O PEN Devices and Supplies

No point where the exposed-conductive-parts are within the building (i.e. inside the equipotential zone in old money) where everything should be at the same potential(ish) even during an open PEN event.

It can see them becoming an option for for other equipment outdoors in some situations - but as most other situations have shown over time to be relatively low risk, I suspect the price of O-PEN devices will have to reduce substantially before there will be much uptake. Not an option for the most obvious cases of caravans or boats of course without a change in Law (in the UK) as the ESQCR prohibits the use of PME earthing anyway.

Generally switching protective conductors has a major disadvantage - switching devices are much more prone to failure than solid joints or continuous unbroken conductors, so there's a risk of losing ADS under non-broken-PEN situations to be balanced against the (rather low) risk of broken PEN events too.

Switching bonding conductors is likely to cause as many problems as it solves. Potential differences can enter an installation from many different sources - e.g. metallic water or gas pipework common with neighbouring properties, as well as the PEN conductors and so there is probably more to be gained by keeping bonding intact rather than switching it off, even during an open PEN event. Also the required current carrying capacity of main bonding conductors in particular is always rather an unknown (especially in PME systems where currents from neighbouring properties can flow in your bonding conductors) so even picking a suitable rating for such a device would be problematic - a 10mm² solid copper bond will generally survive at lot more abuse than say a 100A switching device.

   - Andy.

Yassir Khalil said:

when do you think it will become a requirement to fit these devices on final circuits or supplies where metallic objects are connected, for example kitchen appliances or protective bonding conductors connected to pipe work or the use of appliances outside surely the disconnection of the all conductors and CPC and bonding conductors is requirement in the event of this fault occurring. Any Thoughts ?  

In practice ... never for equipment indoors (cpc's and main protective bonding control touch voltages, plus the open-PEN detection only really is intended to detect specific conditions which are not related to local touch voltages resulting from faults).

Whilst it may occasionally be useful in PME installations for equipment outdoors, in addition to thoughts above, if you consider something like a heat pump outdoors, it will likely have additional or fortuitous bonding via metallic pipework etc. and therefore the open-PEN will be useless (it may disconnect the cpc, but the bonding is still in place, so you will still get the PME touch voltage through the pipework.

The open-PEN detection is intended for a specific set of conditions, and shouldn't be confused with, or used for, any other purpose, including hot tubs. It is only recognized in BS 7671 for Section 722, and as stated above, if used in some other circumstances, especially where there is bonding (fortuitous or purposeful) downstream of the protective device, you may well be offering (selling) a device for a purpose it could never fulfil (now, I'm not an expert in customer sales legislation, but I think that's mis-selling, or mis-representation).

AJJewsbury said:

It can see them becoming an option for for other equipment outdoors in some situations

I think that would have to be considered very carefully, because there are a number of other considerations, not least the issue of fortuitous or purposeful bonding downstream of the device.

If the earth within a premises becomes live and your using an extension lead outside then with the possibility of you coming into contact with a point of true earth then potentially there is a risk. The bonding removes risk of touch voltage but what if the bonding conductors become live. 

All the replies to this post are great. Good to see how other minds work. Great responses 

Yassir Khalil said:

The bonding removes risk of touch voltage but what if the bonding conductors become live. 

If this happens in a building, the arrangement should be such that all of the building rises in potential. There are exceptions discussed in Guidance Note 7 and Guidance Note 5. Outside a building, all bets are off, but there are other considerations as discussed in my previous post. Please could you qualify your statement which I quoted above?

As far as possible equipment out doors could/should be double insulated, so the CPC, and any problem voltages on it, vanishes, and for hand held things like hedge trimmers it is required by product standards.

Cars are most a unusual case in being class 1 (earthed metal case), a high current load and easy to touch and without a self earthing construction, (unlike perhaps a lamp post which folk do not normally touch, and is planted in the ground not on rubber tyres )

So long as we avoid or ignore the live outside tap problem, the only thing that really needs the switched CPC is the car I suggest if you really want to do something to the whole building the correct thing to do is to TT instead.  (and insulating joints on all incoming metallic services.)

