FINAL DISTRIBUTION BOARD ISOLATOR RATING

[abrahamJP]

I couldnot exactly remember which BS standard,but it mentions maximum incomer isolator possible for a distribution board is 125A.Whether
such a rule exists-When I look to well known manufacturers productline,I can see it exists.

My question is on what basis such a rule is made?

choice 1 -->considering arcing factor,means form of construction??

choice 2 -->considering 10A is the minimum mcb fits into at outgoing.So to provide a safety at upstream on short circuit conditions??If more than 125A used, it maynot
be possible to take control by discrimination laws.

I would be grateful if somebody can shed light on this.

tHANKS IN ADVANCE

ABRAHAM

 

[ScottyUK]

Most of your assumptions are untrue.

Arguably the most common range of industrial DBs in the UK - the Schneider Isobar 4 series - has main bars rated for 250A. Outgoing MCBs are available down to 2A, although they're not especially common at such a low size. The Multi 9 MCB on its own is good for 10kA, but when backed up by an MCCB as the incomer to the board the MCB is capable of operation at 25kA fault level.