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Aluminium armoring as earthing

Mark Craig

I'm working on an installation in a new shed at the moment. The incoming cables are 95mm single aluminum armored from a 250A MCCB with vigi. The cables will be glanded into an aluminium plate and run through ducting underground with around a 50m run. My question is.....

Am I ok using the aluminium armoring as my main earth or do I need to run in a separate earth? 

I have seen some reports saying not to use the armoring as earth in singles and to only earth one end of the armoring because of circulating currents, but as these cables are aluminium armored they are non-magnetic so I don't believe circulating currents should be a problem

Any help and guidance will be very much appreciated   

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    The 'big boys' will know better than me on this, but I believe that there is still some voltage induced on aluminium armouring - very much smaller than would have been the case with steel armour but still some (compare with copper winding on a transformer, even if only a single turn). On shortish runs the effect is possibly small enough to be ignored, but on longer runs the voltage difference can sometimes reach hazardous levels. There seem to be a number of techniques to handle such situations - mid-point bonding, insulating ends, and even rotating the armour connections between phases so the voltages cancel out.

       - Andy.

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    The 'big boys' will know better than me on this, but I believe that there is still some voltage induced on aluminium armouring - very much smaller than would have been the case with steel armour but still some (compare with copper winding on a transformer, even if only a single turn). On shortish runs the effect is possibly small enough to be ignored, but on longer runs the voltage difference can sometimes reach hazardous levels. There seem to be a number of techniques to handle such situations - mid-point bonding, insulating ends, and even rotating the armour connections between phases so the voltages cancel out.

       - Andy.

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