My brother

[Dave]

Has contacted me about the Ozzie practice of using mains sockets to connect to wind generated 12 Volts as they are cheaper than the low voltage ones. This could result in a 12 volt device being plugged into a

12 volt mains socket and getting a bit of a shock. I have tried to explain the problem, but I am not an electrician. Would anyone care to correct the following before I post to him please.

start quote.

We call that PME over here. It is basically the same as you describe, but there can be a problem with it. Our power supplies are earthed at the transformers that supply the 230 Volts, from the centre of the 3 phase supply and again at the house input, by strapping the earth and neutral together. (There is no true cable to actual earth) It can happen that if an earth link fails at the incoming supply to a house on the same phase the neutral can rise to the supply voltage, by virtue of all the properties connected to that phase.

End quote

Regards

Dave

[Bob Minchin]

There would appear to be no commonality between the two section of your post.

[Dave]

I have no idea what their low power socket looks like and since they illegally use the same plugs for 12 Volts, as they do for 230 Volts, my brother was getting more than a little concerned. What they do is remove the live pin and consider it as a 12 volt plug, but I was trying to tell him what the problems we have about the neutral rising to mains voltage that we can have when fault conditions happen.

Ignoring the above, how close is my writing after 'We call that PME over here to the truth?

Dave

[John Rumm]

Yup - I was struggling a bit with that as well.

Its not the best description I have read to be fair... sorry!

The particular fault scenario I think you are talking about is where you get an open circuit PEN conductor (Protective earth and neutral) due to a cable fault. The neutral will rise to something close to mains (since it is now free to float, and is connected to mains via the loads in yours and other's properties), however more worryingly, the the earth will also rise toward mains voltage since it is connected to it.

Hence it is very important that the equipotential bonding is up to scratch to ensure that if all the exposed conductive bits of the house are going to float to mains potential - it really is *all* of them.

As a point of interest. the PEN conduction *is* however connected to real earth - not only at the sub station, but quite possibly at multiple other places along the route (hence the "multiple" bit of PME).

What any of this has got to do with 12V wiring and plugs though I can't quite see at the moment.

[Mike Clarke]

From the OP's second post I think they discard the live pin and use the neutral and earth pins when using the sockets in a 12V circuit. So in theory it should be safe if a 12V appliance is plugged into a mains socket since N and E should be at the same potential. But in practice it just needs an error like reversed L and N on a mains socket for plugging a 12V appliance into it to become very serious.

[meow2222]

Daves description / explanation of the problem seems somewhat inaccurate, but yes there are rare situations in whcih the mains earth can become live. However on anything other than a historic installation, mains power would be cut off immediately at the CU / fusebox / distribution board. And if everything in the house becomes live for a moment, the potential addition of a 12v appliance to that event is insignificant.

FYI there is more than one earthing system in use in UK, and the scenario works out differently dependant on the system in use. The nasty possibility is a 1950s install with local earth rod and no RCD or ELCB; these can stay live indefinitely.

The other problem is as Mike says, one trivial mistake on a mains socket or jbox and your 12v appliance can have live mains on it. Checking all wall sockets doesnt prevent this, as extension leads can also be miswired.

What could work is modding the plugs & sockets. Add a pin to the 12v plugs (eg a small bolt) and drill a hole in the 12v sockets, and the

12v appliances cant plug into mains any more. However, easier just to buy suitable plugs.

NT

[Caecilius]

Won't a load between N and E trip the RCD? That's another reason to avoid this practice.

[Graham.]

His newsreader would certainly benefit from some equipotential bonding, but I have a theory as to what he might be alluding to.

During the "Winter of Discontent" I installed some 12v emergency lighting in some of my ceiling roses between live and earth and during the power cuts I would turn off the mains and plug a 12v car battery into a ring socket, also wired between neutral and earth. The cable to the battery had a suitable fuse. I wouldn't recommend anyone tries this now, but for the safety standards of the time it didn't seem too cavalier.

Also, there seemed to be a convention for wiring 5 Amp round-pin sockets for 12Vdc in boats etc., with the earth pin +ve and the neutral pin -ve. Maybe it still happens today.

[Graham.]

That should have read neutral and earth.

[Owain]

Like the ones with the earth pin horizontal or T-shaped, or use electrak or similar.

Owain

[John Rumm]

At risk of getting even further from the intended point;

A short might trip the RCD. A "load" may or may not. There will probably only be small potential difference between L & E - especially on a TN-C-S install - so that might not be enough to push 30mA through the load, depending on the resistance of the load obviously.

[John Rumm]

Yup, your reading does sound plausible.

In which case the concerns over disconnected PEN conductors on TN-C-S / PME is not a particular consideration since both legs of the appliance would have near enough zero volts across them.

Agreed - there are other ways for this to bite you that are more serious.

[Dave]

Go, I'll bite. I wanted to put a smiley here, but since I have done the reinstal on this computer, not every key is where I think it is. Oh, I have found them :-)

What do you mean about, 'His newsreader would certainly benefit from some equipotential bonding, but I have a theory as to what he might be alluding to.' :-)

People of your knowlege can never upset me, by the way. :-)

I have always been open and honest and willing to learn.

Dave

[Graham.]

The equpotential bonding remark was my a light hearted way of agreeing with Bob's remark about the second paragraph of your post not (to us) logically following on from the first, by using an electrical metaphor. I would certainly have used a ;-) if I thought it would stop you thinking my intention was to upset anyone.

You snipped my "theory" about using neutral an earth to distribute the 12V from the wind generator. Was it correct?