27-05-2015, 12:20 AM
(This post was last modified: 27-05-2015, 12:25 AM by ducatiguzzi.)
Hi All
Did you get your workshop wired up nick? A lot of good advice from PPPPenguin but I do wonder how the cable termination would be done at the far end. A cable rated for 100A over 100m points to at least 70mm2 presuming you will be supplying lights (3% VD max) and to allow disconnection times for the fuse. The max cable size for the BG consumer units fitted with the 100A main switch is 35mm2 so a switch fuse must be used to terminate the larger cable, dropping the conductor size and therefor allowable current up to the CU. TT is defo the way to go over that distance, saying that the regular spikes and grids fitted along the way could be used as a PME supply, its what the suppliers are meant to do.... I'm sure you've also learned that the installation must be sign off by a Part-P registered Electrician, Im in Scotland and we don't have part P up here but I did read it recently investigating a PME export job I am doing.
On that note it is possible to export a PME system to an outhouse as long as it is carefully managed as far as earthing conductor sizes, building services and construction are concerned. PPPPengiun is pretty close to the money again but one thing to consider is that you are not just exporting the earthing system but you are actually exporting the equipotential zone. The problem with PME is when a neutral fault happens prior to your main incomer, if this happens anything that is switched on will allow current to flow into the neutral and the earthing system including any extraneously bonded conductive items, i.e. radiators, sinks and the like elevating everything to mains voltage levels. But off course if you are within the zone with enough impedance between you and true earth, socks, shoes, floor etc result - no shock because everything is at the same potential.
The problem comes when you export the zone to say a shed or caravan, an external fault occurs and you can reach a conductive part from outside the zone there's a fair chance of a shock depending on your impedance to true earth. One reason that only double insulated tools should be used in the garden particularly true in a PME installation, not exporting the earth into the garden. PME is cheap, I dislike it but I believe distribution neutral faults are pretty rare.
Hope my blethers help a bit
Cheers
Did you get your workshop wired up nick? A lot of good advice from PPPPenguin but I do wonder how the cable termination would be done at the far end. A cable rated for 100A over 100m points to at least 70mm2 presuming you will be supplying lights (3% VD max) and to allow disconnection times for the fuse. The max cable size for the BG consumer units fitted with the 100A main switch is 35mm2 so a switch fuse must be used to terminate the larger cable, dropping the conductor size and therefor allowable current up to the CU. TT is defo the way to go over that distance, saying that the regular spikes and grids fitted along the way could be used as a PME supply, its what the suppliers are meant to do.... I'm sure you've also learned that the installation must be sign off by a Part-P registered Electrician, Im in Scotland and we don't have part P up here but I did read it recently investigating a PME export job I am doing.
On that note it is possible to export a PME system to an outhouse as long as it is carefully managed as far as earthing conductor sizes, building services and construction are concerned. PPPPengiun is pretty close to the money again but one thing to consider is that you are not just exporting the earthing system but you are actually exporting the equipotential zone. The problem with PME is when a neutral fault happens prior to your main incomer, if this happens anything that is switched on will allow current to flow into the neutral and the earthing system including any extraneously bonded conductive items, i.e. radiators, sinks and the like elevating everything to mains voltage levels. But off course if you are within the zone with enough impedance between you and true earth, socks, shoes, floor etc result - no shock because everything is at the same potential.
The problem comes when you export the zone to say a shed or caravan, an external fault occurs and you can reach a conductive part from outside the zone there's a fair chance of a shock depending on your impedance to true earth. One reason that only double insulated tools should be used in the garden particularly true in a PME installation, not exporting the earth into the garden. PME is cheap, I dislike it but I believe distribution neutral faults are pretty rare.
Hope my blethers help a bit
Cheers
