What is a reference Earth?
I see no difference in an armoured cable powered from directly from an MCB
in a CU or from a fused spur from a final ring circuit. The armoured strands
either supply the earth (an incorrect name I know but it is one people feel
comfortable with) or one of the cores in the SWA supplies the earth
(assuming you are exporting the earth). If one of the cores is used as earth
then it is perfectly reasonable to connect the brass lug on the gland to the
earth cable in the SWA to earth the steel strands.
IMBW, but I think the concern was that the OP might have meant to
connect the earth conductor within the cable to the brass lugs on the
glands at each end of it - and to nothing else. Thus leaving the lugs
and the earth conductor not connected to anything else at all.
I have a junction box in the garage which is fed from a spur in the garage
ring main and is protected by an RCD. The supply is a three core cable
I have used a correctly sized armoured gland with a brass earth tag which is
connected to the earth from the supply cable
At the other end a similar brass tag is used which earths the armour to the
So the system is at present earthed correctly.
If I did use a plastic box I assume that the brass earth tag should be
connected to the supply earth wire for full protection
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