Our Voltimum Experts answer your questions on a daily basis in our Technical Expertise area. This Question of the Day, concerning BS7671 : 2001 Amm2:2004 requiring main equipotential bonding from the original MET to extraneous conductive parts, is answered by the ECA:
1. Where a multi-occupied multi-storey office has each tenancy supplied from the landlords distribution switchboard in the basement (which already has it's own MEB applied to primary services) serving a tenant's MCB board (and where the disconnection times are the same for either 0.4s or 5s), is each of the tenant's electrical installations to be regarded as a separate electrical installation? (ie: with the 'source of supply' being at the termination each of the landlord's sub-mains and, therefore, requiring it's own MEB from the tenant's distribution board MET to services - common and/or dedicated tenants - entering the tenant's area as extraneous conductive parts. Examples in Guidance Note 8, Fig 5.17 only indicate a distributors' distributed sub-mains/service to office spaces).
2. 413-02-02 requires extraneous conductive parts to be MEB and lists service pipes etc... Is it that the parts listed must satisfy the definition of an extraneous conductive part, in that it must be liable to introduce a potential, before it is considered to require MEB? Alternatively, is it that, regardless of the definition, these parts must be bonded, (as the only real instance of central heating and air conditioning systems liable to introduce earth potentials are those that form part of a type of district heating/air conditioning system or pass between equipotential zones)?
Answer: I would suggest that each tenant's distribution board and associated installation should be regarded as a separate installation. MEB should be provided to the district heating pipework and AC ductwork at the entry position within each tenant's area.
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