Getting my head around PEN protection

[iep]

I recently looked at getting an EV charger installed and learned about PEN protection for the first time. I'm not an electrician so it wasn't something I'd previously thought about (also please forgive any misuse of terminology). To help get my head around it, I drew out a few pictures showing different fail scenarios. Curious if I'm missing anything.

It's a long post but if you have time to read over it and answer the questions relating to pictures 9 and 10, I'd be grateful.

1. Basic functioning system.

2. Neutral line failure:

3. Equipotential Zone protection:

4. Danger in non equipotential system:

5. Danger from simple current path to GND(?):

6. Limitted risk in Indoor setting:

7. Increased risk of path to GND in outdoor setting:

8. Risk of non-equipotential hazards enhanced outdoors:

Summary

PEN protection (where voltages of Live, Nuetral and Earth are monitored and all three disconnected during failure) is not a requirement for domestic, indoor electrical systems. This is because equipotential bonding and the generally high resitance to GND through floor coverings etc means, in the highly unlikely event of a neutral fail, occupants will be safe.

There's a definite grey area relating to outdoor 13A sockets in my opinion. Maybe it's different now but we had our house rewired 12 years ago and no PEN protection was aded ot the outdoor sockets fitted back then. I'm not 100% clear on why that is considered 'okay'.

PEN or PME fail protection is rightly mandated for EV chargers since the car is a large metal object which would provide a terrific opportunity for an electric shock either via damp ground or where the user touched the car and another earthed object at the same tinme. The length of time cars are left on charge also increases the likelyhood of their being connected during a supply failure event.

PEN or PME protection seems also to be mandated for high current (comando) style outdoor sockets. It's not clear to me why they should be considered a greater risk than a standard 13A outdoor socket (as illustrated above, the RCBO won't protect you either way) but I guess it may just be down to typical use cases for this type of socket (used to run larger, semi permanent installs of equipment).



Questions. I think the above pictures broadly lay out the risks associated with a PME failure but I have a couple of qurestions related to EV or other 'in garage' install and how earth bonding might actually afford greater protection here.

9. EV charging in garage. Why is it safer (apparetly does not require PEN protect):

10. Does earth bonding to metal water or gas main increase saftey of system betond ensuring equipotential of exposed metal pipework?

 

[JohnW2]

I recently looked at getting an EV charger installed and learned about PEN protection for the first time. I'm not an electrician so it wasn't something I'd previously thought about (also please forgive any misuse of terminology). To help get my head around it, I drew out a few pictures showing different fail scenarios. Curious if I'm missing anything.

OK. You don't seem to have missed much!

Summary
PEN protection (where voltages of Live, Nuetral and Earth are monitored and all three disconnected during failure) is not a requirement for domestic, indoor electrical systems. This is because equipotential bonding and the generally high resitance to GND through floor coverings etc means, in the highly unlikely event of a neutral fail, occupants will be safe.

All true.

There's a definite grey area relating to outdoor 13A sockets in my opinion. Maybe it's different now but we had our house rewired 12 years ago and no PEN protection was aded ot the outdoor sockets fitted back then. I'm not 100% clear on why that is considered 'okay'.

It (and also outdoor water taps etc.) is undeniably a theoretical potential hazard, and one that very much concerns one or two of the regulars here. The problem is not the sockets - it's mains-powered equipment being used outdoors, and would be exactly the same problem if the equipment were powered via an extension cable going out of the kitchen window or door!

I think it's 'considered okay' (which is probably not quite the right way of saying it!) for at least a couple of reasons. Firstly, electrical equipment (e.g. gardening tools) used outdoors is very commonly (probably nearly always these days) 'double insulated' ('Class II'), such that they have no touchable parts connected to the electrical earth system (indeed, commonly have a 2-core cable,so no earth at all). Secondly, despite all the fears about it,'PEN failure' is rare, and usually shows itself immediately by a general malfunction of the electrical installation - so it is very unlikely that someone outdoors would touch something connected to the installation's earth system at the very time that there happened to be a PEN failure.

