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.
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
Rafiq Bulsara
http://www.srengineersct.com
RE: withstand Current rating
Regards
Marmite
RE: withstand Current rating
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
Your thinking is correct.
Rafiq Bulsara
http://www.srengineersct.com
RE: withstand Current rating
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
Regards
Marmite
RE: withstand Current rating
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
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
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