Open PEN causes?

[pc1966]

The risks related to an open PEN in TN-C-S are often discussed here, say in relation to external supplies to out buildings, etc, but I wondered what is the main reason(s) for an open PEN fault?

I assume it is something to do with bad jointing techniques as with concentric cables it would be virtually impossible to accidentally cut the outer PEN and not short out everything, so I wondered if anyone with DNO links knows the sort of underlying causes and what (if anything) could be done to mitigate them.

TL;DR What causes an open PEN?

 

[Julie.]

I was at a conference some years ago, it was stated that there were around 350 open pen faults a year, but they never really went into any detail other than the majority occurred on aluminium cables, the pen being somewhat less protected than the phases, basically any damage to the outer sheeth would allow water along the pen, but it couldn't get though the phase insulation, under the right (wrong?) circumstances water with the impurities from the soil would corrode the pen away to nothing.

 

[pc1966]

I was at a conference some years ago, it was stated that there were around 350 open pen faults a year, but they never really went into any detail other than the majority occurred on aluminium cables, the pen being somewhat less protected than the phases, basically any damage to the outer sheath would allow water along the pen, but it couldn't get though the phase insulation, under the right (wrong?) circumstances water with the impurities from the soil would corrode the pen away to nothing.

Now that is an interesting point I had not considered: the impact of a leak on the sheath and (as you point out) the relatively common use of Al in that large DNO cables compared to the general case where (in the UK at least) Cu dominates.

 

[davesparks]

I think a lot of the problems arise from older cables, joints, systems which are susceptible to failures such as the water ingress.

I think new network installations generally have a copper concentric conductor these days which will resist corrosion far better.

 

[DPG]

There have been a lot of fails in under-pavement joints installed in the 1970s. Happened at least 3 times on our estate over the last few years. Once was at my own house. Guy doing the repair said it was not uncommon.

 

[marcuswareham]

My village is PME, but overhead seperate cables. The lowest cable is the PEN and is jointed at each pole for services. There maybe alot of faults on this type of distribution as it doesnt have the protection of concentric cables in ground etc

 

[LastManOnline]

My village is PME, but overhead seperate cables. The lowest cable is the PEN and is jointed at each pole for services. There maybe alot of faults on this type of distribution as it doesnt have the protection of concentric cables in ground etc

Yes. That is the major issue here as well. Particularly during storms, overhead supplies, are vulnerable in a way underground ones are, nt
[automerge]1597611788[/automerge]

The risks related to an open PEN in TN-C-S are often discussed here, say in relation to external supplies to out buildings, etc, but I wondered what is the main reason(s) for an open PEN fault?

I assume it is something to do with bad jointing techniques as with concentric cables it would be virtually impossible to accidentally cut the outer PEN and not short out everything, so I wondered if anyone with DNO links knows the sort of underlying causes and what (if anything) could be done to mitigate them.

TL;DR What causes an open PEN?

Another question for those with DNO, s links would be about the PME earth drops from the neutral to ground. What resistance do they have? How effective are they when the PEN breaks?

 

[pc1966]

Another question for those with DNO, s links would be about the PME earth drops from the neutral to ground. What resistance do they have? How effective are they when the PEN breaks?

That was something I was wondering in relation to the other current thread where additional installation earth rods on TN-C-S are being discussed - the 'M' in PME should mean that most (or all?) end points of the system's combined neutral/earth have a decent DNO-installed earthing rod/structure.

What are their design parameters, and why do they appear to not be good enough?

 

[LastManOnline]

That was something I was wondering in relation to the other current thread where additional installation earth rods on TN-C-S are being discussed - the 'M' in PME should mean that most (or all?) end points of the system's combined neutral/earth have a decent DNO-installed earthing rod/structure.

What are their design parameters, and why do they appear to not be good enough?

Precisely. What we can say for sure is that they are clearly not adequate to act as a plan B during an open PEN situation. However if we had some idea of their resistance we would at least be able to make a calculation of how many extra, rods would be needed additionally per home in order to design a satisfactory installation.

Is it an Insane idea (I mentioned this elsewhere) to also calculate what the cost of running a separate earth from the DNO, s trafo to each home.Split the cost over each home. Would it not be competitive (even cheaper) than all the extra protective measures we discussed on the other thread to deal with the safety issues that have arisen?
Basically. Go back to TNS.

 

[davesparks]

Another question for those with DNO, s links would be about the PME earth drops from the neutral to ground. What resistance do they have? How effective are they when the PEN breaks?

What are their design parameters, and why do they appear to not be good enough?

UKPN spec is for end of main and other intermediate PME electrodes to be under 100 ohms.

Surprised? I was!

UKPN specs are free to view and download on their website in the G81 document library, you juts have to sign up for a free account.

 

[LastManOnline]

UKPN spec is for end of main and other intermediate PME electrodes to be under 100 ohms.

Surprised? I was!

UKPN specs are free to view and download on their website in the G81 document library, you juts have to sign up for a free account.

Well now. That is Interesting!

 

[davesparks]

Well now. That is Interesting!

I believe as an alternative to end of main electrodes they can connect the PEN of one main to the PEN of another main.

 

[pc1966]

I believe as an alternative to end of main electrodes they can connect the PEN of one main to the PEN of another main.

Soft of "when is a borrowed neutral not a borrowed neutral?" riddle!

 

[davesparks]

Soft of "when is a borrowed neutral not a borrowed neutral?" riddle!

I had never thought of it like that, I'll see if I can dig up the relevant documents from my laptop later

 

[davesparks]

Engineering recommendation G12/4 is where you need to look for the rules covering PME from the DNO point of view.

G12/4 is not a regulation or law, however if a DNO complies with it for a PME supply then they are deemed to have complied with the ESQCR, which is law. Does this merry dance sound familliar?

It can be viewed freely online if you search for it

 

[Simon47]

Is it an Insane idea (I mentioned this elsewhere) to also calculate what the cost of running a separate earth from the DNO, s trafo to each home.

Are we talking about retrofit ? Or only on new installations ?
Retrofit would be maaaaasively expensive, just think of all the digging up that would be needed. Round our way they've been replacing the old cast gas pipes with plastic. There they only have to dig holes at each service tap and then use a mole that splits the old pipe while dragging the new on in. You can't do that with lecky, so they'd be trenching the whole length of all the roads, and trenching across each property to the mains incomer.
I can tell you that I'd be "a tad unhappy" if the lecky board wanted to do that on our street and into our house. For one thing, the amount of stuff I'd need to move, before I could start lifting the laminate flooring the previous owner was so fond of, before I could get the boards up that would be need to get the cable in - no thank you !
Of course, they'd probably insist on fitting a fuglybox on the front of the house - well they'd be told to go forth and procreate on that point.
It would be more possible where it's 100% overhead, but would still involve disruption in customer premises.

But I'll tos in another thing ...
What is the danger from a lost earth in a TNS setup ? It wouldn't be detected because nothing would stop working, but without adequate local earthing, any L-E fault would simply make multiple properties have live exposed parts. So would we be back to having a local earth rod and incomer RCD - as though it was a TT system ?
EDIT: WTF, you can't use t o s s here, as in "throw someone a ball" kind of t o s s ?