what is the difference between ultimate and service breaking capacity of circuit breakers(eg:Icu,Ics)?

Answer 1

Hi Anil,
What sort of breaker are we looking at here ?
Is it a big SO6, spring loaded jobby in, say a substation or a tichy domestic item, and then is it a residual current device, or an older current limited thing ?
Horses for courses, but in general, the nominal breaking current should be exactly what it says on the tin.
It often isn't, though. A margin in manufacturing tolerances is allowed. Plus it can depend radically on ambient conditions, such as temperature, humidity, vibration etc, so something of a grey area often.
I have found that the very same Merlin Gerin 30A RCD would trip nicely in my UK garage at 28A, but was quite happy to leap out at 22A in a Mud East hot workshop.
There again, a 60A, 240V rated thing from Honywell withstood an impressive 85A before giving up.
Not really what it was supposed to be doing.
I therefore err on the side of caution with breakers.
Size them according to the circuit capacity in terms of cabling & wiring, and allow a good 20% on the safe side.
Some don't like a breif surge of 25mS overload, some pop out at the least excuse.
, and some hang on in there regardless, which is not ideally what you want.

Oh I could go into the equations for the designs of breakers, and of course all reputable items are true to their rating, but experience shows me that to be something of a benchmark, as all electical protection included, even good old slow burn fuse wire is subject to variation.
Even AC power frequency can have an effect.
Theoretically, 50Hz or 60Hz should not matter, but some RCD's are seemingly a tad more sensitive in the US (60 Hz), with the same current rating. OK the voltage is different, but so what ?
Sorry for a bit of a vague answer, but the whole subject seems a bit vague sometimes.

Happy Sparks,

Bob.

Answer 2

the best answer is at

www.eng-tips.com/viewthread.cfm?qid=81200