Extraneous-conductive-parts in a TN-C-S (PME) system

Andy H:

 Advice please

Small factory moving their 3 Ø compressors outside the equipotential zone to a small metal shed 1 Metre from main building on a concrete plinth. Compressors fed directly from a dis board within the main building.

Compressed air line is 2” galv steel pipe fed through building metal cladding to feed various pieces of equipment. Also, outside is a dust extractor motor and filter unit on a separate plinth again ductwork fed through the building cladding to various CNC milling machines and supplied from the same dis board.

NB Comp airline and duct work contact Earthed building metal work by fixings at nemours points within the building

System is 420v 3Ø TN-C-S (PME)

Question

1. Does the extraneous conductive comp air pipework and dust extract ducting/filter unit require a 35mm2 main earth bond back to the MET (Main Earth Terminal)?

Yes, as it can introduce a potential (earthy)  - if the steel building frame is already bonded, then link from the frame to the service pipe or duct locally as it enters

1. All service pipework and or ducting is above ground.

Above or below isn't a consideration - the question is "can it introduce a potential ?" - in both cases probably yes, as it connects to an extractor and compressor sitting on a slab at ground potential

1. All equipment is on concreate plinths (as far as I am aware bog standard concreate with no visible reinforcement).

And likely no DPC, so in direct contact with "earth"


 Definition of an Extraneous-conductive -part according to BS7671 is. “A conductive part liable to introduce a potential, generally earth potential, and not forming part of the electrical installation”.

The only reference I can find refers to service pipework entering the building from underground although the regs do make mention of other installation pipework and ducting such as central heating and air conditioning systems, what it doesn’t say is if these enter the building from underground.

Any advice welcomed

Tanks

Andy

Regards


OMS

Andy H:

 Advice please

Small factory moving their 3 Ø compressors outside the equipotential zone to a small metal shed 1 Metre from main building on a concrete plinth. Compressors fed directly from a dis board within the main building.

Compressed air line is 2” galv steel pipe fed through building metal cladding to feed various pieces of equipment. Also, outside is a dust extractor motor and filter unit on a separate plinth again ductwork fed through the building cladding to various CNC milling machines and supplied from the same dis board.

NB Comp airline and duct work contact Earthed building metal work by fixings at nemours points within the building

System is 420v 3Ø TN-C-S (PME)

Question

1. Does the extraneous conductive comp air pipework and dust extract ducting/filter unit require a 35mm2 main earth bond back to the MET (Main Earth Terminal)?

Yes, as it can introduce a potential (earthy)  - if the steel building frame is already bonded, then link from the frame to the service pipe or duct locally as it enters

1. All service pipework and or ducting is above ground.

Above or below isn't a consideration - the question is "can it introduce a potential ?" - in both cases probably yes, as it connects to an extractor and compressor sitting on a slab at ground potential

1. All equipment is on concreate plinths (as far as I am aware bog standard concreate with no visible reinforcement).

And likely no DPC, so in direct contact with "earth"


 Definition of an Extraneous-conductive -part according to BS7671 is. “A conductive part liable to introduce a potential, generally earth potential, and not forming part of the electrical installation”.

The only reference I can find refers to service pipework entering the building from underground although the regs do make mention of other installation pipework and ducting such as central heating and air conditioning systems, what it doesn’t say is if these enter the building from underground.

Any advice welcomed

Tanks

Andy

Regards


OMS