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Assemblies of switchgear and control panels (part 3)Pos ted J U N 3 0 2 0 1 2 b y EDVARD i n L O W V O LTA G E , P O W E R S U B S TAT I O N w i t h 0 C O M M E N T S

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Assembli es of switchgear and control panels

Continued from 2nd part of article: Assemblies of switchgear and control panels (part 2)

Now coverage of distribution panels and switchboards will be presented including molded case circuit

breakers ( MCCB ), safety switches, panelboards, multimetering equipment, ground fault circuit

interrupters. Most of the codes define a circuit breaker as a device designed to open and close a circuit

automatically at a predetermined overcurrent values (short circuit or o verload) without injury to itself when

properly applied within its rating.

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A molded case circuit breaker (MCCB) irrelevant of the manufacturer comprises of:

The molded case (frame)

Operating mechanism

Arc extinguishers

Contacts

Trip elementsThe terminals (connectors i.e. mechanical / compression lugs or solid bar).

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

circuit breaker components . The cases are molded from phenolic material. It provides ruggedness and

high dielectric strengh to the circuit breaker in a compact design.

Maximum current, voltage and interrupting current determine the size and strength of the molded casecircuit breaker (MCCB). Different manufacturers build breakers for the same rating with different physical

sizes i.e. non-interchangeable.

The faceplate ( nameplate ) gives all t he important data such as:

Catalog number (reference)

Serial number of breaker

Interrupting ratings at different voltage classes

Standards registration numbers i.e. for the CSA or ULCalibration temperature for thermal magnetic units

Lugs data and size

Electrical accessories

Date of manufacturing ( usually printed on back/side of circuit breaker )

The function of the operating mechanism is to provide a means of opening and closing the

Engineering Trainings
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breaker. It is of the quick make, quick break mechanism.

These breakers are also trip free i.e. if the breaker operating handle is pushed and hold in the on position

and there is a fault on the system (or the trip test button is pushed, if available) the breaker would trip

open and stay open.

There are three distinct positions for the handle to settle in the ON , OFF , TRIP (which is midway

between the ON and OFF positions). The breaker has to be reset after tripping by pushing the handle all

the way to the OFF position and than pushing it to the ON position.

The function of the arc extinguisher is to confine, divide and extinguish the arc drawn between the

breaker contacts each time the breaker interrupts. The trip element provides the protection in case of

prolonged overloads or short circuits. It actuates the operating mechanism and trip the breaker open. This

element is made up of electromechanical or solid state components.

The electromechanical element is referred to as the thermal magnetic and it includes:

1. The bimetall ic element

2. The electomagnet

The current (load) flows through or heats the bimetal. When a

combination of high current le vel and a long duration exists

which is sufficient to deflect the bimetal far enough to

physically push the trip bar and unlatch the contacts, the

breaker will trip open.

There is no field adjustment available for the thermal element.

The magnetic trip action is achieved through the use of an

electromagnet connected in series with the load

current. When the short circuit current passes through the

coil this causes the electromagnet to attract the armature

(trip bar) and the unlatching action causes the breaker to

trip open. Magnetic trip is adjustable in 250A frame breaker

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and over. Two adjusting methods are availableone is the independent adjustment of each pole of the

breaker thus three adjusting knobs versus one knob to adjust all poles simultaneously.

Thermal magnetic trip elements are temperature sensitive thus a modified unit to compensate for

variations in ambient temperature (with a modified design) can be applied under specific conditions. An

ambient compensated overload trip unit can be used whenever the breaker (protecting a cable or a down

stream device) is located in a different place with different ambient temperature than the protected device

or conductor, thus derating or uprating of breakers becomes unnecessary.

Ambient c ompensation i s obtained by using an additional c ompensating bimetal which counter acts the

effect of ambient temperature changes on the overload bimetal.

There are various methods of connecting the line and load sides of a circuit breaker:

Bus bars

Panelboard strapsRear connected studs

Plugin adapters

Terminals (cable lugs for single or multiconductors for copper only or copper /

aluminum ones).

