Danger of PMR (TN-C-S) and TT earth when used close together.

[ajohn]

Possibly, but I'm not really sure what that has got to do with the earthing arrangements - and one certainly cannot get 'spectacular currents' to flow into a TT electrode.

I should have quoted this
However, what they describe is (relay) switching which includes switching of the earth............................

It's why a combined neutral earth can't be used without the earth switching and why a spike shouldn't be connected to it either - in the case of EV's

It could easily be in excess of 1000amps. Hope they fuse them but then comes the problem of where.

 

[flameport]

I can't see how/why L voltage would rise (relative to true earth) in the event of an 'open PEN'.

Most domestic single phase installations are supplied from a 3 phase system, and when the combined neutral/earth is lost, the neutral point shifts depending on the balance of loads on each phase.

If all the loads on each phase are balanced, there isn't a problem. However with a row of properties with connections distributed across all three phases, unbalanced loading is inevitable. Some will get a voltage that's too low, others far too high (which will cause extensive damage on it's own) and some will get a voltage within the specification but the neutral point is now far away from where it should be, so the voltage between that and the actual Earth can be significant.
Even worse is that as loads in the various properties trip off due to faults being detected or things setting on fire, the actual voltages in the system will shift all over the place so a property that was initially within a safe voltage range can quickly become very unsafe.

There is a description of some of it here: https://myenergi.com/pen-protection/

543.3.3.101 covers switching a CPC, which is prohibited except in certain circumstances, the relevant part being (iii) which states:
'a switching device interlocked with a multipole, linked switching device inserted in the live conductors such that the protective conductor circuit shall not be interrupted before the live conductors and shall be re-established not later than when the live conductors are reconnected'

 

[flameport]

It could easily be in excess of 1000amps.

Only if the electrode resistance was in the order of 0.1 ohms, which will never happen in reality.

 

[ericmark]

I do see how a three pole contactor can switch off all live supplies and earth together and I see the idea of either like the solar panels switch all off if out or voltage spec but that would mean with a standard power cut it would not auto reset, so could loose freezer or fridge supplies, and the sensing earth probe I see how it is difficult to install one where not effected by other services in the ground, would it not be a lot easier to simply phase out TN-C-S?

 

[JohnW2]

... would it not be a lot easier to simply phase out TN-C-S?

In favour of what - TT?

To replace all TN-C-S with TN-S would presumably be a very time-consuming, costly and disruptive exercise, wouldn't it? After all, a fair proportion of TN-C-S supplies exist because the prior TN-S earth failed, and that to repair/replace it would have been much more expensive (and disruptive), don't they?

Kind Regards, John

 

[ericmark]

What would be the difference in cost to fitting every house with an auto switch off system should TN-C-S fail to using TN-S? If in 10 years time most homes will have electric cars then there will need to be a role out of larger supplies to each home so all new cables anyway so what would be the real cost of using TN-S.

If TN-C-S is safe, then why not allowed for boats, caravans, petrol stations, and electric car charging, clearly it is not safe.

 

[ajohn]

They argue that neutral loss wont happen - taken care of by the main supply lines so don't see any point in developing anything to detect it.

People with overhead supplies seem to be the ones most likely to have it happen. They tend to get tingles from things down to earth leakage currents.

We have TN-S and crap Ze. I am told they wont convert it by electricians that have come across this on properties they have worked on. Perhaps it depends on the style of cable running along streets. Anyway RCD's are used to get around resistance to earth issues.

Sure 1000amps needs a low resistance but the point is from the main supply point of view is that it wont be like protecting for 32amps. It will be higher and EV battery voltages if anything are likely to increase over time.

 

[ericmark]

I am not saying TN-C-S is not safe, but one can't have it both ways, either it's safe and no problem supplying boats, caravans, petrol stations, and electric car charging, or it's not and nothing should be supplied using it. Which is it?

 

[JohnW2]

What would be the difference in cost to fitting every house with an auto switch off system should TN-C-S fail to using TN-S?

Obviously quite a lot of money/time/effort - but that is very different from your suggestion of 'phasing out' TN-C-S.

If in 10 years time most homes will have electric cars then there will need to be a role out of larger supplies to each home so all new cables anyway so what would be the real cost of using TN-S. If TN-C-S is safe, then why not allowed for boats, caravans, petrol stations, and electric car charging, clearly it is not safe.

I suppose it's all down to balancing risks, and views about that balance of risks - albeit that money is probably one of the over-riding issues.

Despite all we hear about them, 'lost TN-C-S neutrals' are very rare - and, even when they happen, probably don't often result in the most extreme of possible consequences that we are always hearing about.

On the other hand, 'lost' (or deteriorated) TN-S earths are far from uncommon (and are, after all, a common reason for 'conversion to TN-C-S') - and they cause appreciable other risks, particularly given that, in most cases, the consumer whose installation has come to have an inadequate earth for this reason is very unlikely to be aware of the problem.

Kind Regards, John

 

[flameport]

In terms of risk, what has changed is the equipment used outside a typical dwelling.
Until relatively recently, most equipment used outdoors was portable Class II items that were only used for brief periods, so the risk of electric shock even with a TN-C-S supply was very low.

Electric vehicles are an entirely different situation, as they are very large Class I items and by their very nature will be connected to the supply for long periods.

 

[ajohn]

To be honest I would prefer that it was left down to the car makers. While there are some whoopsies at times they are generally minor and get fixed very quickly via recalls. They have the facilities and a lot of experience of failure modes and a number of other aspects.

All the vehicle should need is plugging into the mains - everything else left to them.

 

[JohnW2]

In terms of risk, what has changed is the equipment used outside a typical dwelling. Until relatively recently, most equipment used outdoors was portable Class II items that were only used for brief periods, so the risk of electric shock even with a TN-C-S supply was very low.

