Hi
Whilst checking the wiring into my garage I noticed that the feed int the garage Consumer Unit is via 3 core armoured SWA cable. The oute galvanized steel armour terminates at the plastic outer case of the C but is not connected to the earth. The earthling conductor i connected to the earth bar in the CU as it should be. Should the oute steel armour be earthed? Thanks Dave
-- daveo
The armour should be earthed at the house end.
There is no need for it to be connected at the garage end, provided some other form of earth is used, such as an earth exported as a conductor within the SWA (which appears to be the case here) or an earth rod.
Christian.
It's unusual to see SWA with an earth wire, are you sure it's not concentric. Consentric carries the earth and the neutral in the armouring, with the strands that carry the neutral coated in a PVC sheath. Typicaly concentric will have no brass glands on each end of the cable.
Regards, Blew
provided some
conductor
Typicaly
Blew,
I've seen quite a few single phase runs using three phase swa and using one of the cores as the earth and the other two as live & neutral.
AWEM
This is something which has long caused me some concern. If, for example, the house is earthed by an earth rod (rural setting), and has a phase to earth fault such that said fault is not sufficient to trip the circuit's mcb, and is sufficient to bring the earth wiring, bonded services and earth rod upto mains voltage - I've seen this happen - then the swa gland at the remote building, if it's not bonded to the remote building's "equipotential cage" will also rise to mains voltage relative to the local earth.
Anyone touching the gland (albeit usually being sheathed) whilst also touching a local earth, is in for a shock in more ways than one...
There are some circumstances (the one that I came across was related to a LPG filling station) where it is required to utilise a special type of swa gland, which incorporates an earth "break", such that a sound mechanical joint can be made, whilst the earth continuety is not. This means that since TT earthing is the only type allowed in such installations, it is possible that full mains voltage could appear across two potentially exposed conductors, very close together, which in my book is more than hazardous...
TN-C earthing is not permitted within a consumer's installation. It is permitted on the main supply line to the property, provided the earth is separated off into TN-C-S within the supplier's cut out.
Christian.
I would say that the chances of electrocution in this failure mode is very rare.
Firstly, it presumes that the house TT earth RCD has failed. Secondly, the fault condition within the house after this failure is so severe (all metal appliances are now live), that the minor risk of an insulated gland being live behind its sheath in the shed is of no significance whatsoever.
Christian.
With concentric the earth and neutral are seperate conductors, the neutral strands of the armouring are all seperatly earthed.
The TN-C system is called PEN when used inside an installation (protective earth and neutral, TN-C refers to the distribution system). It is alive and well in many of the blocks of flats in the area I work in. I belive PEN is the main system used on the continent and that it is still ok to use in new installations, though PME is by far the most popular sytem in the UK.
Regards, Blew
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