Questions about PME

[Steviet]

Hi I'm on my last year of my apprenticeship and throughout my 3 years I've literally asked everyone I've worked with this question about pme systems.

if the PEN conductor fully breaks (near impossible I no because it's concentric but let's say in the metre for example) your return path is going to be anything bonded to earth if it's connected at a met not in a consumer unit as this way the current will still go through the rcd. Therefore causing everything bonded to become live and the system still working without a neatral?

The best answer I've had (from my tutor) is that the resistance would become so high that it would draw more current causing the rcd to trip but would it as the current would still be going in and out of the rcd?


Its obviously one of those things I don't need to no but it bugs me ha Ill anyone have a clear answer cheers

 

[Engineer54]

And that

My understanding is and we were told this on are design course it has multiple earth paths hence the word pme protective multiple earthing so every 15 or 20 metres of the supply cable it has a protective multiple earth down to the general mass so if we loose n it still has a return path

And that is exactly the case where overhead supplies and new new underground supplies are concerned. Not so in many underground supplies that have been converted to PME, many of these are still in progress of being fully PME'd....

 

[Farmelectrics]

Ok interesting and disturbing to read snow head

 

[Knobhead]

Both of these will work. The 2[SUP]nd[/SUP] version won’t cause problems with compliance. A voltage relay between N→E would be high impeadance.

 

[Engineer54]

That may be the case in the main distribution cables but locally it can still be lost.
It happened a couple of years ago on the estate my Son lives on.
Neutral failed in the joint at the entrance to the estate.
The whole estate of around 50 or so houses ended up with a floating "neutral"

It cost central networks thousands to replace anything left on standby and anything switched on before they got there and pulled all the cutout fuses.

The bill for appliances in my Sons house went to over £2 k for new PC, Printer, baby alarm, smoke alarm, Intruder alarm, oven, boiler control panel, fridge, freezer, shower.
And my Daughter in Law got a shock of the screw on a plastic 13a socket.
No RCD's tripped, only the shower MCB went whilst it was in use.

A company called Haste, specialise in recovery of these incidents, and they are very common.

I bet they have installed a monitoring protective relay in that local substation now then....lol!!!!

To be honest, the occurance of such broken neutrals is extremely rare, no matter what you hear to the contrary.

 

[DPG]

I bet they have installed a monitoring protective relay in that local substation now then....lol!!!!

To be honest, the occurance of such broken neutrals is extremely rare, no matter what you hear to the contrary.

My own house and my Father's house have both had broken neutrals. PME supply, bad/corroded joints in the t box under the pavement outside the house in both cases. Houses built in the 70s so not old either. My missus realised ours had gone when she got a shock off the metal clad light switch in the garage. Luckily no real harm done. LUCKILY. Daz
Daz

 

[Guest55]

My missus realised ours had gone when she got a shock off the metal clad light switch in the garage. Luckily no real harm done. LUCKILY. Daz
Daz

ooh , so close to that life insurance payout !
better luck on the next dno fault lol !

 

[snowhead]

I bet they have installed a monitoring protective relay in that local substation now then....lol!!!!

To be honest, the occurance of such broken neutrals is extremely rare, no matter what you hear to the contrary.

Relatively speaking, Yes.
Haste had just sorted out over 1000 houses with the same effects in Liverpool prior to this incident, but I bet the total value of (alledged) faulty appliances per household was far higher there,,,,,

 

[DPG]

I'm currently sabotaging the new t-box that they put in. Only 1 strand left. Daz