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Earth fault loop impedance via Earth stud

Kalvin

Hi,

I am working on a project replacing "like for like" Ex motors and reusing existing cabling. When doing an Amtech calculation the maximum zs is exceeded (long run of cable only using armour as CPC). I can not reduce the fuse anymore and can not get a motor rated 16A Tia fuse.

As it is a long run of cable which goes underground and up at various heights in sections, it does not seem feasible to run a separate earth or upgrade the cable. Currently there is an earth connection to local earth bar instead, where we will do an R1 + R2 test on the cable. I know this isn't best practice. 

Currently this earth form the local earth bar is connected to an Earth stud on the motor, now I thought that was only for equipotential bonding. My question is, if we get a zs reading for the circuit up to the earth stud on the cast iron motor would that be classed as protective earth or only equipotential bonding? Would the earth stud be connected to the earth terminal of the motor via the the metal casing?

Thanks for the help!

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    Is this an installation in Machinery as defined in the Supply of Machinery (Safety) Regulations? If so, there is no distinction, because "protective bonding of exposed-conductive-parts" rather than "protective earthing" is called out by Clause 6.3.3 - BS EN 60204-1 doesn't care whether the loop impedance is met by direct earthing, or bonding, or a combination of both. Having said that, all metal parts of the machinery are supposed to be connected to protective boning, and the protective bonding test of BS EN 60204-1 is required, which is different to the Continuity of Protective Conductors test in BS 7671.

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    Is this an installation in Machinery as defined in the Supply of Machinery (Safety) Regulations? If so, there is no distinction, because "protective bonding of exposed-conductive-parts" rather than "protective earthing" is called out by Clause 6.3.3 - BS EN 60204-1 doesn't care whether the loop impedance is met by direct earthing, or bonding, or a combination of both. Having said that, all metal parts of the machinery are supposed to be connected to protective boning, and the protective bonding test of BS EN 60204-1 is required, which is different to the Continuity of Protective Conductors test in BS 7671.

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