13-02-2015, 04:36 PM
(This post was last modified: 13-02-2015, 04:43 PM by ppppenguin.)
FWIW, It's to do with maintaining equipotential on all earth points. Within a building (including a conservatory) this is easy enough, water pipes etc are bonding to electrical earth. Outside the building things are different becausee there's lots of true earth around. If all's well this will be at the same potential as the electrical earth. In a PME system that's why there's multiple places where neutral is conencted to earth. That's the M in PME. The problem with PME is making sure that the neutral really does stay at earth potential, even under fault conditions. When the earth is carried by the cable armouring and earthed at the star point of the substation (TNS) that's easy enough but a broken neutral in a PME system can cause a lot of current to flow to true earth, thus elevating both neutral and electrical earth above true earth. This can be hazardous.
I'd need to think about this but I'm not certain that an RCD will protect you under these conditions.
Google to see that a lot of ink has been spilled on the subject. More heat than light.
The safe rule of thimb is: If you're thinking of exporting a PME earth then unless you're really sure that nobody in contact with the exported earth can also touch true earth then TT it. SInce it's hard to be sure about that, TT is the safe default.
An IET note on swimming pools. not 100% relevant but discusses the exported PME question:
http://www.google.co.uk/url?sa=t&rct=j&q=&esrc=s&source=web&cd=1&ved=0CCEQFjAA&url=http%3A%2F%2Felectrical.theiet.org%2Fwiring-matters%2F42%2Fswimming-pools.cfm%3Ftype%3Dpdf&ei=YCjeVIjGM8XtaODugrgH&usg=AFQjCNEkQbO5R9aWPLD16Phth0h2EuQzdg&bvm=bv.85970519,d.ZWU&cad=rja
Also useful:
http://www.google.co.uk/url?sa=t&rct=j&q=&esrc=s&source=web&cd=7&ved=0CEIQFjAG&url=http%3A%2F%2Felectrical.theiet.org%2Fwiring-matters%2F51%2F-files%2Fswimming-pools-bonding.cfm%3Ftype%3Dpdf&ei=YCjeVIjGM8XtaODugrgH&usg=AFQjCNF1tJzBjmh4xR3mZno8k7B8XHfGQg&bvm=bv.85970519,d.ZWU&cad=rja
And another: https://www.tlc-direct.co.uk/Book/5.6.4.htm
I'd need to think about this but I'm not certain that an RCD will protect you under these conditions.
The safe rule of thimb is: If you're thinking of exporting a PME earth then unless you're really sure that nobody in contact with the exported earth can also touch true earth then TT it. SInce it's hard to be sure about that, TT is the safe default.
An IET note on swimming pools. not 100% relevant but discusses the exported PME question:
http://www.google.co.uk/url?sa=t&rct=j&q=&esrc=s&source=web&cd=1&ved=0CCEQFjAA&url=http%3A%2F%2Felectrical.theiet.org%2Fwiring-matters%2F42%2Fswimming-pools.cfm%3Ftype%3Dpdf&ei=YCjeVIjGM8XtaODugrgH&usg=AFQjCNEkQbO5R9aWPLD16Phth0h2EuQzdg&bvm=bv.85970519,d.ZWU&cad=rja
Also useful:
http://www.google.co.uk/url?sa=t&rct=j&q=&esrc=s&source=web&cd=7&ved=0CEIQFjAG&url=http%3A%2F%2Felectrical.theiet.org%2Fwiring-matters%2F51%2F-files%2Fswimming-pools-bonding.cfm%3Ftype%3Dpdf&ei=YCjeVIjGM8XtaODugrgH&usg=AFQjCNF1tJzBjmh4xR3mZno8k7B8XHfGQg&bvm=bv.85970519,d.ZWU&cad=rja
And another: https://www.tlc-direct.co.uk/Book/5.6.4.htm
www.borinsky.co.uk Jeffrey Borinsky www.becg.tv