Mike.

unlike perhaps a lamp post which folk do not normally touch, and is planted in the ground

Don't make too many assumptions about steel lighting columns - the root is usually painted with thick bitumen paint against corrosion and in many situations they're planted into plastic sleeves to aid later replacement - so contact with the soil can sometimes be a little precarious.

  - Andy.

Very true, and there are other factors, the  ground may be dry, especially in a concrete or tarmac pavement,  but also the lamp load is physically small, and folk have no need to touch the thing- which is not true for cars which have handles especially for the purpose ;-)  I agree lamp post  earthing is not always going to be sub kilo-ohm, but often it will be, and my point was more that in the car case, every single one of these fortuitous safety factors is absent -  the stars are all wrong for the EV case. For other things such an unlucky alignment is less likely. Which is perhaps why after half a century of PME installed up and down the land, with varying degrees of beautiful and ugly installations, the open neutral problem has only really come to the fore now. That and of course some of those half century old cables are rather past their prime...

Mike.

I’ve been thinking about this a lot recently and I’ve been asking myself why it’s taken EVs to bring in OPDDs…. In the UK there are 400 Open PEN faults recorded by the DNOs annually, 1% of these result in electric shock, 25% of which are serious resulting in hospital treatment or death (death is actually quite rare).

The big problem with our network is the mishmash of earthing, you can go down a terrace and find properties still wired to the 15th edition right next to a recent rewire with a combination of all three earthing styles at different properties every 3 to 4 meters all the way down the street, in this situation is TT really TT if they have bonding on a gas pipe shared with TNCS gas pipe next door? Is a TT property safe next door to a PME installation where they both have outside taps and sockets only a couple of meters apart? It begs the question why so often the DNO doesn’t repair a high impedance fault on a TNCS when I report it, is it always appropriate to recommend an electrician comes to hammer in a rod? Would it be better to get onto converting the whole of suburbia to PME/TNCS? These are interesting questions and I’m sure they have different answers according to different on-site conditions where multiple factors increase and decrease the risks

I think they’re likely to be used more and more and will follow a similar path to RCDs, AFDDs and SPDs. If we look at the career of RCDs first it was recommended in high risk situations, then slowly but surely as they were proven to safe lives and property the places where they were recommended or required was opened up to eventually include pretty much every domestic circuit excepting some distribution circuits. I foresee the same with OPDDs, starting with the next edition of the EV code of practice I suspect they will become the preferred option where an installation is PME and eventually I suspect all new TN installations will require one. 

This emerging technology will save lives and take the pressure off the DNO.

Ill just add that I have found that some makes of RCD will cut out when the supply neutral is lost (even without a load connected which contradicts my understanding of RCDs operation)  but others do not, would this not have been cheaper and easier to implement? 

The big problem with our network is the mishmash of earthing, you can go down a terrace and find properties still wired to the 15th edition right next to a recent rewire with a combination of all three earthing styles at different properties every 3 to 4 meters all the way down the street, in this situation is TT really TT if they have bonding on a gas pipe shared with TNCS gas pipe next door?

I could not agree more.  But the mish mash underground is not fixed either - round our way they have been breaking up the old cast iron gas mains as they have inserted cheerful yellow plastic, so now that bond to the gas pipe is not sharing with next door in a solid metallic sort of way, but more like a horizontal electrode from the house to the first cracked pipe in the street where they made a gap so the yellow pipes could be fusion welded, so now about ten ohms loop instead of previously something too low to measure..
In the same way the addition of a water meter has added an impromptu break of about a foot of plastic in the water pipes, so the scope for parallel support to a dicky neutral with the sub-ohm metal services is not it once was.
The electrode fields of influence certainly overlap at the level of tens of ohms, so any attempt at 'TT' is certainly fault current limited alright, but not exactly pure like the text book diagrams of electrodes in isolation and circular voltage contours radiating outwards  either.

It may be simpler on new estates with all plastic services and pure PME, but give it a few years of hacks and changes and it won't be, and there may be scope for a few nasty accidents when things like water meters are replaced .

Mike.