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RCD Placement - Place In Main CU Or Individual CU

Postby JamesCh92 » Sat Oct 05, 2013 4:48 pm

Hi guys new to this forum so sorry if i get anything wrong!
Just bought a medium side house and rewired some of the outbuildings.

To set the scene i have the main CU in the house which feeds:
A annex which is part of the house.
A log Cabin.
Two outbuildings with two CU units looped off one another.
and another two outbuildings with two CU units looped off one another.

I have wired it all up with the correct MCB's and the earths are all linked to the house.

First question is where do i place the RCD's. Do i put them in the main CU in the house or do i place them in each individual CU?

If in the house will one be ok for all the house and outbuildings? or do i split it between outbuildings and house? also what size RCD do i need?

If in each CU the ones that have been piggy backed can i just use one in the first one? also what size would this be?

Each outbuilding is only having lights and sockets with nothing major running off it. The log cabin does have a water heater.
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Simply Build It

Postby ericmark » Sat Oct 05, 2013 9:49 pm

It would be normal to have either three way split type of main CU or all RCBO type of main consumer unit and when SWA or one of the other approved cables BS 5467, BS 6346. BS 6724, BS 7846, BS EN 60702-1 or BS 8436 then a MCB in the main with RCD or RCBO in the sub CU's however if not one of the approved cables and the cables is hidden anywhere then it needs the RCB or RCBO at the origin.

Where the consumer unit is two way split then no option but RCD in main CU.

Earth type also has to be considered with out buildings and it is not easy to decide if to export a TN earth or go to TT distance, and location have to be considered and also what if any extraneous-conductive-parts exist. If uncertain I will normally go for TT but to work this out does require some skill.

With a TT then I would always fit local RCD not use one in the main building. Of course if the main building is TT then outbuildings will also be TT.

As to linking to house earth with TT not really a problem but with TN supply one has to be very careful it is all down to gradient under fault conditions.

However using a RCD tester and a loop impedance tester and complying with the laid down limits there should not be any real problem if the distance between the RCD and the furthest socket is too great then the trip time will exceed the 40ms or the loop impedance will exceed that allowed for the MCB size and type used.

With a TN supply all RCD's that feed sockets must trip with a 30ma difference and no delay i.e. within 40ms however with a TT supply all before the RCD must be class II so if you want to use SWA or metal enclosures you may also need a 300ma RCD on the main incomer likely a delayed or S type. The type AC will normally be used and the current will match supply or all MCB's added together for the CU supplied. Also of course the prospective short circuit current will not exceed the rated PSC of the RCD normal either 4.7kA or 10kA although where the let through current of the main fuse limits the PSC then that figure may be used.

Personally I would want the RCD in the out building however electrical design takes into account a lot of factors and I am sure there are jobs I have done which others would question. At the end of the day the guy signing the installation certificate for design has to us his skill to decide the method to be used and has to be able to satisfy any court of law if an accident happens that all reasonable steps were taken. Most companies will insist the designer holds a C&G2391 or 2392 this qualification is seen as showing the person has the skill required.

Pre the changes to the English Part P law it would have been down to the LABC to decide if the person has the skill. Changing a CU still needs LABC approval but there have been lots of discussion between sparks as to if a CU fitted after the main one is really a CU or if just a distribution unit. The Part P does not seem to consider fitting a FCU as creating a new circuit however the IET/BSI do.

Personally I think as long as the requirements of BS7671:2008 amendment 1 are meet no LABC with start a court case only when the work is substandard and there attention is brought to the work in question will they start court proceedings and unless some one is injured would not expect HSE to take an interest.

But with the watch my back culture once complete if I didn't have the qualifications I would get a EICR completed by some one holding a 2391 and with professional indemnity insurance or register the work with the LABC so they carry the can for any errors.
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