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withstand Current rating

withstand Current rating

withstand Current rating

(OP)
Gentleman,

When we specify a medium voltage circuit breaker, there are certain short circuit ratings that are part of the spec:

This is what my understanding is and please correct me if I am wrong:

1. Interupting Rating(break capacity)- Symmetrical RMS
2. Close and Latch(Make Capacity)Momentary withstand)- Asymmetrica RMS or Peak- This is the half cycle which includes the dc component

What is this 3 sec momentay withstand rating ?
This is what I have seen recently. Could you kindly advise.
 

RE: withstand Current rating

That is short time withstand rating, which is essential when the breaker trip is to be delayed to allow downstream devices to operate first for coordination.

Rafiq Bulsara
http://www.srengineersct.com

RE: withstand Current rating

Short time withstand current tests demonstrate the ability of a circuit breaker to withstand through fault currents without damage. The standardised time durations of 1s to 3s have been generally derived from what is considered to be a realistic practical operating time for backup protection.
Regards
Marmite  

RE: withstand Current rating

(OP)
Thanks Gentleman. Appreciate the information.

Could you also advise, if this value is normally specified in Symmetrical Amps or Asymmetrical rms Amps.

I think it should be Symmetrical value as by the time elapses to 1 0r 3 sec The fault current is symmetrical anyways rather it becomes after 5 cycle.

Thanks

RE: withstand Current rating

I like to add two comments here:
1) momentary withstand - specifies the ability of the magnetic force it can withstand associated with the short circuit current
2) 3s duration short-time withstand - specifies the ability of the heat it can withstand associated with the short circuit current in the duration of 3s

RE: withstand Current rating

I think you would need to refer to the standard the test has been carried out to. Short circuit withstand tests may be carried out with a full asymmetric current as specified by the Asymmetric current rating.
Regards
Marmite

RE: withstand Current rating

(OP)

Gentleman,

Thanks for introspection.

Why would a vendor publish a 3 sec withstand rating anyways when the momentary withstand is already hight with asymmetrical current.

Could it be because momentray withstand Asymmetrical rms value for 1/2 cyle cannot be sustained by the breaker that long so they need a short circuit parameter that can handle for long.

Please advise.

RE: withstand Current rating

Hi PTCruiser

According to my old JSP Switchgear book the 1 second rating and 3 second rating are related to ratio of symmetrical breaking current to the rated normal current, if the ratio exceeds 40 its a 1 second short time rating, if 40 or less its a 3 second short time rating.

regards

desertfox

RE: withstand Current rating

PTCruiser:

See what pwrtran said. It is also about withstanding total energy during 1-3s plus the peak current breaking duties are much different at 1/2 cycle and at 1s or 3s.

Rafiq Bulsara
http://www.srengineersct.com

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