Apartment block main earth size

captaincaveman · Jun 13, 2018

Hi chaps,

Could any of you help me out with this one please as I don't usually do apartment blocks and haven't come across this scenario before.

Large apartment block built 2004. Looks like the tails enter the consumer unit via Armoured cabling with the armour being used as the main earth & bolted to the top of the metallic CCU (can't confirm as the CCU is up to the ceiling). PSCC = 0.56ka. The linking cable from the SWA bolt to the earthing bar is 4mm2. Now (not taking the earth bar securing/earthing screw to CCU into account) my calculations suggest the size is adequate. But with the tails looking like 10mm (hard to tell as they are is soft flimsy sheaving) my understanding is that 4mm linking cable should be at least 10mm2 to meet the regs.

Anyone tell me if I'm wrong or, as I'm never wrong! ;-) what code it would be. I'm thinking C2?

Cheers,

Richard Burns · Jun 14, 2018

Presumably they have thought that glanding the cable to the board is providing the earth, but is that connection secure?
Therefore you now have a (section of) the earthing conductor at 4mm², adiabatically this is OK for a TNS system and a 60A main fuse, if this is what you have. If it is a bonding conductor as well then you are out of luck even with the SWA armour.
If you are selecting the size from the table then it would need to be 10mm². No code I think so long as the connection is secure and "mechanically and electrically sound"

Toneyz · Jun 14, 2018

Are you doing an EICR? I assume that the SWA is original being from the time of cable colour harmonization. What is the Zs at the switch-fuse? it could be a high resistance due to poor metal to metal contact with the glands. Did you do a test on the gland thread coming into the consumer unit?

captaincaveman · Jun 15, 2018

anthonybragg said:
Are you doing an EICR? I assume that the SWA is original being from the time of cable colour harmonization. What is the Zs at the switch-fuse? it could be a high resistance due to poor metal to metal contact with the glands. Did you do a test on the gland thread coming into the consumer unit?

Hi Anthonybragg,
Yeh an EICR (ergo why posted in the eicr forum

).
My assumption is everything is original with no signs of modifications. Building is only 14yrs old.
By the switch-fuse I assume CCU main linked switch? in which case it was ok (0.34 off the top of my head).
I couldn't inspect the gland thread as it was above the ceiling (which i guess is a fault in itself) but the nut was tight.