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Participant

Join Date: Jan 2014
Posts: 2

Isolator and Circuit Breaker Rating

01/30/2014 1:23 AM

Greetings,

in industrial plant we are using distrubution board including MCCB to power remot equipment which having local isolator , chossing the rating of MCCB is as per the load but did the isolator should have the same rate of MCCB , bigger or smaller ,

kindly help.

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Guru
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#1

Re: isolator and circuit breaker rating

01/30/2014 2:42 AM

This may vary depending on:
-Whether the load is heaters, lights, motors, or other.
-Local regulations.
-The ampere condition when the isolator will be switched (off, on while starting a motor, or on while running normally).

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Participant

Join Date: Jan 2014
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#2
In reply to #1

Re: isolator and circuit breaker rating

01/30/2014 2:50 AM

the load is landry equipment such as like " washer ( including heating and motor) , dryer, ironer) . and the isolators are only for switching equipment on,off and maintainance purpose .

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Associate

Join Date: Oct 2011
Posts: 34
#3

Re: isolator and circuit breaker rating

01/30/2014 4:39 AM

As rightly said by TORNADO , rating of isolator purely depends on teh type of load if it is a on load isolator . Incase of OFF LOAD ISOALTOR it should be rated to cater the rated current of load .

However the rating of MCCB as well as the rating of Isolator should be higher than then the rated current .

Since it is a local isolator , i belive it should be a OFF LOAD ISOLATOR .!

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Guru
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#4

Re: isolator and circuit breaker rating

01/30/2014 6:11 AM

Amazingly, British Standard 7671 has been written and published specifically to answer questions like this. It is available from the British Standards Institution's retail outlets in response to a telephone call with a credit card available during the call.

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#5
In reply to #4

Re: isolator and circuit breaker rating

01/30/2014 6:42 AM

Could you tell me the Reg Nos. please PWS.

I have the BS7671, 17th edition amendment 1 (2011) plus the corrigendum of June 2013 in front of me now.

BTW. It's marketed by the IET (Institute of Engineering and Technology) not the BSI.

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Guru

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#6

Re: Isolator and Circuit Breaker Rating

02/02/2014 10:05 AM

See Wikipedia "Disconnector":

"Unlike load break switches and circuit breakers, disconnectors lack a mechanism for suppression of electric arc, which occurs when conductors carrying high currents are electrically interrupted. Thus, they are off-load devices, 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".

I think it is correct. Except the "load break disconnector" a disconnector which may interrupt the rated load, a regular disconnector opening or closing has to be forbidden.

Since the maximum current expected to flow through this circuit has to be limited by circuit breaker, I think, this current has to be the disconnector rated current too.

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#7

Re: Isolator and Circuit Breaker Rating

02/04/2014 7:40 PM

The isolator has to be a load-break isolator & its continuous current rating should be equal to or more than the aggregate of the rated currents of all the connected loads.

By the by, I am throwing am interesting question: WHAT IS THE DIFFERENCE BETWEEN AN ISOLATOR AND A SWITCH? (Apart from spelling, of course!)

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