The tripping unit can be made up of solid state

components or microprocessor basedelements. These components replace the bi-

metallic and magnetic elements in

a conventional molded case circuit breaker

(MCCB). Internal current transformers in the

breaker are required to feed in the S.S. trip

unit. Each frame size can accomodate a set of

rating plugs, these plugs set the rated

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Compact NSX circuit breaker with Micrologic 2 Electronic TripUnit

continuous current of the breaker.

Some types of rating plugs are adjustable

between 70-100% or 50-100% of the rated

current of the plug itself. The breaker

characteristic curves are similar to the thermal

magnetic trip curves except that the different

tripping actions are more clearl y defined and

can be adjusted. The distinct por tions of the

ch/cs curve are: the long time ampere rating & the long time delay (simulates the thermal section of a

thermal magnetic circuit breaker) the short time pick up and the short time dela y (I2t in or out), the

instantaneous pick up (simulates the magnetic portion in a thermal magnetic curve).

These breakers can come with a builtin ground fault protection (for solid effective grounded neutral

only) with adjustment of ground fault pick up and time delay . The time current ch/cs curves for

current limiting type circuit breaker have basically the same shape as conventional circuit breaker. The

only difference is the current limiting action displayed at higher fault levels. A total interruption time of

from 1/4 to 1/2 cycle isnconsistent with the current limiting operation.

The electromagnetic repulsion created by closely spaced parallel contact arms carrying current in

opposite directions intensified by an O shaped magnet surrounding the contact arm causes the contacts

to blow open in an e xtremely short period of time.

Circuits which produce high f ault currents typically are associated with low short circuit power

factors (i.e. high X/R ratios). The high speed contact separation enhances the ability of the circuit

breaker to limit both the peak current and the let through energy. The integral current limiting breaker is

similar to the standard thermal magnetic except that it incorporates a current limiting device. This device

consists of 3 current limiters specially designed to coordinate with the breaker of which they are an

integral part.

All normal overloads and short circuit faults are interrupted by the thermal magnetic portion of the circuit

breaker. When a large fault occurs the trigger type current limiters will blow and operate the trip bar to

open the breaker, thus preventing single phasing.
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Safety switches panelboard

The standard molded case switch , which is a more compact design than the conventional

disconnect switch, has no trip unit. It consists of the standard breaker contacts, bussing

and lugs for the highest ampere rating in each frame size and is manually operated only.

The safety switches can be single throw or double throw . All switches

to have visible blades and safety handles (interlock bypass lever, cross

bar and operating spring).

There are two types of switch contacts the butt and the knifeblade/jaw

construction. In the former, only one of the blades contact surface is held

in tension against the conducting path, electromagnetic forces due to high

current faults will tend to force the contacts apart therefore arcing and

contact burning may be the result.

In the latter the jaws distribute a uniform clamping pressure on both sides of the blade contact surfaces,

the electromagnetic forces due to the high short circuit currents will tend to squeeze the 2 sides of the

jaw together onto the blade.

The action of the blade moving in and out of the jaws aids in cleaning the contact surfaces. The contacts

are made of copper and the current carrying parts of the switch are plated, to reduce heating.

Fused switches will have fuse clips/holders plated and either spring reinforced or not, the classes

found on these switches are J, R, or L for 800A and over . The molded case switches and the safety

switches are installed in a variety of enclosures to meet practically all applications. The enclosuresare designated according to Nema standards (1,3R,4,4X,5 etc.) or CSA (1,2,3 etc) all the

enclosures have to go through a set of qualifying tests before been given such a designation.

Panelboards

Now, we shall cover panelboards. These types of boards can be classified, broadly, into two categories:
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1. Lighting

2. Distribution / power

Another method of c lassification is whether t he board contains breakers, switches ( fused or unfused ) or

a combination there of.

Left: Distribution panelboard; Right: Lighting panelboard

The panelboards can also be classified according to the main, accepting the incoming cable, whether it

is a breaker a switch or just a set of lugs. The rating ranges, for single phase (three wire) or three phase

(three or four wire) lighting panels, from 120/240V to 600V 50A 800A main breaker or main lug.

