--------because of a gas leak?----------

I've had experience of terraced cottages with shared services.
I'm of the opinion that all gas meters should be isolated from the street/other peoples' supplies with a plastic section of pipe fitted in the gas inlet.

so that if there is a fire the plastic melts causing a raging fireball

-------------------------
Keep up with the antics of the UK,s most humoured electrical company
Facebook: https://www.facebook.com/RtIndustriaLtd
Twitter: @rtindustria

Because people keep stealing parts of the network and network operators cannot guarantee that parts will not be stolen.

Copper thievery, because of this I would give PME the boot.

You may wish to give PME the boot but that will not save you from a fire due to over voltage caused by the loss of the supply neutral.

-------------------------
John Peckham

http://www.astutetechnicalservices.co.uk/

I think it's time that it became general practice in this country to install a local earth rod along with TN-C-S, as is the requirement in many other parts of the world in which PME is effectively the norm.

Over voltage.
Any chance of you explaining this one a bit more to me?
I mean how would a 230V supply become more than 230V due to the loss of a supply neutral?
Thanks.

"where PME is the norm"----------------its not PME

Yes well how is your maths

In short this will occur when the neutral is lost from an 3 phase circuit supplying an unbalanced load. The DNO distribution main is 3 phase.

If if the neutral is lost there is no reference to the star point of the supply transformer and a voltage will develop between the supply transformer star point and the load star point (the neutral). You have to think of the houses as being the 3 phase load.

This voltage is Vsn and it will vary in magnitude and phase angle dependant on the loads. If the system is in a steady state you could calculate it using Millman's theorem

Vsn = [(Vrn * Yr) + (Vyn * Yy) + (Vb * Yb)] / (Yr + Yy + Yb) where all values are complex numbers and the Y terms are admittances which are 1/impedances.

However, the system is unlikely to be in a steady state as televisions etc will be blowing up all over the place and the load will swing around.

So we would have to assume the worst case which would put 400 + volts across your appliances .

Regards

Geoff Blackwell

point taken but there are plenty of fire resistant materials that are non-conductors.

What isn't?

Thanks for the answer I appreciate it.

My maths is a bit rusty but I think I could get my head around what you are saying if I had some good reading matter to help explain it.

Can you or anyone recommend some good reading matter to explain the theory you are suggesting.

I would like to know more about this subject and the subject of three phase distribution as employed by the network operators.

Thanks.

Postman,

The neutral doesn't disappear throughout it's length.
Houses on different phases share the common neutral where it has not been stolen.

So what you end up with is two different houses with L on different phases BUT joined in series via their common neutral which is floating.
400 V phase to phase through the appliances but inseries with each other.

Nice answer.

I can dimly see it.

Thanks.

Still at least with a TT system you wouldn't have to worry about all your metal work becoming live.

It's still a safer system if you ask me.

Actually, that is not quite true. If the RCD fails to operate, it is easier to make the metalwork live. It is also possible to put a significant voltage on the neutral compared to true earth, which will affect every property connected to the same distribution substation.

TNS is always going to be the safest option. PME or TT will be safe in the correct situations - PME will always be safe within an equipotential zone (such as inside a correctly bonded property) but not necessarily as safe outside it, TT will always be safe with a functioning RCD that breaks the neutral conductor as well as the phase conductor(s).


Regards,

Alan.

When supplies are from pme using underground cables, where does the ME come into it ?

The distribution main is connected to the general mass of earth at a number of defined points - usually via earth rods - so that is mutiple earthing (ME).

The DNO people will no doubt give you chapter and verse about all of the variations.

Regards

Geoff Blackwell

I would love it if someone explained this one a bit more.
How would the metal work become live?
I mean the earth circuit is completely separate from the neutral circuit in a TT installation.

Thanks.

In the situation Alan described where an RCD fails to operate in the event of an earth fault, with a TT system there is unlikely to be enough fault current flowing to open the overcurrent device (fuse or MCB). The result will be that the entire earthing system of the house will rise to a potential which, due to the resistances involved, is near full supply voltage.

Right thanks for that.

I had a horrible feeling he was talking about a fault on the suppliers side.

This earth fault danger can easily be rectified by adding another layer of protection namely an S type 100ma RCD to cover the whole installation.

If one of the 30ma RCD's fails to operate because it is faulty then the S type will switch everything off.

I still think TT is safer than PME.

But I am still just a beginner at this game.