Customer distribution board

[Designmen90]

Does anybody read 61439-3 normative? There wrote: "Verification of the cabapility of the CDB (customer DB) to withstand a 16kA fault". Does it means that all CBs (lighting, fridge, climate, oven, etc...) in any houshold DB should be at least 16kA?

 

[Midwest]

I thought the main fuse provided the ability to comply with withstanding 16kA, as long as the devices were 6kA?

 

[Simon47]

This came up elsewhere when we were discussing what the standard does - and in particular doesn't - say.
What that requirement means is that the device has to withstand a 16kA fault under the given test setup - which would include the upstream fuse in the DNO's service head. I think it's basically a case of a fault that large is going to blow the fuse "quite quickly", and so the CU devices don't need to actually be able to break 16kA - and they wouldn't be trying to as they'll be slower at tripping. I'll admit that I'd missed that subtlety.
At some point, with a lower fault current, the tripping time for the CU devices will become faster than the upstream fuse - and that's when the fault interruption capability of the devices comes into play.
So the CU devices don't need to be able to break 16kA - just to survive carrying it for as long as the 100A fuse takes to blow.

It's a riveting read isn't it

 

[pc1966]

What you have to remember is that the PFC limit is not usually what the fuse actually interrupts, it is what could happen if it were not for the OCPD. You get 0.5A fuses rated to break a 120kA PFC, the resistance of that fuse when cold is not going to let that sort of current through!

If you look at the fusing curves for a typical 100A BS88 fuse, say page 5 of this data sheet:
http://docdif.fr.grpleg.com/general/MEDIAGRP/NP-FT-GT/F00192EN-08.pdf

You will see at 20kA PFC the peak let-through is limited to around 10kA which is equivalent to the non-fused case (the left-most straight line part of the curve where the fuses branch-off as they start current limiting) for around 4.5kA. Even at a PFC of 120kA (symmetric RMS) the peak let-through is still only about 16kA, equivalent to around 8kA PFC.

So in a typical DB that has a 100A fuse feeding it the MCB, etc, need only be rated in the 6kA (typical domestic as DNO cable unlikely to approach 16kA) to 10kA (typical commercial, where you might have a local transformer giving many tens of kA PFC) case since there is no way they will have to interrupt the full supply fault current.

Down stream of the MCB there is some additional limiting, but MCB/MCCB are very poor at fault limiting, so the real help to limit the fusing requirement of end equipment is the R1+R2 that takes anything around 6kA quickly down to 1-2kA by time you reach the sockets. In any case BS1362 fuses are rated to break a PFC of 6kA so again they are safe even if immediately connected to the DB providing it is fed from a 100A fuse or similar.