Asking DNO re supply characteristics

[bernardgreen]

Some interesting and informative documents will be found if you google with the search term "diverted neutral" ( includes one which is not supposed to be in the public domain )

 

[JohnW2]

The complication is when the "official" Earth for the property is achieved by TN-C-S ( PME ) and there is also another low impedance route from the internal Earth wire to Ground. Most commonly this other route is a metalic water or gas supply pipe that has been bonded to the internal Earth at the Main Earthing Terminal. This is safe until there is a fault in the supply network which causes the supply Neutral to rise just a few volts above the potential on the bonded supply.

A lost Neutral ( metal theft at substation ) caused this damage, .... The source of the fire is the Main Earth Terminal

If DNOs wanted to protect again such (very rare) events, presumably that could achieved by installing an RCD upstream of the cutout (or, at least, upstream of where the installation's earthing conductor was derived from the neutral)?

P.S. Don't tell the LFB about that incident, otherwise we might find ourselves with a requirement for 'earthing terminals' to be contained within 'non-combustible' enclosures!

Kind Regards, John

 

[JohnW2]

In our case, plastic water pipe but metal pipes to a big metal oil tank sat on lumps of saturated wet timber. Some interesting safety implications there under fault conditions.

Some safety implications, yes, but in terms of what bernard has been talking about, I don't think you would ever get enough current flowing through saturated timber to melt cables and/or start fires.

Kind Regards, John

 

[bernardgreen]

I don't think you would ever get enough current flowing through saturated timber to melt cables and/or start fires.

Never exclude even the remotely possible exception.

 

[JohnW2]

Never exclude even the remotely possible exception.

The possibility of some things is so 'remote that it makes no practical sense not to 'exclude' them.

Kind Regards, John

 

[SimonH2]

I would guess that this sort of thing is behind the latest idea that earth rods are back in fashion even when PMEd. If every house had an earth rod, then between them they'd restrain the neutral and limit the current passed in each house.
But then there comes the discussion : if every house had an earth rod, and the neutral was sufficiently constrained so supplies were more or less normal - could the loss of a neutral go un-noticed for a while ?

 

[SimonH2]

Some safety implications, yes, but in terms of what bernard has been talking about, I don't think you would ever get enough current flowing through saturated timber to melt cables and/or start fires.

Maybe so. But several things come to mind ...
Firstly is the issue of having an outside tank that is live. Yes it's the same question over outside taps etc - if the "earth" becomes live and someone is stood on wet/damp ground then it's a significant shock risk. Different inside where the floor is dry and (in theory at least) anything conductive is either not connected to anything or is equipotential bonded to the live "earth".
And then there is another question as to what happens to the copper pipe. Would enough current pass to warm it up, and at what point does this become dangerous ?
And finally, consider that the conductive area of these lumps of timber (old railway sleepers) is substantial - considerably more than that of a thin earth rod - but insignificant compared with a metal water pipe. As it's been sat there for probably over 40 years, we can assume pretty good contact by now with both the ground and the tank.

 

[JohnW2]

Maybe so. But several things come to mind ... Firstly is the issue of having an outside tank that is live. Yes it's the same question over outside taps etc - if the "earth" becomes live and someone is stood on wet/damp ground then it's a significant shock risk.

As I said, there are some safety indications (but, in my opinion, not the one bernard was discussing) - and that is the most obvious of those 'some'. However, you can't have it both ways, if your saturated wooden blocks provided a low enough impedance to earth to result in bernard's melting earthing/bonding conductors and fires, then the 'shock risk' to which you refer would be minimised.

And then there is another question as to what happens to the copper pipe. Would enough current pass to warm it up, and at what point does this become dangerous ?

My personal view remains that even your saturated wooden blocks are not going to result in enough current (under 'lost TN-C-S' neutral conditions) to result in a dangerous rise in the temperature of even bonding conductors, let alone copper pipes.

And finally, consider that the conductive area of these lumps of timber (old railway sleepers) is substantial - considerably more than that of a thin earth rod - but insignificant compared with a metal water pipe.

My personal opinion is as above.

Kind Regards, John

 

[JohnW2]

.... But then there comes the discussion : if every house had an earth rod, and the neutral was sufficiently constrained so supplies were more or less normal - could the loss of a neutral go un-noticed for a while ?

If the number and quality of earth rods were such that supplies were 'more-or-less normal', I would imagine that a 'lost neutral' might go un-noticed more-or-less indefinitely.

However, for at least a good few decades, I would imagine the only way we would arrive at a situation (at least, for 'now existing' installations) in which "every house had an (adequate and remaining adequate) earth rod" would be if DNOs rushed around installing (and maintaining) them - and that's not going to happen!

