I note the drama of a serious fire in the picture associated with the text! A good bit of journalistic research by the team at E and T to obtain a photo of a serious fire caused by an open PEN condition!!!

I think that might be GK and JP skulking in the background, likely discussing the pros and cons of earthing systems! 

But what to do about it?

Wholesale return to TN-S?

Or do like the French and simply not provide an Earthing facility at all (and make everyone go TT)?

Or stick with PME and modernise the cables?

Or stick with PME and add some kind of mitigation - e.g. devices to detect a broken PEN and report it and/or disconnect (could even be built into the new smart meters with modest changes).

(BTW I see diverted N currents not being quite the same thing as broken PENs - the former can occur even in a fault-free system under some circumstances)#

   - Andy.

That fire is not caused by a broken PEN, there are image look up tools that will identify it for you. Fine art America has it listed as an image of a practice drill in Calafornia, from 2013, and shutterstock and adobe both list it as part of their collections of 'fires and firemen' but with later date stamps.
II is just a 'clip-art' image.

But yes, diverted neutral current is pretty much inevitable if you have more than one property on a PME supply sharing a metallic service like gas or water assuming main bonds to BS7671 are in place. The bigger question is at what level it becomes a hazard worth worrying about.

Clearly a broken PEN leads to 100% current  diversion and is a far more serious matter. I can well believe as the 1970s Aluminium clad cables (CONSAC) suffer from moisture ingress and general age, this will be more common. I also think that the regime of reporting such things has became more formalized. - a great many instances were probably silently fixed in the past, when there was not an associated serious incident.

While we are renewing street mains to handle electric vehicles, heat pumps etc, all this will need to be kept in mind.

Mike.

I thought the recommendation at the bottom of 411.4.2 was the answer to "what to do about it"..

"it is recommended that an additional connection to earth, by means of an earth electrode ..............."

Yes?

I assume most of us are avoiding this recommendation? I tend to do commercial works in big concrete jungles - so this doesn't come up, or at least I've never considered it........perhaps I should. I can barely tell what sort of supply it is in a big switch room and would probably be wrong, let alone even beginning to decide if an earth electrode is required. The building steels will be bonded which is the same thing, so scratch that - no need for me to consider it. 

"it is recommended that an additional connection to earth, by means of an earth electrode ..............."

Yes?

That might be useful for preventing normal earth leakage currents raising the installation's metalwork to a hazardous voltage if the means of Earthing the installation is lost entirely (e.g. break in PE, but not N), but I can't see it being a sensible mitigation to a broken PEN in general. Generally you'd need to achieve something like  Ra * I < 50V - where I is the installation's load - to have any sensible effect - so fine where the installation draws less than an amp (single street lights for example) - but even for a modest domestic installation Ra would have to implausibly low to have any sensible effect.

   - Andy.

AJJewsbury said:

Ra * I < 50V

But if a number of installations downstream of the break have an earth electrode, then R_a(effective) is the parallel resistances of Ra for each installation (assuming no other earthing of the PEN conductor, and no extraneous-conductive-parts).

The good thing about that, is we often hear that there could be high numbers of properties downstream of the break. The higher the number of properties affected, the greater the population at risk ... yet if all had electrodes (and/or extraneous-conductive-parts), the lower R_a(effective) ...

Now, you could also argue that I goes up with the number of installations ... BUT it's also the more likely the break is in a three-phase portion of the distribution network, so if there are n properties downstream of the break, it's only n/3 times as much current (not n times), and also the overall earth electrode resistance requirement is quite possibly lower (depending on the maximum phase imbalance downstream of the break).

I'm pretty sure we've debated this regulation and its recommendation of use of the additional earth electrode for a TNC-S system.

Since some time has passed - are guys ACTUALLY installing this additional earth electrode? Seems like a very good recipe to eventually hit SOMETHING, gas, water, sewerage drain, electricity supply, mole burrow, badger set......something will get hit eventually if we're all doing this every day. 

Is anyone actually adhering to this regulation? 

tattyinengland said:

Is anyone actually adhering to this regulation? 

Whilst it is a "recommendation" I can't see it happening wholescale, in that respect  it is bit like the AFDD recommendation -  not going to happen very often.

One of the reasons the PEN fault detection were recommended for EV charge points was to avoid the potential damage to electrical and other services an earth electrode might cause. This seem to contradict that way of thinking. 

tattyinengland said:

Since some time has passed - are guys ACTUALLY installing this additional earth electrode? Seems like a very good recipe to eventually hit SOMETHING, gas, water, sewerage drain, electricity supply, mole burrow, badger set......something will get hit eventually if we're all doing this every day. 

Only if by 'electrode' you mean 'drive a rod' (which I'd agree is very risky).

There are other options.

But if a number of installations downstream of the break have an earth electrode, then R_a(effective) is the parallel resistances of Ra for each installation (assuming no other earthing of the PEN conductor, and no extraneous-conductive-parts).

The good thing about that, is we often hear that there could be high numbers of properties downstream of the break. The higher the number of properties affected, the greater the population at risk ... yet if all had electrodes (and/or extraneous-conductive-parts), the lower R_a(effective) ...

Now, you could also argue that I goes up with the number of installations ... BUT it's also the more likely the break is in a three-phase portion of the distribution network, so if there are n properties downstream of the break, it's only n/3 times as much current (not n times), and also the overall earth electrode resistance requirement is quite possibly lower (depending on the maximum phase imbalance downstream of the break).

Indeed. But on the other hand if you happen to be the only house on the street that's been upgraded to have an additional electrode, I could be a lot higher than your own load... All in all it seems to be of least use in the very circumstances where it should be most useful, which doesn't really recommend it as a solution.

   - Andy.