Advise / Logic check please..

[tonycoysh]

Ok, it's the first time I've worked on an arrangement such as what I am about to explain so could someone check my logic.

The Installation: TT earthing arrangement, CU1 supplied from service head, 100mA RCCB Main switch with various CB's for original house. CU2 Supplied via 16mm T&E connected straight to the RCCB main switch in CU1. CU2 supplies various circuits in the extension which is about 13 yrs old.

Currently extension is on one 32A ring, which also supplies the garage via a 13A FCU. Customer wants to segregate garage and ring as well as some other work which will require me to improve / upgrage the RCD protection of CU2.

Am I right in thinking the 16mm T&E needs to be protected by a 30mA RCD and a 100mA time delayed is not sufficient? Especially as I am not sure where the cable is run (in or out or perscribed zones etc.) And that the 100mA RCD was only required to satisfy disconnection times of a TT system and it is ok to replace with 30mA.

If so the route I was going to go down is this: As there is room in CU1, fit a DP Main Isolator switch to CU1, Change the 100mA RCCB in CU1 for a 30mA RCCB (to improve protection) and add a 2nd 30mA RCCB in CU1 to supply CU2. Then CU2 will remain as is (DP isolator and CB's)

Not ideal but hopefully satisfy's everything required whilst providing some segregation. And yes the 2 RCCB's will be in paralell (such as a '17th edition' CU) not in series.

Otherwise I could just replace the 100mA RCCB for a 30mA one and leave it as it is but I'm not so sure I like that way as there is not enough segregation.

Ze of CU1 and Zdb of CU2 was both around 100 Ohms when I tested. Gas bonded direct to the rod and, water bonding all in place.

Thanks for any replies and advise and pointing out anything I have over looked.

 

[telectrix]

if you feed a RCD from another RCD, then they will be in series and 1 or both could trip in the event of a fault. you need to establish whethert or not the feed from CU1 to CU2 needs RCD protection, and go from there.

 

[tonycoysh]

I wasn't going to feed one RCD from another I was going to feed them both from the isolator switch. Like you would find in a so called '17th edition' dual RCD board. Then feed the 16mm T&E from one and the Busbar for the CU from the other. (if possible.)

My gut feeling is that the feed from CU1 to CU2 does need RCD protection.

 

[telectrix]

sorry, misread OP. sounds the best of a bad situation. as long as all circuits are RCD protected ( 30mA on TT) then you just have to do the best you can within the customer's budget.

 

[tonycoysh]

Kind of what I thought, thanks Tel!

 

[EIPM]

Agree with Telectrix, as long as you incorporate 30mA protection and do the best you can with the situation.

 

[Engineer54]

Gas bonded direct to the rod and, water bonding all in place.

Well that's not right is it?? Gas and water bonding cables should be run to the installations MET, not the earth rod!!
In reality you haven't a clue now what your true Ze is, unless you disconnected the gas bonding from the rod before you tested for Ze, ...which i very much doubt!!