All true.

Electric vehicles are an entirely different situation, as they are very large Class I items and by their very nature will be connected to the supply for long periods.

Do they actually have to be Class I?

Kind Regards, John

 

[ajohn]

PAT limits
Class Type Max Leakage Current
1 Handheld & Portable 0.75mA
1 IT, Movable, Stationary, & Fixed 3.5mA
1 Heating & Cooking 0.75mA per kW*
2 All Types 0.25mA
* to a maximum of 5mA

These can also be seen as "fault currents" of items with a disconnected earth at the power source and the disconnected earth connected to a metal casing. They wont kill so to provide a "decent shock" another fault is needed. A severe leakage or a direct internal live to earth short. The woolly area is 0 to 30ma

I tried to explain this to a well known man on youtube who measures with a 10 megohm dvm and says look high voltage so can kill. He's measuring the voltage via a voltage divider formed by leakage resistance and the meters resistance not current flow. Edit - he argues that PENs should be banned. Actually it more relates to how they are wired and used.

If I buy a used item and earths of this type are connected to a casing I always disconnect it and measure the current flow - also touch the casing providing it's not a high reading. It never has been worryingly high. I do it because used items may have been sold because they have failed PAT testing.

When a single item is connected to a protected circuit there is no need to use a 30ma RCD it could be lower so there is scope for improvement. I have received a shock from a low current one while working on a live panel years ago - just felt like my hand touched something sharp.

 

[ericmark]

All true.
Do they actually have to be Class I?

Kind Regards, John

This is what I said about boats, there is a problem with a boat in that any copper earth rod can cause electrolysis and eat away at the boats hull, so there are two common methods, one is a set of diodes which gives a 1.2 volt differential before current can flow, the other is the isolation transformer.

Since you can get class II battery chargers the easy way around the problem is have a large battery charger as the only direct connection to the shore supply, and all 230 volt within the boat is from an inverter connected to the battery.

So as already said

To be honest I would prefer that it was left down to the car makers. While there are some whoopsies at times they are generally minor and get fixed very quickly via recalls. They have the facilities and a lot of experience of failure modes and a number of other aspects.

All the vehicle should need is plugging into the mains - everything else left to them.

However this means every car needs to be designed not to require an earth, which means there is a problem as unless every electric car is made class II the outputs still need to be class I, this has happened with narrow boats, all shore connections have an earth one hopes TT with a steel not copper electrode and the boat may either import the earth from shore supply or not, it is up to the design of the narrow boat as to if class I or class II well an isolation transformer does not really make the boat class II the boat can have its own earth return, all it needs is to be sitting in a good conductor and it is.

It seems even boats have some odd things, a boat which has an earth problem using a shore earth can kill some one swimming past it in fresh water, but they need to touch it in salt water.

So as long as the car body is not connected to the bollards earth, and the bollard is not metal, it would not matter what earth is used, as no one can touch it, the earth would be needed so in the case of a car fire the supply would trip out, but it would one be required in the case of a fault, like burnt out isolating transformer.

Charging a car is more like using a shaver there is a transformer some where to step the voltage up or down, all it needs is for that transformer to be of an isolating type.

Why the car is supplied with 230 volt AC rather than DC at under 70 volt I don't know, maybe there will be a car designed for use where there is an earthing problem, you simply park it over and induction loop or something?

However as it stands there is a problem, and any electrician who installs a charging point needs to be aware of the problems and take measures to minimise them, on another forum there was an "electrician" asking questions about earthing after just having completed the installation course, so it seems "electricians" are unaware of the dangers. OK I would not personally call the guy an electrician, I would assume some one who has retrained for what he thinks is a job with prospects, fitting charging points. But that worries me, if I trip and touch a car or charging point I don't want to get a shock.

 

[ajohn]

However this means every car needs to be designed not to require an earth,

It very probably does have it's own earth as far as it's power source(s) is concerned as a none conductive body can have problems. Take 12v for instance and fibreglass cars. Both polarities are wired around the vehicle. This seems to cause more problems than the usual way of doing it, also costs more though. Some one knocked on my door once and said excuse me your car is on fire. Not much of one fortunately but would have been if given more time. The makers later probably took a lot more care running cables through panels thanks to type approval. The grommet wasn't really up to many thousands of miles of vibration

I have worked on an EV that used a 200v battery pack and charging to well over that. To be a problem some one needs to be connected to both polarities. They could touch either without any problems. It's effectively a floating supply so in some respects it doesn't matter if one polarity is connected to the body. I doubt if they would do that. Say a charger is plugged in and that earths the body - no different to plugging many items in. Simplistic view, say they installed and rcd on the charge line internally. They should also sit down and work through possible faults and find solutions for them. It's a little different to working to part P - a set of instructions some one else has set and in real terms they are pretty good at it. If something is needed that doesn't exist - simple they will make it.

They were not that good at it years ago but an american lawyer got cheesed off with them and that resulted in type approval and many cars could no longer be made and a number of techniques used in them went as well. Then there is crash testing etc. Problems when the crop up are fixed pretty quickly and are rare. One company I had dealings with wanted to use sodium sulphur batteries - imagine what would happen if that spilt onto the road and it was hosed down with water. The sulphur needed to be molten as well. They were purely battery makers.

In some ways I wish I was still working as could then easily find out what they are getting up to. To be honest I suspect the government is steering things in a direction that isn't sensible for the foreseeable future. They have interfered before and may well have taken the wrong route. One that interests me is catalysers on diesels - one well respected maker said no - a tank of urea - the stuff some have used to cheat but bigger and used all of the time. Cats have their implications on petrol as well. When those went in the business were getting there with lean burn and saying hang on. They always try to work some years ahead.