higher PFC than MCB rating.

[septiclecky]

Think you should all look at the back of the regs in the appendix as the BS1361 standard is no longer as it has been withdrawn it,s a BS 88-3 now and has been since 2007 harmonisation again

 

[Sidewinder]

Peeps, we need to be careful in calculating high levels of fault current from d.c. measurements made of Ze.

The DNO MUST be contacted as they have clever ways of limiting a.c. fault currents in their network to equal to or below that they quote.

There is a significant difference between d.c. measuring the Ze of an installation and the impedance seen by a high level of a.c. fault current.

I spoke to one of my friendly DNO guys recently about a LARGE install, 1600A ACB incomer, look at KME's pictures to get an idea.

I measured Zs at my db about 30m into the install, Zdb-0.01 ohms, work this out 230kA. Gave DNO the MPAN & the premises details and the 11kV sub station details from the plate on the door, the sub is the other side of the wall from the main incoming switch room about 10m from the ACB.

Guy says I can calculate it for you if you want but you are fine to use our standard quoted figures of 25kA.

My response was thank F***, (he laughed!) because the MCCB I'd fitted was only rated at 25!

Why I said, he replied that they may reinforce the network and increase the PFC above that he would calculate now, however, it would never exceed their quoted values due to the design of their network.

 

[binky]

I suggest the niceic guy is wrong, it wouldn't be the first time, you give him the let through value at 16ka of a 1361 type ii fuse which is 9 ka and ask him if a mcb rated icu 6ka is protected if he says yes then he shouldn't be on the help desk.

regards chris

Phone and argue yourself

 

[user 6351]

But that's where the maximum pfc is 16.5ka, you had 22ka

Though at 22ka, a busman 1361 type ii 100 amp would only let through 8ka at that fault level so in my opinion no issues with a 60439-3 board but iwould expect mcbs with an icn/icu of (10000)

Cheers Chris, I agree... but the point they made to me was that 22k was above the maximum stated by the DNO, and that we only need to work to the maximum PFC stated by them - which does fall safely within the limits of the suppliers fuse.