat any in terl ock and refer to I nstruction Manual when in doubt).

11.3.0 Maintenance procedureManufacturer's instructions for each circuit breaker should be carefully read and

followed. The following are general procedures that should be followed in the

maintenance of low-voltage air circuit breakers:

a) An initial check of the breaker should be made in the TEST position prior to withdrawing it from

enclosure.

b) Insulating parts, including bushings, should be wiped clean of dust and smoke.

c) The alignment and condition of the movable and stationary contacts should be checked and

adjusted according to the manufacturer's instruction.

d) Check arc chutes and replaces any damaged parts.

e) Inspect breaker operating mechanism for loose hardware and missing or broken cotter pins, etc.

Examine cam, latch, and roller surfaces for damage or wear.

f) Clean and relubricate operating mechanism parts such as pins, bearings, and the wearing surfaces

of cams and rollers, etc.; with a recommended lubricant or equivalent.

g) Set breaker operating mechanism adjustments as described in the manufacturer's instruction

book. If these adjustments cannot be made within the specified tolerances, it may indicate

excessive wear and the need for a complete overhaul.

h) Replace contacts if badly worn or burned

i) Inspect wiring connections for tightness.


41/53



temperature could be a factor of operation. Synthetic lubricants last longer, which is an advantage

over petroleum lubricants, particularly with today's requirements for extended service without

maintenance.

Be sure to remove all traces of the old lubricant before you apply a new lubricant product for

the first time. Use a commercial cleaner - kerosene, mineral spirits, etc. If possible, soak the part in

the solvent and use a soft-bristled brush to loosen the old lubricant. After cleaning, parts should be

dried carefully and re-lubricated as soon as possible.

While penetrating oils are useful for loosening stuck parts, they should not be used as a

lubricant in electrical equipment because they attack, dissolve and wash out factory-installed

lubricants, hastening equipment failure. Also, penetrating oils are flammable and should not be

applied in areas where sparks or arcing may occur.

Electrical contacts should not be lubricated with metal filled lubricants unless tested and

proved to be effective long term. Many metal-filled lubricants can accelerate corrosion, createconductive paths and eventually cause failure. It is better to avoid graphite and Molybdenum

Disulfide containing lubricants for electrical contacts, because it could cause a resistance rise after

multiple operations. For switches that operate infrequently, keeping the contact just clean and dry

with no lubricant might be a viable option.

11.6.0 Testing of ACB

11.6.1 Insulation resistance test

A megohmmeter may be used to make tests between phases of opposite polarity and from

current-carrying parts of the circuit breaker to ground. A test should also be made between the line

and load terminals with the breaker in the open position. Insulation resistance of control circuit, trip

circuit and protection circuit should also be measured. Resistance values below 1 megohm are

considered unsafe and the breaker should be inspected for possible contamination on its surfaces.

11.6.2 Contact resistance test

The d.c. resistance of each pole of the circuit breaker is tested with the breaker closed. The

resistance across terminals of each pole may be measured by means of micro-ohm-meter. Thecontact resistance is of the order of a few tens of micro-ohms. Manufacturers manual may be

referred for the recommended value and adjustments, if required.

11.6.3 Function test:

Thecontrol and protectioncircuit of the breaker must be read and understood before carrying

out the test. The circuit breaker is racked to the Test position. Control and protection circuit fuses

are reinstated. Closing and tripping tests are carried out by giving appropriate signals. Various



faults/abnormal conditions are simulated by shorting the relevant protection relay contact and

tripping of circuit breaker is observed



12.0.0 Testing and Maintenance of Motor Control Centre (MCC)Preventive maintenance is a scheduled periodic action that begins with the installation of the

equipment. At that time, specific manufacturers instruction literature should be consulted, then

stored for future reference. Follow-up maintenance should be at regular intervals, as frequently as

the severity of duty justifies. Time intervals of one week, or one month, or one year may beappropriate, depending on the duty. It is also desirable to establish specific checklists for PM on

MCC/Starter unit, as well as a logbook to record the history of incidents.

Care must be exercised to comply with local, state, and national regulations, as well as safety

practices of the operating unit. Authorized personnel may open a unit door of a motor control center

(MCC) while the starter unit is energized. This is accomplished by defeating the mechanical

interlock between the operating mechanism and the unit door.

When servicing and adjusting the electrical equipment, refer to the applicable drawings

covering the specific motor control center (MCC) and any other related interconnection drawings.

