Our Voltimum Experts answer your questions on a daily basis in our Technical Expertise area. This one, regarding a site three-phase PME supply and whether the main equipotential bonding be disconnected for Ze measurement, is answered by the ECA:
A three-phase PME supply feeds a main site distribution board in a GRP cubicle, and 60m and 125m long sub-mains from the main site distribution board feed two separate buildings. The incoming distribution boards in the two buildings feed both internal distribution boards and distribution boards in other separate buildings. Apart from the 125m sub-main and its distribution boards in building one, which were installed two years ago, the rest of the installation is new.
A question has now arisen as to whether the existing incoming distribution boards in building 1 should be treated as the source of supply for the outgoing sub-mains and a Ze measurement made with the main equipotential bonding disconnected, rather than a Zs with the MEP in place. Is this so?
The reason for the question is because the same value of Zs has been recorded on the Installation Certificate at the incoming distribution boards in building 1 as that recorded for the Ze at the main site distribution boards.
Note that there is a large amount of 'earthy' structural steelwork in building 1, which has been bonded to the MET in the distribution boards at one point using 35mm earth cable.
Answer: The Ze should be measured at the origin of the building to which the certificate relates without the bonding connected.
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