Consumer box RCD/MAINS Isolation switch


Postby 306james » Fri May 06, 2011 1:23 pm

Hi all. An electrician when inserting a new Meter noticed that my consumer box did not have a RCB in it. He advised me to remove the red isolation switch and replace it with a RCD. Is this correct or do I need both in situ.The consumer box is a GE 8 way. (Not split). I purchase the RCD 80a 30mA
but my builder handyman says their is not enough room to put all the wires in.
Any advise would be grateful.
Thank you in advance. :)
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Postby ericmark » Fri May 06, 2011 7:12 pm

There are two reasons for using a RCD.
1) To protect personal.
2) To reduce risk of fire and ensure a fault will cause auto disconnection with in 0.1 seconds.

The first is normally 30ma and will work in 40ms but they are prone to tripping due to a build up of leakages so we normally fit at least two. In some cases we use RCBO's which is a RCD and MCB combined and every circuit has it's own.

The second is linked to earth type and today is only found with old TT installations. It works at between 100 and 300ma and often delayed (S Type) and normally protects the whole board.

Since 2008 when BS7671:2008 became the new regulations the use of 100ma RCD's has reduced as normally we will fit 30ma which do both jobs. However to replace the Main switch we would use a 100ma type not a 30ma type.

Since noted when changing meter very likely you need the 100ma type S to comply with supply regulations. Although that may have been what the supply guy was thinking about an electrician would have a problem modifying a system with a 100ma as sockets under 20A and all in bathroom and outside now needs 30ma protection.

So likely it will need either a consumer unit change or fitting of RCBO's if they will fit in old unit.

There is a second problem. With a TT supply the RCD should be on all live wires. This means two pole. But most RCBO's are single pole. So it may need both.

There are two things.
1) The regulations
2) Your safety
Although both are similar they are not quite the same. Electricians are normally members of a scheme to be able to self certify. The scheme operators may stipulate he does work better than required by regulations. To DIY will cost more because of LABC charges.

So step one select 3 electricians and ask for a quote. See what they think needs doing. Then decide which one you feel is looking after you the best.
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