Miniature Circuit Breaker Sizing _ Electrical Engineering Centre
Transcript of Miniature Circuit Breaker Sizing _ Electrical Engineering Centre
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04 February 2011 ~ 6 Comments
Circuit Breaker 0
When we design or build a electrical installation,wemust know how to choose the suitable circuitbreaker for the any application that we design.Forelectrician,this is compulsory knowledge.So here iwant share my little knowledge about circuitbreaker sizing method.
This calculation for sizing the miniature circuitbreaker.For other circuit breaker likesMCCB,ACB,VCB,OCB and etc,it have a additionalfactor and calculation that we must consider.Forthat i will discuss it in my next post…so this time iwant explain the simple way how to sizing theminiature circuit breaker.
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Why it so important? Ok..it have many reason as below :
For electrical safety purposeTo not over cost to purchase circuit breaker when we over sizing it. or not suitableTo prevent the equipment from damage before it can use it.To easy when we want purchase the mcb,because we have all detail information about it.To make the design matter more effective and faster to complete.
How to select a suitable MCB for the application?
we must focus in 3 main points to choose a right MCB for electrical circuit :-
1) Ampere rating ( In )
2) Short circuit breaking capacity ( If )
3) Application type
Calculation for Miniature circuit breaker ( MCB ) sizing
Example : We want install 1 unit single phase motor.full load ampere is 20 ampere. Power supply240VAC,50 Hz
1) Ampere Rating ( In )
Formula : In = 0.25 x Iz
In = ampere rating Iz = Load ampere
Calculation :
In = 0.25 x 20 ampere
In = 25 ampere x 0.25 ( safety factor )
In = 31.25 Ampere
Refer the ampere rated in technical data for MCB from manufacture
From the technical data,we selected the ampere rated for MCB is : 32 ampere
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2) Short circuit breaking capacity ( If )
Formula : If = V / 0.276 (R cable)
If = short circuit breaking capacity
V = Voltage
R cable = resistance cable ( 0.276 )
Calculation :-
If = 240 / 0.276
If = 870 ampere
From the answer,we can decide to select rating for one ( 1 ) kA
3) Application type.
From application,it state the single phase motor,so we can choose TYPE C for MCB.It for generalapplication and for small motor.Please refer the table below for MCB type for the application.
* For more detail,please refer the manufacture Technical data / user manual or contact the supplier.
ABB MCB Technical data
MCB Type,Instant trip and Application table
—————————————————————————-
1 2.7 – 4 In Domestic and commercial
B 3 – 5 In
—————————————————————————
2 4 – 7 In
C 5 – 10 In General use,small motor
3 7 – 10 In
—————————————————————————
D 10 – 20 In High in rush equipment
4 10 – 50 In
—————————————————————————-
K Frequent short duration
—————————————————————————-
Z Semiconductor devices
—————————————————————————-
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So…the conclusion from the calculation,we get the specification for MCB to install on our application :-
1) Rated ampere is : 32 ampere
2) Short circuit breaking capacity is : 1 kA
3) Type of MCB is : Type C
For more detail,please contact the electrical engineer for their suggestion.This is a simple way to sizing thecircuit breaker.Please refer the IEE manual for electrical guideline.
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Tags: abb, ampere, clipsal, mcb, mccb, safety, schneider, voltage
Sinnadurai Sripadmanaban 24 April 2011 at 11:41 PM Permalink
Please give an example for selection of MCB(type C or D)for a 3phase induction motor,say10HP,15Amp- if started using an across the line(DOL)as well as wye-delta starter.
Reply
1.
lemau 25 April 2011 at 1:34 PM Permalink
from your example :-10 hp;15 ampere
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the calculation is :
15 ampere x 0.25 = 3.75 + 15 ampere = 18.75 ampereSo we can choose the MCB ampere rating = 20 ampereFor induction motor with DOL stater,the suitable type is C or D
Reply
Sinnadurai Sripadmanaban 3 May 2011 at 11:13 AM Permalink
As the starting current is not the same for DOL and star-delta starterscan we use smaller MCB for star-delta starting?. Is it correct if I say 20A for star-delta and25A for DOL starting
Reply
lemau 4 May 2011 at 9:42 AM Permalink
U can use a smaller MCB but make sure it not below from the full load ampererating.One more important thing is the TYPES of MCB must suitable with yourapplication.Example MCB type C or D for motor stater.For more safety about overloadprotection,we still have the overload relay :D
Reply
Sinnadurai Sripadmanaban 5 June 2011 at 10:11 AM Permalink
For frequent start/stop or DOL starting, OL relay won’t protect. MCB will trip for inrush current.Sowe used MCB type C rated 2xFLA for DOL and 1.5xFLA for star delta starting. Another importantaspect is the rating and type of MCB should match the duty of motor -AC1,2,3 etc. For frequentstart/stop like pressure switch operated water boosting pumps should be use type C duty AC3 ortype D AC1?. Therefore the formula 1.25xFLA is for which duty class and will the tripping andcontact wear problem will be solved for some class of duty by using class D MCB?. Pleaseelaborate.Manufacturer’s literature will be a useful guide.
Reply
3.
saouita 18 June 2011 at 9:18 AM Permalink
stl
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