Can anyone tell me - what is the name of the fault condition where the neutral stops working and the earth takes over as the return path and there is then a problem because (a) the earth wire is thinner than the neutral wire and (b) the electricity could make its way through the gas meter and cause a fire? I read about it somewhere but I cannot find it on a search.
Diverted Neutral Currents?
Never, ever bond an electrical earth to a gas pipe.
That's total bollocks. Bonding the gas is one of the principal things that must be done if the incoming pipe is metal, or can give an earth potential. The cable size is important, and must be 10mm CSA for most domestic installations. An existing 6mm cable could be permissable under certain conditions.
"PEN fault" or "lost neutral"
There ought to be insulated washers on the pipes in and out of the gas meter
^^^ Ignore that advice ^^^
Never use a gas pipe as an earth, but always bond metal gas and water pipes to the main earth terminal!
I suspect you are referring to what is, generally known as, a PME fault - although it generally relates to any ‘hazardous’ route to earth for the fault current when the Neutral fails.
For example, if you have an outside tap which isn’t isolated and the copper pipes are bounded to the Neutral to provide your local Earth ( a TN-C-S or PME system) and there is a Neutral break, your pipes ‘rise’ to the Live voltage.
Touch the outside tap and there is a current path via the earth ( the stuff you are standing on) and the numerous PME points prior to the break.
The good news is, fatalities from such incidents are very rare. That said, Neutral faults are less so.
Ours was bonded by the DNO.
I believe that the gas bonding should also be within 600mm of the meter outlet.
Industry sources say many reported appliance failures, which have led to fires, were in fact a result of NCD. The fault can cause gas explosions, according to Mackenzie. While the cause of the majority of gas explosions are recorded as gas leaks, he noted that NCDs are so prevalent on gas pipes that the gas industry attaches jump leads between gas pipes before changing them, because they generate sparks.
In its written response, the government added that the Gas Safety (Installation and Use) Regulations require additional electrical bonding when cutting conducting gas pipes to minimise the risk to workers.
However, Mackenzie said these measures were not adequate and would not mitigate the risks associated with the heightened levels of current that can flow through gas pipes where NCD exists. He also pointed out that any metallic piping could carry the current, including water and oil pipes, which are not covered under these regulations.
It would be a pretty naff installation if it had live wires anywhere near the gas in the meter. Would this not be picked up by modern circuit breakers though?
The difficult part would be finding the break. I'd imagine a double insulated device that had no earth, enough to not work on part of the system where the neutral is disconnected.
Brian
I'd assume that would be so obvious, it should not need saying. Most smart gas meters I've seen of late are battery powered and use plastic pipes. Brian
Thanks - that is the term I was looking for.
Thanks, Brian. It's just I read about it somewhere in the media and I wanted to know more about it. When I read it, I was as sceptical as you are. To my mind, if I take away the neutral for the washing machine there is no amount of AI that would allow it to substitute the earth wire to allow it to continue with the spin cycle.
Neutral is not a 'live' wire. Earth and Neutral typically come into the home as a single cable and only separated at the incomer.
The issue and I recommend doing some research of your own, is that when a neutral on the incomer goes o/c, the neutral and earth are then dependent on any local bonding, such as to a gas pipe. I recall a description to a block of flats where the gas pipe was running hot, because the gas pipe was supplying the neutral current into the block.
This is nothing about live wires, except that the Neutral/Earth conductor, in a fault o/c condition, can creep towards Live depending on bonding.
AIUI the issue with a PEN fault is that a turned-on appliance causes the neutral to go live. If you have, say, an incandescent light bulb that is switched on, a loss of neutral will cause the current to drop to near-zero (so it's not illuminated) but the resistance of the bulb is low so the neutral wire is now at live potential. If the neutral is bonded to earth, so is all your earthed exposed metalwork, and so your metalwork is now live.
It's only if there's another conductive path back to the neutral will enough current flow to illuminate the bulb and drag down the neutral voltage to a safe level. If the neutral is bonded to earth, that means the earth needs a good enough conductive path back to the neutral bypassing the fault (via the earth to next door's bonded neutral that is still functional, or an earth path all the way back to the substation), then the neutral will be viable and not at a dangerous voltage.
When installing an EV charger, you either have to provide it a separate earth rod (not part of PME), or a device that shuts off the live if a PEN fault is detected. There are noises about making such a PEN fault detector part of a standard consumer unit - sounds like a good idea to me.
Theo
But it'll be out in the street, so supplier's responsibility.
If it's like mine, then on a fault it disconnects all three: L, N and E before the EV wall box.
nib
There are noises about making such a PEN fault
In my experience my bonded earth was quite a low resistance and only by putting a kettle on did the neutral rise by 80VAC or so. This affected 2 other neighbours, and given the location of the fault, would have meant their mains voltage would have risen by sqrt(3) x 80V
There are rumours of an extra component in a consumer unit that measures earth current to disconnect when there is a measurable flow. It's closely reminiscent of an earth leakage breaker.
*and* earth
Come to think of it, this was also mentioned in the original piece that I had forgotten about. There *may* have been a suggestion of mandating this in the future.
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