Follow any instructions, which may be given for each device.

Followings are the general guideline for the preventive maintenance. These instructions may

not cover all details, variations, or combinations of the equipment, its installation, safe operation, or

maintenance.

12.1.0 Safety precautions:Maintenance of control components requires that all power to these components be

turned OFFby opening the branch circuit isolating means and withdrawing the unit to the partial

isolation position or removing the unit entirely from the MCC. When units are fully inserted into

the MCC, the line side of each isolator is energized. Do not work on fixed units unless the main

isolator for the MCC is OFF. When working on portions of a branch circuit remote from the MCC,

lock the disconnect means for that circuit in the OFF position to positively lock the operating

mechanism in the OFF position.

Separate control sources of power must also be disconnected. If control power is used

during maintenance, take steps to prevent feedback of a hazardous voltage through a control

transformer. Be alert to power factor correction capacitors that may be charged. Discharge

them before working on any part of the associated power circuit.

Current transformer primaries must not be energized when secondary is open circuited. Short

all CT secondaries

Soot or stained areas (other than inside arc chutes), or other unusual deposits, should be

investigated and the source determined before cleaning is undertaken.



12.2.0 General guidelines:The whole purpose of maintaining electrical equipment can be summarized in two rules:

a. Keep those portions conducting that are intended to be conducting.

b. Keep those portions insulated that are intended to be insulated.

Good conduction requires clean, tight joints, free of contaminants such as dirt and oxides.

Good insulation requires the absence of carbon tracking and the absence of contaminants such as salt

and dust, which may absorb the moisture and provide an unintended circuit between points of

opposite polarity.

12.3.0 Visual inspection of MCC

12.3.1 Frame/Enclosure

Ensure that the nameplate/label data is legible.

Inspect the overall exterior for missing screws, bolts, nuts, fasteners, retainers and keepers.

Inspect for unused openings.

Inspect for improper covers.

Inspect for rust and corrosion.

Inspect main lugs for signs of overheating and missing and defective parts.

Inspect insulation structure for signs of overheating and deterioration.

Inspect for proper alignment of each section.

Check that cabinets are plumb and square.

Record results on appropriate Inspection and Test Form.

12.3.2 Phase/ Neutral/ Ground Bus

Inspect for signs of overheating.

Inspect for rust and corrosion.

Inspect for missing and defective parts.

Inspect all connection points.

Inspect insulation structure for signs of overheating and deterioration.

Inspect for loose connections.


12.3.3 Bus Support


Inspect for signs of deterioration.

Inspect for chips, cracks and broken insulators.




12.3.4 Panel Indicators/ Instrumentation/ Control Wiring

Check all lens covers.

Check all light bulbs for operation.

Check all function switches.

Inspect all meters.

Inspect all control wiring.

Inspect for signs of deterioration.


Inspect for loose connections.

Check all interconnecting wiring terminal blocks.

Verify accuracy and legibility of all applicable wiring schematics and drawings.


12.3.5 Cabinets/Cubicles/ InterlocksCheck all cabinets for interlock function.

Check all pad lock systems.

Check operation of all racking mechanisms.


12.4.0 Cleaning of MCC/ Starter unitsVacuum or wipe clean all exposed surfaces of the control component and the inside of its

enclosure.Starter unit may be blown clean with compressed air that is dry and free from oil. If air

blowing techniques are used, remove arc covers from contactors and seal openings to control circuit

contacts that are present. It is essential that the foreign debris be removed from the control center,

not merely rearranged.

Control equipment should be clean and dry. Remove dust and dirt inside and outside the

cabinet without using liquid cleaner. Remove foreign material from the outside top and inside

bottom of the enclosure, including hardware and debris, so that future examination will reveal any

parts that have fallen off or dropped onto the equipment.

If there are liquids spread inside, determine the source and correct by sealing conduit, adding

space heaters, or other action as applicable.

12.5.0 Mechanical checks of MCC/Starter unitsTighten all electrical connections.

Look for signs of overheated joints, charred insulation, discolored terminals, etc.


47/53



13.0.0 Testing and Maintenance of Motor starter (controller)

13.1.0 Preventive maintenance on MCCBMolded case circuit breakers are designed to require little or no routine maintenance

throughout their normal lifetime. Therefore, the need for preventive maintenance will vary

depending on operating conditions. As an accumulation of dust on the latch surfaces may affect the

operation of the breaker, molded case circuit breakers should be exercised at least once per year.

