Earthing cable from MET to other distribution boards

hillbillie · Jan 12, 2011

I'm working in a warehouse, that was originally a factory. The electrics were put in in 1965 and were added to by successive owners over the next 20years and since then have been neglected.

Some of the boards have no earth so I thought I'd run some cable from the MET in the meter room to these boards. The problem is that the runs will be up to about 90 metres. Should I use the same standard that the OSG uses for bonding conductors (i.e. to ensure the circuit resistance is kept under 0.05 ohms) for these?

The earthing system for the premises is TN-C-S

Thanks.

IQ Electrical · Jan 12, 2011

If you are talking about a CPC then you'll have to size it according to section 543, either from selection or calculation.

If the system is PME then 544.1.1 needs to be satisfied.

When you say the boards have no earth, are you sure trunkings/conduits aren't being used as a cpc?

hillbillie · Jan 12, 2011

The conduit is being used as the earth but this doesn't go everywhere. There are a couple of boards whose circuits don't go through the conduit.

the building has had bits bricked off and new bits added, it's a nightmare.

IQ Electrical · Jan 12, 2011

hillbillie said:
The conduit is being used as the earth but this doesn't go everywhere. There are a couple of boards whose circuits don't go through the conduit.

the building has had bits bricked off and new bits added, it's a nightmare.

If that's the case then you need to recommend a periodic inspection.

The problem arises when you upgrade some of the earthing but miss other bits that you maybe weren't aware of, but as far as your boss/duty holder is concerned, "I thought everything was ok because hillbillie sorted it your honour".