Consumer unit RCD queries

4 Jan 2004
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United Kingdom
I have been told the following: "When I install my new consumer unit, I should use a split-load CU, where the lighting circuits can be protected by a 'slow' RCD, and the power circuits can be protected by a 'fast' RCD"
1) I assume the difference between 'fast' and 'slow' is the rated trip current: 30mA being 'fast' and 100mA being 'slow', but why has my 'advisor' recommended different ratings of RCD for lighting & power circuits?
2) What's the reasoning behind a split-load CU? What's to stop me protecting all circuits with one RCD on the incoming mains?
3) My leccy supply comes from overhead cables. Does this dictate what rating of RCD I should use, and why?
4) My existing CU has an 80A/30mA RCD but no separate mains switch. I thought RCDs were double pole, so do I need both?
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The way you are supplied from your electricity supplier is known a TT System, that’s why your supply is supplied via the over-head cable. No earth has been supplied so you get yours by using an earth electrode “Rod”…

Now this means of earthing could be better and to provide the extra safety measures TT systems are usually covered by a residual current device (RCD), suitable rated. In the day 100mA RCD’s used to be used to provide protection for the whole installation but with sockets outlets “that are reasonably expected for outdoor use” this cannot be satisfied as current regulations state that 100mA RCD’s cannot be used to provide supplementary protection to socket outlets, that’s why a 30mA is used.

The way you are set up now (ie, 30mA covering all) is fine, but over-done as usually lighting, immersion etc are not required to be covered, but your system is TT so a 100mA is needed and adequate to cover these circuits. Why split-load…well you don’t want your lights going out if there was a fault on one your appliances and it provides discrimination between the RCD’s. RCD’s are not meant to be relied upon to isolate an installation, I would recommend to install an double pole isolator between the meter and your new distribution board…..

Hope it helps…well a bit anyhows…..


il78 said:
...... I would recommend to install an double pole isolator between the meter and your new distribution board…..
Thanks, li78. Most useful information. My understanding has improved considerably, but regarding this isolator switch: Is this a separate device that the leccy board should install (as I have read elsewhere on this or another forum?), or is it one of those red switch modules that's going to occupy another two valuable positions in my CU? Or is it a separate device that I can install to vacate the necessary CU module positions?
I would go for the isolator between the meter and your board (as discussed in other topics) but a "select few" on this site will say its not required as you can use the RCD as a isolator......every spark is different and all advice will differ........
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For what it's worth, I would always have a 100A DP isolator inbetween the outgoing meter terminals and the CU. This way you can isolate your installation without the need to break seals etcetera. As that jazz club geezer on the Fast Show said, "Grreaaat......"

Some electricity companies have a policy to replace old meters, and the new ones have their own iso built in, either a rocker switch or a plastic cylinder that to switch on the suuply you have to push in and twist.

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