Routine trip testing should be performed every 3 to 5 years.

13.2.0 Routine maintenance tests on MCCB:

Routine maintenance tests enable personnel to determine if breakers are able to perform their basic

circuit protective functions. The following tests may be performed during routine maintenance and

are aimed at assuring that the breakers are functionally operable. The following tests are to be madeonly on breakers and equipment that are deenergized.

13.2.1 Insulation resistance test

A megohmmeter may be used to make tests between phases of opposite polarity and from

current-carrying parts of the circuit breaker to ground. A test should also be made between the line

and load terminals with the breaker in the open position. Load and line conductors should be

disconnected from the breaker under insulation resistance tests to prevent test measurements from

also showing resistance of the attached circuit. Resistance values below 1 megohm are consideredunsafe and the breaker should be inspected for possible contamination on its surfaces.

13.2.2. Millivolt drop test

A millivolt drop test can disclose several abnormal conditions inside a breaker such as

eroded contacts, contaminated contacts, or loose internal connections. The millivolt drop test should

be made at a nominal direct-current voltage at 50 amperes or 100 amperes for large breakers, and at

or below rating for smaller breakers. The millivolt drop is compared against manufacturer's data for

the breaker being tested.

13.2.3 Connections test

The connections to the circuit breaker should be inspected to determine that a good joint is

present and that overheating is not occurring. If overheating is indicated by discoloration or signs of

arcing, the connections should be removed and the connecting surfaces cleaned.

13.2.4 Overload tripping test

The proper action of the overload tripping components of the circuit breaker can be verified

by applying 300 percent of the breaker rated continuous current to each pole. The significant part of



this test is the automatic opening of the circuit breaker and not tripping times as these can be greatly

affected by ambient conditions and test conditions.

13.2.5 Instantaneous tripping test

Refer to manufacturers instructions for instantaneous trip time. The automatic opening of

the circuit breaker is verified at the set value as per the current rating of the circuit breaker. It is also

important to verify that the circuit breaker will not automatically open for the current below the set

value.

13.2.6 Mechanical operation

The mechanical operation of the breaker should be checked by turning the breaker on and off

several times.

13.3.0 Isolator (Disconnect switch):The external operating handle of the disconnect switch must be capable of opening the

switch. If visual inspection after opening indicates deterioration beyond normal wear and tear, such

as overheating, contact blade or jaw pitting, insulation breakage or charring, the switch must be

replaced.

13.3.1 Fuse holders.

Deterioration of fuse holders or their insulating mounts requires their replacement

13.4.0 Contactor

Contacts showing heat damage, displacement of metal, or loss of adequate wear allowancerequire replacement of the contacts and the contact springs.

If deterioration extends beyond the contacts, such as binding in the guides or evidence of

insulation damage, the entire contactor may be replaced.

Irregular surfaceReplacement not

Worn out

contact

Replace

Curling

Repla

Conditions of the contacts



13.5.0 Protective relaysIf burnout of the current element of an overload relay has occurred, the complete overload

relay must be replaced. Any indication that an arc has struck and/or any indication of burning of the

insulation of any of the protection relay also requires replacement of that protection relay. If there is

no visual indication of damage that would require replacement of the relay, the relay must be tested

using proper test set to verify the proper functioning of the relay contact(s).



14.0.0 Automatic transfer schemeThe power interruptions can result in the unplanned plant shutdowns which causes loss of

production measured in thousands of dollars and may also cause severe damage to some of the

critical plant equipment, where continuous process industry (Oil & Gas, Petrochemical, Fertilizers

etc.) are involved.While power interruptions can not be totally eliminated by transfer scheme there duration

and effect can be minimized by providing an alternate power source and some method of load

transfer to the alternate source such as emergency generator.

Electric supply companies are very conscious of the need of the service continuity to their

customers and put their best effort to eliminate the factors contributing to the outage. Despite efforts

made by the supply companies, outages do occur. Industrial power user must therefore do all that is

possible to eliminate effects from outage.

There are many ways to design auto transfer schemes; some are simple, and some are quite

complicated and expensive. The main purpose of auto transfer scheme is to keep entire bus

energized. This is accomplished by switching on/off the proper breakers and load

shedding/reconnection at the proper time. The automatic transfer scheme becomes more complicated

when motor loads are involved.

The logic of such two schemes is shown in the diagrams on the next page.


52/53


53/53

module no e04 lv switchgear

Documents