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

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

Bob Minchin wrote:
Dave wrote:
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
There would appear to be no commonality between the two section of your
post.

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

Dave wrote:
Bob Minchin wrote:
Dave wrote:
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
There would appear to be no commonality between the two section of
your post.

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

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?

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.


--
Cheers,

John.

/================================================== ===============\
| Internode Ltd - http://www.internode.co.uk |
|-----------------------------------------------------------------|
| John Rumm - john(at)internode(dot)co(dot)uk |
\================================================= ================/

John Rumm wrote:

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

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.

--
Mike Clarke

Mike Clarke wrote:
John Rumm wrote:

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

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.


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

On Wed, 01 Apr 2009 10:26:01 +0100, Mike Clarke
wrote:
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.

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

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

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.

%Profound_observation%

During the "Winter of Discontent" I installed some 12v emergency lighting
in some of my ceiling roses between live and earth

That should have read neutral and earth.
--
Graham.

%Profound_observation%

Caecilius wrote:
On Wed, 01 Apr 2009 10:26:01 +0100, Mike Clarke
wrote:
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.

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

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.

--
Cheers,

John.

/================================================== ===============\
| Internode Ltd - http://www.internode.co.uk |
|-----------------------------------------------------------------|
| John Rumm - john(at)internode(dot)co(dot)uk |
\================================================= ================/

Mike Clarke wrote:
John Rumm wrote:

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

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

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.

needs an error like reversed L and N on a mains socket for plugging a 12V
appliance into it to become very serious.

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

--
Cheers,

John.

/================================================== ===============\
| Internode Ltd - http://www.internode.co.uk |
|-----------------------------------------------------------------|
| John Rumm - john(at)internode(dot)co(dot)uk |
\================================================= ================/

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

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

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

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

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

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?

--
Graham.

%Profound_observation%