So, it's a genuine theoretical risk, but one of very low probability.

PEN or PME fail protection is rightly mandated for EV chargers since the car is a large metal object which would provide a terrific opportunity for an electric shock either via damp ground or where the user touched the car and another earthed object at the same tinme. The length of time cars are left on charge also increases the likelyhood of their being connected during a supply failure event.

All true.

PEN or PME protection seems also to be mandated for high current (comando) style outdoor sockets. It's not clear to me why they should be considered a greater risk than a standard 13A outdoor socket (as illustrated above, the RCBO won't protect you either way) but I guess it may just be down to typical use cases for this type of socket (used to run larger, semi permanent installs of equipment).

I think you'd have to ask the person(s) who did the 'mandating' to get an answer to that question. The situation is, again, obviously probabilistic, so it's all down to whether the probability of harm is considered high enough to warrant pre-emptory action/regulations, given that life is full of risks, of varying probability (some of which we choose to 'accept') - for example, in relation to your previous harm, even if there were not 'mandated PEN-failure protection' feel sure that people would be much more likely to be harmed when driving (or travelling in) their car than when charging it

9. EV charging in garage. Why is it safer (apparetly does not require PEN protect):

Primarily simply because the car is totally within (and, in fact, is part of) the equipotential zone.

10. Does earth bonding to metal water or gas main increase saftey of system betond ensuring equipotential of exposed metal pipework?

Not really. That's what (and really all) the bonding is there for - to ensure that everything within the equipotential zone is equipotential!

I hope some of that helps a bit. You seem to have a pretty good understanding of the issues.

Kind Regards, John

 

[iep]

Love this from @JohnW2 :

'I think you'd have to ask the person(s) who did the 'mandating' to get an answer to that question. The situation is, again, obviously probabilistic, so it's all down to whether the probability of harm is considered high enough to warrant pre-emptory action/regulations, given that life is full of risks, of varying probability (some of which we choose to 'accept') - for example, in relation to your previous harm, even if there were not 'mandated PEN-failure protection' feel sure that people would be much more likely to be harmed when driving (or travelling in) their car than when charging it '

Fair to say then, probably don't have PEN-failure among your top ten list of daily worries

 

[iep]

Not really. That's what (and really all) the bonding is there for - to ensure that everything within the equipotential zone is equipotential!

Kind Regards, John

@JohnW2. Thanks for your detailed response.

Just on your last point above. While the connection to gas and water mains is intended only as part of the equipotential bonding scheme (and so shouldn't be relied upon to perform other functions), does the current path I highlight in picture 10 not provide some theoretical protection in the event of PEN failure (by providing preferential current path to the relatively high resistance of human body)?

Not for a second suggesting this should be considered sufficient to replace actual PEN protection but, doing the thought experiment, curious if it might be offering an unintended benefit.

 

[EFLImpudence]

While the connection to gas and water mains is intended only as part of the equipotential bonding scheme (and so shouldn't be relied upon to perform other functions),

That is the intended purpose nowadays but other consequences are not removed by being ignored.
What I mean is if the bonding does indeed form a satisfactory earth then nothing can alter that; it is just not to be relied on as the pipes might be removed.
No doubt John will tell you of his own installation.

does the current path I highlight in picture 10 not provide some theoretical protection in the event of PEN failure (by providing preferential current path to the relatively high resistance of human body)?

Yes.

Not for a second suggesting this should be considered sufficient to replace actual PEN protection but, doing the thought experiment, curious if it might be offering an unintended benefit.

Why not?

PEN (PME) has only been necessitated because of the DNOs failure to replace their faulty cables satisfactorily.

