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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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    There doesn't have to be a particular distinction between bonding and c.p.c.s - it's common for a single conductor to perform both tasks at the same time (e.g. the c.p.c. in a distribution circuit feeding an outbuilding with extraneous-conductive-parts) and you can even use an extraneous-conductive-part as a c.p.c. (543.2.1(vii)) (at least in principle, in certain circumstances). The important thing is to make sure you can get an adequate loop impedance using just reliable parts - using c.p.c.s of other circuits in parallel with your own might be a good way of doing that provided you're in an environment where you can guard against changes to the other circuit, similarly with extraneous-conductive-parts (although 543.2.6 sets the bar a bit higher).

    If all else fails and upping the size of the c.p.c. isn't on, you can always use an RCD for earth fault protection - as it (presumably) won't need to provide additional protection, just ADS, it can have a high rating (300mA, 500mA say) and be time delayed, if required, so shouldn't pose a nuisance tripping problem.

       - Andy.

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    There doesn't have to be a particular distinction between bonding and c.p.c.s - it's common for a single conductor to perform both tasks at the same time (e.g. the c.p.c. in a distribution circuit feeding an outbuilding with extraneous-conductive-parts) and you can even use an extraneous-conductive-part as a c.p.c. (543.2.1(vii)) (at least in principle, in certain circumstances). The important thing is to make sure you can get an adequate loop impedance using just reliable parts - using c.p.c.s of other circuits in parallel with your own might be a good way of doing that provided you're in an environment where you can guard against changes to the other circuit, similarly with extraneous-conductive-parts (although 543.2.6 sets the bar a bit higher).

    If all else fails and upping the size of the c.p.c. isn't on, you can always use an RCD for earth fault protection - as it (presumably) won't need to provide additional protection, just ADS, it can have a high rating (300mA, 500mA say) and be time delayed, if required, so shouldn't pose a nuisance tripping problem.

       - Andy.

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