6 mA DC testing and 9 mA AC testing with reference to RCD's

[JohnW2]

It's very likely that open PEN conductors go unnoticed. The path via common bonded ECPs will inevitably be of very low impedance, as it will be bonded at every installation.

I acknowledged that but didn't think that those paths would be of sufficiently "very low impedance' to render the fault 'unnoticeable'.

Consider a typical street with 15 properties, all with a common metal gas service bonded in all of the properties. PEN conductor fails at a point which results in 10 of those properties not having any neutral connection. All of the neutral current for those 10 properties will now return via the individual bonding conductors, through the metal gas service and through the bonding conductors of the 5 properties which are still connected to the PEN. Everything works as normal, no one suspects anything ....

Yes, that was the sort of scenario I was envisaging - although I admit that I didn't consider the additional complexity of the situation in which some properties which were 'connected by metal gas pipes' were not suffering from an open PEN conductor.

However, at least in terms of the issue which bernard raised (of hazardous N-'true E' pds), I don't think one can 'have it both ways'. IF, in a given property which is suffering from an 'open PEN', the path to earth via extraneous-c-ps is of such "very low impedance that everything in the property works totally normally then, essentially by definition, there will not be a substantial N-'true E' pd.

... until bonding conductors in the remaining properties get hot, gas meters start to melt and so on. With low or moderate loads it could be weeks or months before anything seems wrong.

There is so much 'it depends' here that I don't think it makes sense to attempt a general answer.

At one extreme, IF all of the properties 'connected by a metal gas pipes' are suffering from an 'open PEN', and IF all those properties have bonded extraneous-c-ps of "very low impedance", then each of those bonding conductors will, on average, only carry one property's total load current, and (given the 10mm² minimum for bonding conductors with TN-C-S) I very much doubt that the bonding conductor will come to any harm. I can't speak for gas meters, but if a 10mm² conductor doesn't melt, I can't believe that a gas meter would - and if gas sup[pliers thought there was credible risk of gas meters 'melting' (which would be very bad news!) as a result of any credible current, they would surely isolate them (electrically) from electrical installations, wouldn't they?

At the other extreme, if you are envisaging a vast network of properties (electrically) interconnected by metal gas pipes, of which a very small proportion are suffering from an 'open PEN', then the neutral current from the affected properties would be spread so widely that the amount of that current in the bonding conductors (and gas meters!) of the properties would presumably be of no concern - and, as above, if the situation in the 'affected' properties is such they 'everything is working totally normally';, then they wouln't be seeing a particularly high pd between their neutral and (true) earth.

Between those extremes are various shades of grey - and, of course, as utility supply pipes become progressively changed to plastic, the scenarios you describe will presumably become increasingly uncommon.

Kind Regards, John

 

[ebee]

It's very likely that open PEN conductors go unnoticed.
The path via common bonded ECPs will inevitably be of very low impedance, as it will be bonded at every installation.

Consider a typical street with 15 properties, all with a common metal gas service bonded in all of the properties.
PEN conductor fails at a point which results in 10 of those properties not having any neutral connection.
All of the neutral current for those 10 properties will now return via the individual bonding conductors, through the metal gas service and through the bonding conductors of the 5 properties which are still connected to the PEN.
Everything works as normal, no one suspects anything until bonding conductors in the remaining properties get hot, gas meters start to melt and so on. With low or moderate loads it could be weeks or months before anything seems wrong.

Agreed.

I often wonder with the old TNS there must have been numerous occasions where N & E were joined together (Besides the DNO practice of joining at a repair to make seemingly only a small area TNC-S) . Whether by a fault or by somebody incompetent doing something very silly you would effectively have a something similar to TNC-S but the accidentally PEN having two conductors (example the N conductor and the E armour) joined.

I suppose that in this instance the lower the resistance (impedance) of such fault would make it more TNC-S like.

Similarly, not the same as but some of the principles might give rise to some similar thoughts :- Some years ago a customer rang me "XXX (old regional electricity board) has blown me up!". I was at a alcoholically propelled evening with about 20 others from the building trade - annual Xmas Bash of a small firm plus self employed contractors. So next morning I rang him back. Seems that he`d bought some Xmas presents and some were plugged in to test/charge etc prior Xmas. Came home, could smell burning. Lots of houses locally suffered similar . I advised him to ring Electric Board and ask for compensation. Coincidentally one of the venues we had visited that night meant that we passed his house in a fleet of taxis and noticed road being dug up by the Electric board.
Electric board were advising customers to claim on their household insurance.
I told him to ring them back and say "Hey you are responsible" anyway they sorted him and others out eventually and agreed to pay my bill when I renewed his Intruder Alarm Control panel. At that time he had a report of the situation that they had issued to affected customers, it was 2 pages of A4 paper and admittedly I read it twice to get an understanding.

Several days previous one of their guys was connecting something and on making metered tests had noticed odd things happening so aborted the connection and reported to a team to investigate within next few days.

That team found all appeared to be in order so no further action taken.

A few days later wham. two thirds of some local properties had a 415 voltage and the other third had nothing. Hence some transformers etc burnt out in connected appliances etc etc.

The report concluded that the N had fractured and that is why the first guy had noted odd readings. However the break was under the road and the fault had compressed back together to re-make a decent connection by the time the second inspection was made. So all seemed well.