Kind Regards, John

 

[SimonH2]

I guess my second email got to the right people. I got a call today to make an appointment for an engineer to call and confirm the earthing arrangements and fuse rating. Booked for a couple of weeks time.
No charge

 

[SimonH2]

You speak as if 'having to maintain a PME earth' is some sort of burden - but it surely is a total 'non-burden', since it merely means that they have to maintain the neutral connection to the property (which they obviously have to, anyway)?

Well, yes and no.
It might be a burden if they've agreed that it is PME, and then the electrician helpfully points out that Ze is above their limit. I'll find out on Tuesday what happens next ...

One of the questions in the 18th exam relates to 132.2 and includes getting that info by Enquiry, which obviously seems not that easy.

Yes, it does seem to be harder than some of the documentation suggests. But, when looking for something else in the BBB I spotted in section 2 that Ze references 313.1 where there is a note that says "The above information should be provided by distributors on request (see Appendix a sec 2)". App2 sec 2 helpfully points out that Regulation 28 of the ESQCR 2002 requires distributors to provide the following information to relevant persons free of charge - with a list that includes everything I asked about.

This was the Voltage Operated Earth Leakage Breaker ( VOELB ) which compared the voltage on the Earth wire inside the property with the voltage on the ground rod.

The pedant in me has been ruminating on that for a bit. The device isn't voltage operated, it's current operated - with fault currents going to the earth rod via the device - and the current creating a magnetic field that operates the trip mechanism. The voltage is a result of the resistance & inductance of the operating coil - but the operating characteristic is the current required to trip the mechanism rather than the voltage across it.
So apart from sensing current in the earth rod connection rather than difference between line & neutral currents, both perform the same function - disconnect the supply if a fault passes enough current to cause a potentially dangerous voltage on the protective conductors. The similarity is particularly notable where an RCD is being used for protection where Zs is too high to rely on tripping of fuse/breaker.

But then, this this could be a whole thread of "discussion" on it's own

 

[JohnW2]

Well, yes and no. It might be a burden if they've agreed that it is PME, and then the electrician helpfully points out that Ze is above their limit.

I'm not sure that that would be so much a case of 'maintaining a PME earth being a burden' as a complete hash-up of 'agreeing that it was PME' in the first place - since it's very unlikely that the impedance of their neutral conductor would ever increase.

[of VOLECBs vs RCDs] .... So apart from sensing current in the earth rod connection rather than difference between line & neutral currents, both perform the same function - disconnect the supply if a fault passes enough current to cause a potentially dangerous voltage on the protective conductors.

In the case of an L-CPC fault, a VOELCB should certainly operate if the pd between exposed-c-ps and true earth rises to a 'dangerous' level (>50V or whatever) (although no level of such a pd should really be 'dangerous' if there is a proper equipotential zone), but an RCD will result in that threshold pd being lower (even with TT and very much lower with TN).

More generally, it depends on what one is expecting of the protective device. An RCD will operate, and clear the fault, whether the fault is from L to CPC or from L to some other path to 'true earth' (e.g. via an extraneous-c-p). On the other hand, a VOELCB will only operate in the case of an L-CPC fault. If it is, partially or completely, a fault to 'truth earth' which doesn't travel through CPC and VOELCB to an earth rod, if that non-CPC path is of very low impedance, a high current can travel through that path (indefinitely, unless an OPD operates) without the VOELCB ever operating and clearing the fault.

Kind Regards, John

 

[bernardgreen]

The device isn't voltage operated, it's current operated

Correct, The only truely voltage operated "mechanism" is the gold leaf electroscope but that device is so mechanically weak it is unable to do anything usefull. Explained HERE

 

[SimonH2]

Engineer from DNO came as arranged. Confirmed that it's PME with a proper link and not some DIY arrangement. Got the fuse details for the EIC.
While he was there with the fuse out, checked all the connections - and removed the redundant extra neutral cable. The workmanship by the meter fitter was worse than terrible - it was outright dangerous
I hadn't noticed that the reason the neutral from meter to block had loads of copper showing was because a strand had missed the hole - and by the time he'd sorted that, it was very obvious the the other end of the cable was loose in the meter. All 6 connections (for some reason the meter fitter split the load across 2 phase connections in the meter) in the meter were loose.
It's a good job we don't normally have much of a load

 

[SimonH2]

Oh yes, AIUI Ze for a PME supply should be 0.35Ω max, I measured 0.48Ω. The DNO chap said something like "that's OK, pribably why it's only an 80A fuse"