ELECSA Certification Scheme Elecsa Assessment. Trip times and radial MCB

Nicktric · Jun 13, 2011

Hi again,
I have done a CU change for my assessment and just want to run a few more things past you if I may before the big day? I have a 2.5mm socket radial supplying two points for my minor, I would have put this on a 20A breaker but the board (my major) only came with a 16A should I upgrade to 20A or not necessary and why don't they supply 20A MCBs with new boards when the regs state 20A for radials?

I don't want to get caught out on this as I know it is a common question:
Are these correct?
General purpose RCDs to BS61008 should trip under 300ms
GP RCD's to BS4293 should trip under 200ms
Socket RCD to BS7288 should trip under 200ms
An RCD not exceeding 30ma should trip at 40ms at 5 times operating current
But the max disconnection time on a TN is 400ms so why should a GP RCD to BS61008 trip under 300 and not 400ms?
When he asks me what time should the rcd trip on a TNCS is it 300 or 400ms as EGtoBR states 300ms and 7671 states 400ms, I know I'm missing something..

Cheers!

KFH · Jun 13, 2011

All looks good. As long as your load does not exceed 16A thre is no problem with a 16 instead of a 20. I have used 16 in the past on radials when installing garage CUs to try and provide discrimination from the 20 or 32A MCB in the main CU.

As to the 7671 giving 0.4 sec disconnect time for TN circiuts not exceeding 32A compared to 61008, I have always assumed they were different standards and as long as 300mS is less than 400mS why worry? If your assessor can tell you please let us know, unless someone else does first.

Good luck

steamboatwilly · Jun 13, 2011

Agree....as long as you can 'justify' fitting a 16A there should be no problem. My NICEIC inspection last year was on a 16A radial to a conservatory. There were 20A MCBs on the RFCs too and they'd never tripped apparently. Deals with 2.5mm through insulation quite nicely!

telectrix · Jun 13, 2011

the disconnection time of 400msec is for overload devices to trip on a fault of neg. impedance. the 300msec is for rcd's to trip at x1 ( 30mA )

Nicktric · Jun 13, 2011

Thanks for that Telectrix, great answer!

SirKit Breaker · Jun 13, 2011

Trippedit,

You are getting all hot under the collar about nothing. You dont need, nor will be expected to know BS7671 inside out and back to front. You do need however the ability to find the appropriate answer using the relevant publications. So have a beer and chill out. If you are asked a question that you are not sure about, then say so, with the reply "I am not sure, but i will look in my regs book / OSG and find out".

You are having a Part P self certification assessment, you aren't sitting the C&G 2391.

Cheers...........Howard