 

[blup]

Look at one of John Wards YT videos on the subject for a concise overv

 

[JohnW2]

Love this from @JohnW2 : ...... Fair to say then, probably don't have PEN-failure among your top ten list of daily worries

Indeed - that's certainly true of me (although, to be fair, I don't have a TN-C-S earth!) - but, as I implied, there are at least a couple of regulars here for whom it might well be one of their 'top ten worries' (at least, they are always 'going on about it')!

In other words, I probably 'worry' more ('am more conscious of a risk') when IN a car (driver or passenger) than when touching one that is being charged

 

[JohnW2]

@JohnW2. Thanks for your detailed response.

You're welcome. I hope it helped a bit.

Just on your last point above. While the connection to gas and water mains is intended only as part of the equipotential bonding scheme (and so shouldn't be relied upon to perform other functions), does the current path I highlight in picture 10 not provide some theoretical protection in the event of PEN failure (by providing preferential current path to the relatively high resistance of human body)?

If all required bonding s present and adequate, such that the entire inside of the building is more-or-less 'equipotential', there should not be any possibility of a significant current flowing through a human body. I suppose you're sequentially saying much the same thing, in that what I say is true because the current is flowing through the 'preferential current path' - but that is really just a different way of thinking about 'equipotential bonding'.

However, what you say does highlight one lof the other potential concerns about PEN failures (other than risks of electric shock). If the bonded gas/water/whatever pipes have a very ,low impedance to 'true earth', then the majority of all the total ('neutral') load current (possibly including that of several other nearby houses) might go via that route, potentially serious overloading the bonding conductors and hence representing, at least theoretically, a fire risk.

Not for a second suggesting this should be considered sufficient to replace actual PEN protection but, doing the thought experiment, curious if it might be offering an unintended benefit.

See above - I think you're just using different thinking/words to refer to what is essentially just equipotential bonding.

 

[ericmark]

It does raise a question about any class I equipment used outside, but most outdoor equipment is class II, so not a problem. As to why the electric car is not class II, I have never worked out.

However, there is a lot more involved with loss of PEN, and to protect the whole house would seem more prudent. But how is not so clear. The idea of PME is multiple earth points, and one hopes if the PEN is lost there are enough earth rods, so the supply earth does not exceed the real earth by more than 50 volts.

I have worked with an IT system, 220 volt phase to phase, from a delta transformer, and I have never had so many shocks. It also means both lines need protections, and there is no way one could use the British 13 amp plug with an IT supply.

Using a TT supply does remove danger to personal, but does not protect equipment on loss of neutral, which can cause voltage swings of zero to 400 volts, and one has to ask if having PME is a better system, as although the voltage can still swing, the amount it swings is likely a lot less. As the earth rods limit the amount.

But the problem today is some households are drawing far less than in the past, and some a lot more, and this varies so much depending on the time of day, it is impossible to balance the phase loads.

 

[plugwash]

It does raise a question about any class I equipment used outside, but most outdoor equipment is class II, so not a problem. As to why the electric car is not class II, I have never worked out.

There are various approaches to making a class II device.

One is simply to put "double/reinforced insulation" between everything electrical and everything a user can touch. That is fine for something like a lawnmower but gets tricky for bigger stuff. Maybe it could be made to work for an electric car, but I suspect nonmetallic couplings in the drivetrain would reduce reliability and it would complicate connection to accessories.

another is to have an isolated power supply. The problem is that there are very real tradeoffs between leakage, efficiency, size and EMC. There is a a reason why 20W power supplies are nearly always class 2 while 1KW power supplies are nearly always class 1.

For high-power devices class 1 is usually much easier/cheaper to design.

 

[flameport]

9. EV charging in garage. Why is it safer (apparetly does not require PEN protect):

It's not required in a garage because those who wrote that section of BS7671 decided that the risks in a garage were lower than those outside.
In some cases they could be, but certainly not in all. Garages typically have concrete floors, and if a vehicle was driven in there while it was raining that floor would be wet and not really any different from being outside.

