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Short Circuit Duration

01/30/2014 6:00 AM

Someone please clarify whether it is possible to use breaker of 20 kA for 3 sec. instead of breaker of capacity 40 kA for 1 sec. Are they same or not?

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Guru
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#1

Re: Short circuit duration

01/30/2014 6:43 AM

In what context?

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#2

Re: Short circuit duration

01/30/2014 6:57 AM

You can only determine this by reference to the manufacturers 1 & 3 second ratings.

If this is homework I suggest you look up various manufacturers stated figures and draw a correlation between them for a mean value.

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Guru

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#3

Re: Short circuit duration

01/30/2014 8:36 AM

Try it and let us know so we won't have to check the specs either.

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Guru
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#4

Re: Short Circuit Duration

01/30/2014 11:15 AM

Obviously these two breakers are not identical. The specifications are different. Can one use either without knowing anything else, I would not. I would not use any breaker without knowing where and how it will be used. Can the OP use either breaker without knowing anything else? Sadly they can. Nobody here can prevent the OP from risking anybodies life and property. Can either breaker provide sufficient protection? Maybe.

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#5

Re: Short Circuit Duration

01/30/2014 1:22 PM

If it is in IEC World then this rated current is the thermal short-time withstand current as per IEC 60865-1 ch.3.2.4 Calculation of the thermal short-circuit strength for different durations of the short-circuit current sub.ch.3.2.4.1 Electric equipment.

Ith=thermal equivalent current Ithr=thermal rated short-time withstand current

Tk=short-time Tkr= rated short-time

For Ith<=Ithr and Tk>=Tkr then Ith<=Ithr*sqrt(Tkr/Tk).

If Tk<Tkr then Ith<=Ithr

However it is not the maximum interrupting time [this has to be less then 100 msec].

For USA see ANSI 37.

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#6

Re: Short Circuit Duration

01/30/2014 11:17 PM

20kA can not interrupt fault current of 40kA - interrupter will burst/explode.

The structure of 20kA will deform if subjected to 40kA. Bus Bar and CT will explode due to Electromagnetic forces.

Only one parameter matches that is short time thermal withstand capacity for through faults.

Hence do not use 20kA for 40kA application.

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#7

Re: Short Circuit Duration

01/30/2014 11:19 PM

Thanks to you all for replying. If not clear, I would once again state the problem. Actually, we have some existing outdoor 11 kV switchgear board of 25 kA for 3 sec. we want to connect this board to another 11kV board of 40 kA for 1 sec rating which is kept in underground power house. Now, I just want to know whether the short circuit withstand capacity of both the board would be same or not. Do I need to upgrade my existing boards capacity to 40 kA for 1 sec. or is it sufficient?

Further, 25 kA for 3 sec. board can withstand 40 kA for 1 sec. or not. It seems possible if I use I^2t= constant. Is it possible to convert 3 sec. rating into 1 sec. rating and vice-versa

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#10
In reply to #7

Re: Short Circuit Duration

01/31/2014 12:20 AM

Sorry, I meant you may use 40 kA 1 sec for 20 kA and 3 sec. but not conversely!.

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Guru

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#11
In reply to #10

Re: Short Circuit Duration

01/31/2014 12:55 AM

Compare I^2t for both-20KA for 3sec<40kA fo1sec

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#12
In reply to #7

Re: Short Circuit Duration

01/31/2014 2:43 AM

what is system short circuit current at existing 40kA switchgear. If this exceeds 25kA you can not use it to extend 40kA switchgear.

In your latest input you are looking only at one out of three factors of my previous contribution, that is, short time thermal rating.

Besides other factors, once again to use available spare switchgear, you have to ensure that all of following capability are fulfilled:

1. Interruption duty (Fault breaking capacity of interrupter),

2. Short time withstand capability against mechanical stresses

and 3. Short time thermal withstand capability.

I will suggest that you are not fully conversant with requirements and either shall consult your senior who has the required knowledge or a system designer.

Besides matching fault level, there are other duties such as C -O- C -O- 3 Secs. - C... etc. defined for re-closing duty and if that is the case then must be checked.

Similarly if CB are used for synchronising application of gen sets and/or Grid, then closing time is important and has to be less than 120m Sec from the time the closing signal is given by synchronising relay.

Better you consult the right person, feed him with Specifications of both Switchgears,and application and advise you the suitability..

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Guru
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#8

Re: Short Circuit Duration

01/30/2014 11:26 PM

The 1-sec and 3-sec ratings are used when deciding the discrimination scheme. If there are a lot of downstream circuit breakers, each is set at progressively longer delays so that only the faulty feeder is isolated. As more CBs end up in series, the upstream CB may need a 3-sec delay. This is extremely unlikely, it dates back several decades. Mostly in medium and high voltage CBs now.

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#9

Re: Short Circuit Duration

01/30/2014 11:45 PM

I would not, personally, use a 20kA breaker in a 40kA application. Apart from the fault response curves there will be structural differences in the two, the forces generated by a 40kA fault current are 400 times higher than those in a 20kA fault. Unless your incoming fault level is limited to 20kA or the equivalent power at the supply voltage there will be a bang under fault conditions.

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Guru

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#13

Re: Short Circuit Duration

02/04/2014 7:31 PM

A strict NO-NO! A breaker with 20kA short time withstand current rating (even though the duration is 3 sec.) should not & must not be used for any short time withstand above 20kA (even for 21kA, leave alone 40kA).

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