Is this a load of hokum

[pompeygit]

The following is an excerpt from P65 of The Electricians Guide to The Building Regulations......its the Green Book published during the time of the 16th Edition:

Broken PEN conductor. Under very exceptional circumstances, the supply PEN conductor connection to an installation could be lost to a failed joint. Where the phase conductor remains unbroken, a risk of electric shock from exposed-conductive parts and extraneous -conductive parts could forseeably arise. In most installations, the main equipotential bonding plays an important role in protecting against the danger from the loss of a PEN conductor.

So lets imagine that your supply has a broken PEN conductor, somehow the digger in the road managed to sever the PEN but left the phase in tact (haha). Your water supply is MDPE, and the gas board have just upgraded your meter (current practice of shoving a plastic sleeve through your old metal pipe, and then changing to iron or copper). You have a split load board. Is there any chance any safety features will activate? Are all your radiators live? Will you get a belt of the toaster etc etc??

 

[Spark123]

There is little chance a safety device will operate. Items will probably cease to work (assuming the open circuit PEN is on part of the single phase network) as there is now no path for current to flow, bar any sort of leakage back via mother earth.
The main bonding comes into play as even though everything is now at near mains potential with respect to mother earth, the bonding causes nearly the same potential to be aparrent on all exposed and bonded extraneous conductive parts. This in theory reduces the risk of fatal electric shock by maintaining the voltage between them to less than 50v.

 

[pompeygit]

Get your drift spark123 but lets say there is some earth leakage, earthed back boxes for example and they all add to around 300ohms, I toch the toaster........am I a gonna?

 

[Spark123]

Theres a lot of ifs and buts, the leakage from the socket back boxes isn't a really massive issue to you. In the event of an open circuit PEN, if you picked up a class 1 piece of equipment and there was a path through you to mother earth then you will more than likely recieve a shock, the severity depending on the effectiveness of the path.
If you were not in contact with a path to mother earth and touched an equipotentially bonded metal pipe whilst holding your toaster then you should not recieve a shock - OK you might get a tingle!!
If the pipe was not bonded, the pipe nicely in contact with mother earth and you touched it whilst in contact with your toaster you'd get a big belt!

 

[Lectrician]

It is nasty when it happens, and I have seen it a couple times.

Far more likely with an overhead supply and the lugged neutral at the pole corrodes and goes high resistance.

This is why caravan parks cannot use a PME earthing facility at the hookups - a steel chassis on rubber wheels......the soil being at 0v, and chassis being at 240v (or somewhere near). Would you like to get in or out of the caravan?

 

[Spark123]

Applies to marinas too, ESQCR iirc prohibits it.
Have seen the devistation caused by an open circuit neutral on a 3 phase supply to a housing estate before, every 2 of 3 houses got 415v across all their appliances! What a mess. Not sure if it was a TN-S or TN-CS in that case tho.

 

[pompeygit]

Thanks for your illuminating replies. Makes you think TNS is more desireable. Suppose the 415v was caused by neutral back feeding to other houses? Is TNCS an economic choice for supply companies? Or are there real benefits to it?

 

[RF Lighting]

It is attractive to the DNO as it requires less copper to supply each property, and is therefore cheaper.

The advantage to us it you will generally get a much lower Ze reading from a TN-C-S supply.

 

[pompeygit]

RF you are right about the zs issue. Most of the housing stock in our area(old) is TNS and generally get a measured zs of between 0.18 - 0.30, much less than if you bother to enquire and get the standard reply. However, I can see that the cables must be very expensive at todays prices. They appear to be lead sheathed wrapped in a sticky fabric tape, just not a viable proposition these days. Have only seen a few TNCS, looks like concentric cable, and cheap as chips in comparison.

 

[securespark]

Would you like to get in or out of the caravan?

Suddenly you're in the mood for a spot of quick-step....

 

[ban-all-sheds]

As the sheath on old TN-S cable rots away, you may find you're gradually, and unknowingly, converting to a TT supply...

 

[ebee]

Yep,
seen that happen on TNS,
High Ze,
Supplier contacted,
they dug up outside the property and fixed it, Apparently it was affecting next doors too.
Could have been a nasty situation existing for years and nobody knew.

 

[sm1thson]

Would you like to get in or out of the caravan?

when in france years back the caravan we were in gave a distinct tingle when you had one foot on the step and one on the ground outside, we reported it straight away (was Haven or Eurocamp or something similar), i dont recall them sorting the situation whilst we were there. We found having the fridge turned on caused it (and presumably some other earth or neutral fault).

 

[holmslaw]

..

 

[sm1thson]

and you should not use sp rcd's.

what about SP RCBO's? i thought they were the future? i know the point has been discussed before but Ive not seen the conclusion.