PME and M30+OSN

[Gareth's Downstairs Computer]

It can be very difficult, if not impossible, to determine what it is that M30+OSN is trying to say about PME when there is so much QRN in the shape of spite and nastiness also coming from his keyboard.

However, in the interests of technical correctness and safety, he is absolutely correct, IMHO, to debate AGAINST Spike's suggestion of a fuse between the PME earth and the RF earth because should that fuse blow, or there be no such connection in the first place, in the event of the posited neutral break, then the fault current, being the neutral return from all properties set up as TCNS downwind of the break with no further earthing-off connections, will travel from the earth terminal at the consumer unit, all round the house, and into the earth wire of the mains lead feeding all equipment also connected to the RF earth, where the relatively small CSA of such wires could result in overheating and a fire risk.

One thing puzzles me; in a PME house with no other earth were there to be such a neutral break, would that not mean that all mains-powered euipment with metallic earthed exteriors switched on would be at mains potential with the possibilty of a fatality were it to be touched?

If so, how real is the occurrence of such neutral breaks, for the obituary columns are silent on this issue?

[Jeff]

You are correct Gareth, under those conditions exposed metal work will rise up to possibly mains potential.

The theory goes that because all metal work is bonded together there is not a shock risk as there is no path to 'real' earth. In effect a Faraday cage.

That is why introducing an RF earth (or any other path to real earth) is a hazard, even if it is isolated from mains earth.

Jeff

[Brian Reay]

Les has already commented on the Faraday shield misconception so, other than agreeing wit him, I won't delve into that.

True but needs some more detail. The real fly in the ointment is the fact that our mains supply has the Neutral and Earth linked in the supply system- before it reaches the consumer. Therefore the 'real' earth always provides a potential part of return path in the event of a neutral break. This is the part of the basic theory usual rejects don't seem to grasp- the concept of a return path and that, due to the inherent nature of our system (regardless of whether the end consumer has a PME installation) the real earth is always potential return path if there is a neutral break.

What is more, RCDs use this return path, if it didn't exist, they wouldn't work.

[Roger Hayter]

I am not sure what you are trying to say here. In a non-PME system you either have a good low impedance local earth, or a very good low resistance earth provided by the utility separately from neutral. In either case, even in a neutral fault condition, the protective earth conductor of the installation is *not* likely to present a hazard by having a markedly different potential from another local earth because it is not connected to the neutral.[1] So this hazard is hardly a concern. If you are saying that any real world supply has a significant potential to earth and is a shock hazard (rather than being isolated from earth) I think most people realise this (though I agree some confusion exists about the detalis).

[1] An RCD is important to make the relatively high resistance of a local earth arrangement safer under some other fault conditions.

[Brian Reay]

Oh dear Roger, draw a diagram.

If you introduce a 'real' earth (eg a RF earth) into your shack which is in a PME supplied home and there is a neutral break you face two hazards, unless you take precautions. Electric shock and the normal (not bonding) earth wires in equipment cable carrying excess current*.

*It was these cables Spike's hare-brained fuse idea was intended to protect against not the heavy 'direct' bonding wire. Check the archives.

I've used the example of a shack but the same applies to an unprotected outside tap. The only 'PME fatality' I was able to find related to such a tab- a child standing on the ground, probably damp, touched the tap.

As for you claim re 'most people', when this topic first arose your chums and others were totally unaware of the issues. Having finally being forced to accept there was an issue, they insisted on reinventing the wheel - Spike's current current antics are an extension of this.

Of course, you will (as usual) ignore the facts and blunder on supporting the petty stupidity of your chums.

[Gareth's Downstairs Computer]

Wear a condom because it's an electrical insulator? :-)

Important to stress that where the shack is in a shed, then it has its own TT earth (assuming regulations were followed) so not a problem.

[Roger Hayter]

You do realise that this can only be a TT earth if there is *no* connection in the shed to the incoming protective earth from the house supply? It is a brave person who sets up such a system themselves without hte approval of the electiricity board (or whatever they call themselves now.)

[Gareth's Downstairs Computer]

Hence my comment about regulations because it is *VERBOTEN* to export the PME earth outside the premises containing the incoming supply.

[Buttons]

OH NO IT'S NOT!

[Brian Reay]

You (and I) live in the real world. Evans doesn't. In his strange world, his rules apply ;-)

[Roger Hayter]

So you think it is allowed to export the PME earth then? It does depend what to, caravans and sheds are a bit dubious. A proper extension or separate living accommodation to building regs standards would probably be ok, with relevant bonding of services.

[John Rumm]

You are, however you may also need to extend the equipotential zone into wherever you export the earth - especially if there is access to an independent earth in the new location.

Caravans have different rules.

Sheds if all timber with timber floor may be ok.

More detail here:

[Brian Gaff]

Look behind you. This post for some reason reminded me of a pantomime. Brian