Broken PEN conductor

[snipersmith87]

Attended a seminar today based on EV charging points. The broken PEN conductor was mentioned and converting to TT.

2 things.
1. How often does a PEN conductor go down?? My understanding was that they use PME to reduce this risk further.

2. I asked the question as to should we be installing class 1 external light fittings / exterior water taps (no plastic joints) on a tn-c-s setups with the same potential to become live in the event of a broken pen. I honestly dont think they knew how to answer it. They just fobbed me off. So am i missing something?? Surely the risk is exactly the same. We dont go and TT mrs smiths metal light fitting beside her front door??

 

[Dustydazzler]

Worms and can spring to mind

 

[Nigel]

The Health and Safety Executive report that a broken PEN is reported every day with 10% of reports resulting in injury.

When designing an installation we need to consider likelihood as well as safety. The likelihood of touching a metallic light and true earth at the same time a broken PEN conductor occurs is very low. The likelihood of touching the car door is much higher.

In principle there is no difference. The risk severity is the same. It is the likelihood that changes.

 

[Dustydazzler]

Does anyone actually know anyone who has received a full on belt from a lost pme ?

 

[mattg4321]

I would've said touching an outside tap and true earth is pretty likely at this time of year.

 

[cliffed]

Well said Essex,if Tncs was that bad why are we still using it as a Earth.
Charging points by all likelihood will have to be tt anyway.
Loving the new set up on this forum,by the way.

 

[Bobster]

Well said Essex,if Tncs was that bad why are we still using it as a Earth.
Charging points by all likelihood will have to be tt anyway.
Loving the new set up on this forum,by the way.

We use TNCS because it's cheaper to install. Also, on failed TNC-S cables it is easier to convert for the DNO if the cable is damaged.

 

[shaun1]

Why do we not just install an earth electrode to the MET on all TNC-S installations, at the point it becomre TNS? It doesn't seem that much extra work in most cases, and if it had been written into the regs earlier on, a lot would have been done by now with board changes etc. It would also be a much simpler option when it comes to adding charge points, outbuildings, hot tubs etc.

I guese there is a risk of carrying the load current of several other supplies through your earthing system, but if everyone had one it would be divided between them. And I suppose that there would have to be some kind of monitoring at the substation for current returning via the earth there instead of the neutral or these faults would take a while to be detected. It sounds like a lot of work now but if it had been done from the start... Am I missing something?

 

[Andy78]

Why do we not just install an earth electrode to the MET on all TNC-S installations, at the point it becomre TNS? It doesn't seem that much extra work in most cases, and if it had been written into the regs earlier on, a lot would have been done by now with board changes etc. It would also be a much simpler option when it comes to adding charge points, outbuildings, hot tubs etc.

I guese there is a risk of carrying the load current of several other supplies through your earthing system, but if everyone had one it would be divided between them. And I suppose that there would have to be some kind of monitoring at the substation for current returning via the earth there instead of the neutral or these faults would take a while to be detected. It sounds like a lot of work now but if it had been done from the start... Am I missing something?

It was included in the draft of the 18th but didn't make the final cut. I think if it is deemed necessary it should be in the regs, but not BS7671, the onus should be on the supplier.

 

[Bobster]

It was included in the draft of the 18th but didn't make the final cut. I think if it is deemed necessary it should be in the regs, but not BS7671, the onus should be on the supplier.

I'll put money on it being in one of the amendments.

You're right though, should be done by the DNO, but I believe the reason it was included under the electricians scope of work, is to try an save money for the DNO who is already struggling to just maintain our national grid.

 

[Strima]

Outside a property you may well have metallic parts in contact with earth that are at a different potential to the installation.

Think of a row of terraces with a charging point and some metallic street furniture. There's a possibility of someone touching the vehicle on charge and the street furniture at the same time, if there's a fault on the charging system you could well receive a shock.

As for supplemental electrodes for properties, often it may well be difficult to effectively add an electrode especially in built up areas, much easier on new builds as the electrode could be part of the foundation.

 

[Dustydazzler]

I go back to my original question

Has anyone on here or does anyone even know of anybody that has received a full blown shock from a failed PME system

??

 

[Andy78]

I go back to my original question

Has anyone on here or does anyone even know of anybody that has received a full blown shock from a failed PME system

??

Nope. I have reported high Ze readings and seen a few failed joints, including my own house and both next doors, but never heard of accident or shock directly from a failed PEN.
I have seen quite a few potentially dangerous situations from badly installed TT installations though.

 

[DPG]

There have been at least two PME issues on our estate caused by failed joints where the house feed is tapped off under the pavement. It's a 1970s built estate and both times the guys fixing the fault have told me it's a common thing with that era of joint.

One was my own house and my wife received a shock when touching the metal clad light switch in the garage whilst stood on the garage floor. Luckily it was a tingle rather than a full on belt.

The DNO were very quick to come out and check things out. They did a quick fix by running a bit of concentric from my neighbour's outside meter cabinet and then came back the next day to dig up and replace the joint under the pavement.

 

[Dustydazzler]

So what are we actually looking at ? When we come across a faulty PME

Lost/deteriorating N connection somewhere in the ground ? Or problems back at the sub station

Has anyone ever measured the touch voltage on a tingly tap during one of these faults