The ratings range for single phase (two or three wire) or three phase (three or four wire) power panels

from 120/240V through 600V a.c. or 250V d.c., main breaker or switch or lugs, up to 1200A.

The major components of the breaker panelboards are the boxwhich comes in standard

dimensions, the interior which holds the breakers and comes in standard current carrying


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capacity & mounting spaces, the solid neutral which is a bare conducting bar with lugs, the

trim either flush or surface which covers the gutters and has the door that covers the

branch/main breakers, finally the breakers plug on or bolted type.

For the fusible panels the components are: the fusible switch unit, the interior assembly ,

the enclosure the box and the front trim parts, the cover plates (blanks)and the neutral.

There are metering equipment (centres or sockets) that are used for single phase and three phase

systems. They are units that act as the point of placement for the P .U. to place its detachable

meter (watthour). The single phase 120/240V, 3wire units are used in conjunction with the load centre

which is located inside the single family resident.

There are three types of meter sock ets:

1. The non-circuit closing

2. Automatic circuit closing

3. The manual circuit closing

The first ( non-circuit closing ) means when the watthour meter is removed the service to the down

stream load is interrupted, the second ( automatic circuit closing ) means that a mechanism to short the

line to the load jaws of the socket when the meter is detached is provided, the third ( manual circuit

closing ) offers the ability of shorting the line to the load jaws of the socket manually through the use of

a slider or lever before removing the meter.

All these mechanisms are meant for t emporarily shorting the socket and usually rated 50% of the

continuous current carrying capacity of the socket.

The jumper bars are not to be used to make / break current. The 4 jaw meter socket is used on

120/240V a.c., 1 phase, 3 wire. The 5 jaw used with single phase loads fed from 3 phase, 4wires

120/208V a.c. system. For 3 phase, 4 wire systems the 7 jaw meter socket is used with the proper

meter to measure the energy consumption of the connected loads. The type of socket trim will suit the

type of socket installed.


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Ground fault interrupter (GFI)

These meter sockets can be assembled together in one lineup to meter different loads in one location

like shopping centers, apartment buildings, etc. They are built for indoor or outdoor installations.

Protection Devices

In this paragraph people protection rather than

equipment protection from ground faults will

be discussed. The device used is known as

ground fault interrupter (GFI) . The hand to

hand resistance of an adult lies between 1000

to 4000 ohms function of voltage, moisture

levels and muscular structure. The effect of a

60 c/s shock on a normal adult are as follows:

over 5 mA painful shock, over 15ma

is sufficient to cause freezing, over 30 mAcauses difficulty in breathing that can lead to

suffocation, 50-100 mA possible ventricular

fibrillation, 100-200 mA definite ventricular

fibrillation, over 200 mA severe burns & muscle

contractions.

A standard overcurrent device will never s ense

such low levels of current despite the fact that

it is dangerous to the human being. Groundfault circuit interrupters are covered in the UL

standard 943 and the CSA C22.2 no. 144 .

The GFI should trip when the ground current

reaches 6 mA or more, the time it should take

to trip = [20/ground fault current] .

This device should not trip below 4 mA, it also contains a means of testing the ground fault circuitry. The

standards classify the GFI into class A (which is briefly covered above), portable and non classified ones.


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The GFI sensing components are the zero sequence current sensor that monitors the balance of the

current in either the single phase 2 wire circuit (the hot and neutral wires) or the single phase 3 wire i.e.

2 pole breakers (the 2 hot and the neutral wires).

When the sensor detects an unbalance, as low as 6 mA, it will send a signal to the control circuitry.

This will initiate a trip of the breaker feeding the faulty circuit (having the leakage current to ground).

The rules that govern the application and use of the GFI are given in the local applicable code, eg. CSA

C22. 1.

Resource: Unknown

TAGS

breaker, circuit breaker, ground, ground-fault, magnetic, mccb, m olded case, thermal, trip

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Definitions from the StandardUL 489 (MCCB)J U LY 1 2 , 2 0 1 2

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