PME & Broken neutral

[Knobhead]

I know some countries use protection to guard against a broken neutral in PME systems.
Anyone know how they work?

 

[Edd]

Doesn't the white wire on the RCD help with this?

The old MEM RCD did have a functional earth but that was years back. the only time Ive seen the functional earth now is on RCBO's. With any normal front end RCD if the neutral is lost before its split then the RCD wont become unballenced.
Supply comes down the live through all your equipment, back up the neutral/earth and as theres no neutral back to the substation then its only return path is your bonding conductors, or what ever the lowest resistance it can find to earth!

 

[dean.diy]

Afternoon Snowhead , interesting thread you introduced. One of the contributors mentions this "pre" rcd as unsuitable for UK'S tncs system.This must be due to DNO's
not wanting sparks fitting an rcd to the(concentric) supply cable.

 

[Jimmy Boy]

Found an interesting PDF on this now I am thinking if 10 houses are on the same phase as me and the N drops off will I get all their fault load as well because my nice shiny new earth rod will provide a lower path of resistance than all their bonded extraneous water and gas ?
J
<snip> It is not a simple matter to retrofit TN-C-S distribution systems with such a facility. This is true for
TN-C-S systems (such as those in South Africa) that do not require the installation of a foundation
earth at the service entrance. Unless every consumer each have their own service entrance
foundation earth facilities, in the event of a break in the PEN conductor, the load current of all
other consumers may find a return path through those foundation earth connections that happen
to be present.
An extreme example would be the case of only one consumer having a foundation earth. In such
a situation, all the load currents of all consumers connected to that distribution system would try
to flow through the single foundation earth present. The consequential overheating together with
the related possibilities of incendiary ignition will be obvious.

View attachment Loss of Neutral.pdf

 

[DPG]

It happened to my house years back.

Happened to mine last year. Broken neutral in the joint box under pavement at end of drive. Guy who fixed it reckons there's loads of them happening. Daz

 

[Knobhead]

I am thinking if 10 houses are on the same phase as me and the N drops off will I get all their fault load as well because my nice shiny new earth rod will provide a lower path of resistance than all their bonded extraneous water and gas ?

This is the problem I had years back. I had 40A+ going down to the rod.
Will have a read of the PDF later
Thanks

 

[ackbarthestar]

Supply neutrals do fail and either go high resistance or open circuit.

I can't understand why everybody says its a rare occurrence since everybody I know says they have had PME problems. So its not as rare as some people make out.

Farms are your best bet for fireworks. I had one a few years ago where all the showers went live, the lights all blew and we had glowing 'stars' in the corrugated metal shed where the current was using the metalwork of the shed as a return earth path.

Now the IET, in one of their periodicals recommend that you should stake the pme supply at the MET. There is one small problem with this and that is everybody and his dog down stream of the fault will use your earth stake as part of their neutral return which could result in quite a few amps flowing through your green and yellow wires.
However, its wise to make sure your MPB is of a suitable size for the neutral and connected to the incoming services

 

[Jimmy Boy]

And if you have plastic gas/water coming in it gets worse.
J

 

[SPARTYKUS]

But as we all know, the laws of physics changed when coincidentally it became a tad expensive using TNS, so it'll be fine to use the N as an earth path.

 

[ackbarthestar]

And if you have plastic gas/water coming in it gets worse.
J

As long as your final circuits do not have a parallel earth paths via extraneous parts then MPB is not required. Gas pipes will require MPB if they exist in the property!

 

[Knobhead]

It would seem some countries use a RCD with a built in over voltage trip to protect the consumers equipment.



This vector diagram shows what happens with a floating neutral.

 

[Engineer54]

You are far more likely (if ever) to come across a high resistance supply neutral fault than a completely broken neutral, and that's a fact like it or not... You can check all the IEC and ITE and anyone else's reports/studies you like. They will all say the same thing, ''the incidence/occurrence of neutral breaks in low voltage supply distribution systems is so low, as not to even deserve consideration''!!! That's not to say that you should not protect against such an occurrence....

I take with a large pinch of salt, those that state they have seen numerous instances of broken neutrals on PME systems, it just doesn't happen, fortunately!!! I have a recently retired friend (DNO regional manager) that has only known of 3 such instances since joining, as was then, Eastern Electricity through to the time of his retirement in 2011, approx 35 years in total!!!

One of the easiest and cost effective ways of mitigating such problems to some extent, is to have a local earth rod at ''Each'' and every PME/TNC-S supply head, as they do in many European countries. But not at a numbty 200 ohm level, that would be a complete and total waste of time!!

Can't see why the combined over voltage/RCD devices that's been mentioned in this thread can't also be utilised in the UK either, that sounds like a reasonably solid idea, one that should be actively promoted.....

 

[ackbarthestar]

I can't help suspecting that there is a conspiracy of denial going on here.
Every electrician known to me, who's been in the game for a while has come across at least one failed neutral, that's not to say that it was open circuit, but more high resistance. As soon as the resistance rises beyond a few ohms it will begin to cause problems and will get progressively worse unless it is recognised and dealt with pdq.
I would certainly agree that spiking the MET at every supply intake will alleviate certain problems but will also hide them as well

 

[SPARTYKUS]

Bring back TNS

 

[johnboy6083]

i cant see how it would be a problem if all of the neutrakl curent was trying to go down one electrodwe. the laws of a certain mr ohm would take care of that. Aint you supposed to size the cable to the rod by best case resistance anyway? So in tother words, itys very unlikely that you would have a sub 10 ohms electrode. so the current that would flow would be 2.3 ohms. a 10mm cable for example would take that indefinately.
thy problem comeswhen people have undersized the cable to the rod, and you get a low current, long duration fault.

 

[Chr!s]

i cant see how it would be a problem if all of the neutrakl curent was trying to go down one electrodwe. the laws of a certain mr ohm would take care of that. Aint you supposed to size the cable to the rod by best case resistance anyway? So in tother words, itys very unlikely that you would have a sub 10 ohms electrode. so the current that would flow would be 2.3 ohms. a 10mm cable for example would take that indefinately.
thy problem comeswhen people have undersized the cable to the rod, and you get a low current, long duration fault.

You on the :54:


Its the touch voltage you need to consider.