A few days later it had opened up again so the N was floating depending upon phases imbalance. By coincidence, at the same time, someone had done something silly in one of the houses making a near dead short/extremely heavy load between N and Phase.
Result, One phase was held at Zero volts to N because of this sillyness and the other two phases were 415V.

I know a NE joint is not the same as a LN joint combined with an N fracture but in both cases strange things happened.

 

[ericmark]

It's very likely that open PEN conductors go unnoticed.

In the UK only found a loss of PEN once, a frantic call on 2 meter radio, my earth wires are melting, what should I do, already turned off the electric at isolator but insulation still melting.

By time I arrive the earth wire from house to radio shack was in balls of copper on the floor, rang DNO and said we had a lose of PEN, and told impossible, but some guys did turn up at the road works and the power was turned off.

Volt meter between his radio aerial ground (copper tape around his garden with 2 meter earth rods at each corner, so rather good) and the DNO earth showed zero to 380 volt approx, every house except his lost electrical equipment, he had turned of his supply before the earth wire melted.

When we had water and gas with metal pipes to each house then with loss of PEN they would all share a little of the load, but as we went to plastic pipes, it relies on the multiple earths of the PME system, and if the DNO has not fitted multiple earths, any single property which has a good earth will try and earth the lot.

So with your 10 houses, if all 10 have the earth bond wrong side of the isolation block on the gas pipe then they share the load, but if only one has it the wrong side, then it melts the gas pipe. I looked for a picture of the isolation block, but could not find one, where the gas pipe is plastic and any metal bits are plastic coated then likely not enough current to melt the pipe, had the installer used the SWA as well as the earth wire with the radio shack supply likely it would have taken the current, one can see why 10 mm² is recommended, I would guess he had 4 mm² but hard to tell from copper balls, he went to TT after that event.

Most people don't have earths like that in their garden, but the big issue was, he could not turn it off, there is no isolator in the earth, so even if the AFDD opens, many do have auto opening with excess voltage, it may save equipment, but does not remove the earth connection, so touch that patio heater, and your in for a shock.

 

[bernardgreen]

his incident was caused by a broken PEN, broken by metal theft.

Amazing footage of house explosion as fire brigade, police and utilities spell out dangers of metal theft: Watch video here

WEST Yorkshire fire bosses have released footage of a huge explosion just yards from firefighters and residents they rescued seconds earlier.

www.examinerlive.co.uk

I cannot find the original and clearer video of the event,

A better report

 

[flameport]

Industry must acknowledge dangerous network fault, warn experts

While networks and the government are well aware of the dangers of neutral current diversion, they have failed to publicly acknowledge the extent of the fault. This is delaying potentially lifesaving work, E&T has learned.

eandt.theiet.org

 

[flameport]

Home Page| Northern Powergrid

Northern Powergrid ensures the electricity you buy from your energy supplier gets to you safely and whenever you require it. Find out more online now.

www.northernpowergrid.com

 

[ericmark]

This incident was caused by a broken PEN, broken by metal theft.

I dispute caused by copper theft, that was an excuse, the cause was more likely earth or bonding wires wrong side of the insulating block on the gas pipe, or insulating block missing.

Yes the metal theft caused the over voltage and many item likely damaged as a result, in the past the earth/neutral path would have been shared between gas and water, and unlikely enough current to melt the gas pipe, and PME or protective multiple earthing should have a series of earthing points so loss of PEN may cause the voltage to fluctuate, but should have stayed within safe limits.

But TN-C-S does not state multi earth points, and clearly often there are not multi earth points, so loss of the main PEN means not just a few volts variation but a massive volt variation and any points which are earthed take a massive amount of current.

With patio heaters, and other earthed items in the garden including EV charging points, the earths fitted should be enough to stop over 50 volt from true earth. So a 60 amp supply, 50 volt = 0.8 Ω and a 600 amp supply should have enough earthing points so they can carry that 600 amp, so 10 at 0.8 Ω or 40 at 3.2 Ω it is up to the DNO to ensure enough earth rods, or separate neutral and earth.

Petrol stations are not allowed to use TN-C-S and neither should any premises with a gas supply unless precautions are taken to auto disconnect with loss of PEN, it is no good trying to blame others, be it road works or metal theft, if the DNO want to use TN-C-S then up to them to make it safe.

 

[ebee]

Well said that man!

 

[JohnW2]

his incident was caused by a broken PEN, broken by metal theft.

We know that 'it happens' (and no-one disputes that), and that, when it does it may get significant media (maybe national media) coverage. However, the same is true of plane crashes, earthquakes and many other 'very rare' events (in the UK, even including murders), so that these media-covered 'anecdotes' give no indication of frequency/probability.

Kind Regards, John

 

[bernardgreen]

I dispute caused by copper theft, that was an excuse, the cause was more likely earth or bonding wires wrong side of the insulating block on the gas pipe, or insulating block missing.

There are many proven cases of metal theft disrupting electrical supplies, disruption that has resulted in damage to properties from burnt out bonding conductors.

Metallic gas and/or water supply pipes do not have to have isolating sections ( block ) but fortunately most supply pipes installed in recent times have not been metallic.

I had several conversations with the team replacing the gas mains in the village.

The possibility of high currents flowing through metallic pipes is a serious concern when metallic gas mains are being replaced. One of the first actions after excavation and before any pipes are cut is to install a bonding connection either side of where the cut is to be made. The bond in green/yellow can be seen in this image,