RCD discrimination

sirocco · Oct 3, 2010

I am trying to design a wiring layout for an outbuilding which will be fed from the main house supply.
The main house supply is protected by a fairly old (20yr) Crabtree 100A RCCB with a 30 ma trip. (No time info marked on the unit but is to BS 4293)
I want to protect the outbuilding with a single RCD which will trip out 1st and not affect the RCCB at the main house. Any ideas on how I can achieve discrimination with the existing RCCB or will I need to replace it with something more modern/suitable.

pushrod · Oct 3, 2010

AFAIK the only way to guarantee rcd discrimination is for one to be a time delay which wouldn't be suitable in this case. Having different IΔn values isn't certain to work. Best is to have the distribution cable coming from an unprotected way.

telectrix · Oct 3, 2010

suggest you take your supply from the incoming side of your house CU (Henley Block the tails) come off in SWA then the outbuilding doesn't affect the house installation in case of a fault. You can earth the SWA at the house end, and use that for your earth or TT the outbuilding

fredthetester · Feb 12, 2011

is the rccb stand alone or part of the cu

sirocco · Feb 12, 2011

totally standalone, between meter and cu.
Graham

fredthetester · Feb 12, 2011

in which case you need to take your feed before the rccb into the henly block

malcolmsanford · Feb 12, 2011

Graham is it a RCCB or could it be an old ELCB. What type of earthing system have you sounds like a TT or an old TN-S

pushrod · Feb 12, 2011

fredthetester said:
in which case you need to take your feed before the rccb into the henly block

and then probs into a switch fuse unit before going off to the out building, unless there is some other means of isolation ?

Knobhead · Feb 12, 2011

For gods sake get rid of the Crabtree ELCB I wouldn’t trust one as far as I could throw it! They are a death trap! See CR4 - Thread: Difference between ELCB and RCCB (RCD)

malcolmsanford · Feb 12, 2011

Tony said:
For gods sake get rid of the Crabtree ELCB I wouldn’t trust one as far as I could throw it! They are a death trap! See CR4 - Thread: Difference between ELCB and RCCB (RCD)

Not established 100% if that is an old Crabtree ELCB but as I ask I got a feeling it might be, then he as a whole new set of problems to deal with as you said Tony.