RCDs & Earthing

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I was planning on installing a 100mA RCD to protect my consumer unit & a 30mA RCD protecting selected circuits on that unit.

I have now been told that I would need 'special cross-bonding' as well. As my circuits are already earthed & cross-bonded is this necessary? The first reference to this came wth the quotation & I have seen no mention in RCD installation literature about additional bonding.

Obviously, I want a safe electrical supply. It has been pointed out that the use of the 2 RCDs is over cautious.

 

[plugwash]

the 100ma type S rcd covering the whole box is only normaly used on TT systems (earthed with a rod) where earth loop impedende is high (though there is nothing to stop you using one on another earth system they are bloody expensive though)

if your earth is supplied by the rec (TN-S or TN-C-S) then you really don't need the main rcd

all sockets that can feasiblly supply equipment outside (all sockets if your earth system is TT) should be on a 30ma rcd

anything metal that enters the property (water and gas pipes) should be connected to your main earth terminal by 10mm or larger earth cable (if your earth is PME then the RECs often reccomend 16mm even though wiring regs only demand 10mm)

within bathrooms anything metal that enters or leaves the bathroom and could carry a potential between it and another room should be bonded together with 4mm or larger earth cable this includes pipes and the earth conductors of any curcuits serving the room

note that short tails of copper pipe that join to plastic under the floor do not neeed to be bonded as it is not feasible for them to cary a potential into a room

 

[securespark]

The NIC (boooo!) have a good book about earthing.

Snags & solutions, earthing & bonding is 15 quid from the NIC. 01582 539 719.