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https://www.askthetrades.co.uk/cgi-bin/yabb/YaBB.pl DIY Forum >> Electrical Questions >> Changing MCB in domestic consumer unit https://www.askthetrades.co.uk/cgi-bin/yabb/YaBB.pl?num=1453150686 Message started by Dave1948 on Jan 18^th, 2016, 8:58pm

Title: Changing MCB in domestic consumer unit
Post by Dave1948 on Jan 18^th, 2016, 8:58pm

Hi, I want to up-rate a 6A type B MCB in my domestic consumer unit to a type C in an attempt to stop newly fitted LED lights tripping. (A common occurrence I have found out since fitting them) I feel quite confident around electricity but the thought of groping about inside a mains board is a bit unnerving. Can you tell me; will throwing the main switches on the unit ( the big red ones), isolate the unit from the mains coming into the unit to allow me to change the MCB safely? If not, how can I make myself safe to do it?

Thanks

Dave1948

Title: Re: Changing MCB in domestic consumer unit
Post by baz on Jan 19^th, 2016, 12:26am

I guess it is the MCB that is tripping at switch on? MCBs trip because of over-current so it is a rather strange occcurance as the LED lights have a much lower current than the lights you have replaced.

I have seen this problem with a large number of halogen lights on one circuit as the in-rush current can trip the MCB, but never with LEDs. I rather think you may have some other problem.

To return to your query. Do you have a multimeter or two-probe volt tester? Before putting your fingers inside a consumer unit you must use safe isolation procedures to make sure the power really is off.
Read this guide http://www.electricalsafetyfirst.org.uk/mediafile/100117573/Best-Practice-Guide-2.pdf especially the part from page 9 onwards. Take special care if you have solar panels or other auxilliary sources of supply!

Having isolated the consumer unit using the MAIN SWITCH, note that there will still be lethal voltages on the terminals of the switch itself.

The next issue will be to confirm that the EFLI (Zs) of the circuit is within the limits of a Type C breaker. Remember that
Type B - trips between 3 and 5 time full load current
Type C - trips between 5 and 10 times full load current
so the allowable EFLI for a C breaker will be about half of that specified for a type B breaker.
This table gives you the figures http://espm.co.uk/Max%20Zs%20tables%2017th%20Ed.pdf

Not so easy huh?