withstand Current rating

[PTCruiser85]

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.

 

[rbulsara]

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

 

[Marmite]

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

 

[PTCruiser85]

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

 

[rbulsara]

PTCruiser:
Your thinking is correct.

Rafiq Bulsara

http://www.srengineersct.com

 

[pwrtran]

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

 

[Marmite]

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

 

[PTCruiser85]

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.

 

[desertfox]

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

 

[rbulsara]

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