adequate earth?

lionelmcc · Oct 4, 2009

Hi chaps,
I visited a house the other day which has a granny flat. this is supplied by a 16mm T&E On a 50A Breaker with 30mA RCD from the CU in the house. There would not appear to be any extra earthing run along with the submain for the granny flat.
would i be right in thinking that the 6mm in the T&E is inadequate for the main earth between the two CUs?
Should I run a 10mm alongside the T&E?

NoSparks · Oct 4, 2009

The section of the Regs that cover this is 543.1.3, but to use the equation you need to know more about the installation. The pfc at the sub board is the main controlling factor feed this into the equation and it gives you the cross section of the sub-main. I did a few assumptions and came up with 9.7 sq mm.

Also the rukle of thumb I was taught in class was that the earth should be not less than 1/2 the size of the phase conductor. So again for a 16mm cable the earth should be 10mm.

However, is it a very old cable? most modern cables would comply with these requirements, so I would have thought the earth with a 16mm T&E would already have been a 10mm. Personally I cannot look at a cable and say if it is 6 or 10. (and neither could my inspector!) It may be worth a comparison with a known cable.

xraytek · Oct 4, 2009

If you use the adiabatic equation does the fact that an RCD is present come into it?

For instance if the Zs figure was at the maximum allowed for a 50A breaker (0.92) then you would have a fault current of 250A.
If the RCD operates within its maximum allowed time of 40mS then the adiabatic calculation gives a minimum size of 0.5mm

NoSparks · Oct 7, 2009

That's a good question. It seems to be an anomoly in the Regs, although they do say that you must not rely on an RCD as the only means of ADS. So I guess the answer is no, you still need the full size of earth.

Has anybody tried the NIC tech line on this one?

xraytek · Oct 17, 2009

NoSparks said:
they do say that you must not rely on an RCD as the only means of ADS. So I guess the answer is no

Hi NoSparks,
That was my thought too but wondered about TT installations.
If you had a Ze of 20 Ohms the PEFC will only be 11.5A, so the only protective device that will operate would be the RCD which makes it the only device that can be used in the equation.

I have not phoned any technical helpline as I am an employee and not a member of a scheme, but certainly curious about this senario.