10. Does earth bonding to metal water or gas main increase saftey of system betond ensuring equipotential of exposed metal pipework?

If entirely within the property and everything that should be bonded is, then in most cases safety is improved as all of the exposed and extraneous conductive parts will be at the same potential (voltage) as everything else.
Floors inside properties are not usually conductive, so the risk is contact between two or more conductive parts. Connecting them together (bonding) ensure they are at the same potential.

However bonding things that do not need to be bonded makes this worse, as now you can have metal objects connected to the electrical installation that don't need to be, resulting in more things with various voltages on them when faults occur.
See bonding to radiators, baths, window frames and all the other nonsense that used to go on decades ago and unfortunately still does.

When outside, it's totally different. The main risk is voltages between the conductive part(s) and the Earth itself. Ideally all electrical items used outside should be Class II or better, with no exposed conductive parts at all.
Unfortunately electric vehicles are Class I by design, with the metal frame of the vehicle connected to the earth contact in the charging socket, and therefore to the electrical installation earth terminal when plugged in.

There are other risks such as an outside tap supplied by metal pipework, in the event of a broken PEN / CNE that could have dangerous voltage on it. That is fixed by using plastic pipework,

, does the current path I highlight in picture 10 not provide some theoretical protection in the event of PEN failure (by providing preferential current path to the relatively high resistance of human body)?

It will, but in most instances it won't make any difference to anything.
To have any meaningful effect, the resistance of those pipes / other metallic parts to Earth would have to be well under 10 ohms. Even lower to prevent shock-level voltages from appearing. Such a thing is described in BS7671, but will be rarely used due to the impracticality of ensuring such a low resistance.

Water pipes to UK properties have been plastic for the last 50+ years. Gas pipes have been plastic for almost as long. Older properties with metal pipes are routinely upgraded to plastic.
Absolutely no way that these can be relied on for anything.

 

[plugwash]

Generally, ground/floors are not bonded. It is not impossible to bond them, but it's expensive and rarely done in domestic situation.

Indoors floors are usually dry and hard, and in potentially wet areas are often surfaced with non-confuctive materials like vinyl. There is often another non-conductive "damp proof" layer between the top level of concrete and the ground. Outdoors the ground is often wet and water on the surface is common.

Garages are somewhere in-between. They often have bare concrete floors which may not have a damp-proof layer and may be more conductive than a typical indoor floor, but usually are at-least designed to avoid standing/flowing water on the surface.

 

[JohnW2]

Garages are somewhere in-between. They often have bare concrete floors which may not have a damp-proof layer and may be more conductive than a typical indoor floor, but usually are at-least designed to avoid standing/flowing water on the surface.

Indeed - and people in garages virtually always will be wearing footwear, a lot of which has soles which are pretty (or very) non-conductive, even when the floor is very wet.

In many garages (like mine) a far greater potential 'hazard' than the floor is probably 'structural metalwork' which penetrates the floor.

However, in an attempt to keep this all in some sort of perspective, let me repeat what I recently wrote - that I feel sure that a person is far far more likely to come to harm when driving, or travelling in, their car than by touching it whilst it is being charged (in any environment)!

 

[JohnW2]

It will, but in most instances it won't make any difference to anything.

As I recently wrote, I personally wouldn't put it quite like that.

As I wrote, to say that nearly all of the current ('heading for earth') from someone touching a 'live' exposed-c-p preferentially takes a route to earth through conductors bonding extraneous-c-ps, rather than through the body of a person touching the extraneous-c-ps, is really just another way of saying that the bonding is doing it's job in creating an equipotential zone - so really actually 'making a big difference'.

 

[plugwash]

Indeed - and people in garages virtually always will be wearing footwear,

People outdoors will also "virtually always" be wearing footwear.

I think the key difference between outdoors and a garage is the potential for water in sufficient quantities to overtop or soak-through footwear.