TT supply Help required

[EddieB]

This is the first TT installation I've worked on and I am adding a couple of sockets to an existing circuit, the main switch is a 80amp Wylex RCD 100ma I cannot make out the BS No , I've had a look at the earth loop impedance at the socket and it reads 10ohms. Is this ok, and will I need to replace the existing contactum CB with a RCBO. The RCD trips within the correct times I've checked that.Any help will be greatly received.

 

[Mr Mark Sparks]

everything sounds fine the way it is the circuit is already covered by the rcd main switch.
10 ohms is fine you just need to do a mwc after


ignore this i was wrong

 

[Guest123]

Hey Eddie.

The max permissable Zs on all circuits in a TT installation is 1667ohms, however a reading this high should not be accepted, as the accepted max for the earth electrode resistance is 200ohms. So your 10ohms is fine.

The BS number for the RCD will either be 4293 or 61009 depending on how old it is.

You will also need to install a 30mA RCD/RCBO to protect the circuit you are working on as the Zs is too high to achieve the disconnection times for an MCB. Ideally all circuits should have 30mA protection.

Also as usual check all earthing/bonding etc etc.

Hope this helps.

 

[Mr Mark Sparks]

lenny is the circ not already covered by the100ma rcd main switch if the times are ok or are you just being extra safe

 

[Spudnik]

lenny is the circ not already covered by the100ma rcd main switch if the times are ok or are you just being extra safe

All circuits worked on now must have 30mA RCD protection, regardless, so the MCB will need an 30mA RCBO in its place.

As for the 10ohms at the socket, i would take a Ze at the Cu with main bonds disconnected and see what the rod resistance is.

Its VERY rare to have a reading that low in a TT, although not impossible.

It could be the bonding that is giving the false reading.

 

[Guest123]

Hey.

As we all know just about all circuits need 30mA protection under the 17th, especially ones that we have carried out work on. I wouldn't be happy leaving it on 100mA protection as even in the 16th, socket outlets required 30mA protecion.

On a TT theres an even higher shock risk with the earth being so poor which is why 30mA is required.

 

[EddieB]

Boys
Thanks very much, Len good work as usual many thanks

 

[johnnyb]

Lenny, a long long time ago in the days wooly underpants were worn, i worked on an explosives store on a quarry and they obviously had earth rods, but the lad that came from the university to test these had a special test meter for testing the rods, cannot for the life of me remember what sort of meter it would be, anybody else know, i have racked my 1xcell for yrs trying to remember, but to no avail, he had to prove the rods infront of Lancashire County Council, Lancashire Fire Brigade, Police, to verify results, but its annoyed me for yrs.

 

[Guest123]

Lenny, a long long time ago in the days wooly underpants were worn, i worked on an explosives store on a quarry and they obviously had earth rods, but the lad that came from the university to test these had a special test meter for testing the rods, cannot for the life of me remember what sort of meter it would be, anybody else know, i have racked my 1xcell for yrs trying to remember, but to no avail, he had to prove the rods infront of Lancashire County Council, Lancashire Fire Brigade, Police, to verify results, but its annoyed me for yrs.

Was it one of those clamp type resistance meters?? Or possibly something a lot more technical with him being from the university and all.

Cant think what though.