Broken PEN conductor

[dobby1]

Is there any viable method to test for and quickly identify issues that are instigated by a broken PEN conductor ? Obviously, this is a potentially fatal flaw with TN-C-S configurations, but how would you go about identifying it in the event it did arise ? I would imagine things would go into lock down pretty quickly since fatalities would be quite likely, but it's good to brood over such issues in any case; life has the uncanny ability of throwing curve balls at you.

In my mind, I would probably test for more "routine" blunders, and probably narrow it down, but in the real world it always tends to be more nuanced and complicated.

 

[mwatsonxx]

There is no practical test possible due to the numerous parallel earth connections. If it goes dark, and the main neutral is not making a circuit, you have a broken PEN.

 

[bernardgreen]

Using a ground rod as a reference for true Ground potential measure the voltage between the rod and the CPC ( "Earth" terminal ) in the house.

As you say a broken PEN does create hazardous situation as the "Earth" in the house could be 230 volts above true ground potential. Safe for anyone in the house ( except the lights won't work ) but hazardous for anyone touching an "Earthed" item and something else at true Ground potential.

The hazard is for someone using an outside water tap, pipe bonded to the CPC "Earth" in the house and people like gas meter readers who are standing on the Ground and touching meters that are bonded to the CPC "Earth" of the house.

Another hazard is when the water supply pipe is metallic and it has a very low impedance to true ground. The wire bonding the water pipe to the "Earth" may have to carry the current that would have flown along the now broken Neutral. This could result is a hot, possibly melted, bond wire.

 

[mwatsonxx]

Using a ground rod as a reference for true Ground potential measure the voltage between the rod and the CPC ( "Earth" terminal ) in the house.

Very dangerous, you could be connecting stray voltages to the house that were previously carried away by the unbroken PEN.

 

[bernardgreen]

Very dangerous, you could be connecting stray voltages to the house that were previously carried away by the unbroken PEN.

Please explain that,

 

[Lectrician]

Isolate DP isolator at CU. Remove main earth from CU earth bar. Loop test from L to E(removed from earth bar) and L to N. If this is high or open circuit, you have an open PEN conductor, or issue from head to CU.

This test is done SUPPLY side of main switch and assumes no henleys etc between the head and CU, including in the earth.

An open PEN conductor can FOR SURE be identified via testing.

 

[bernardgreen]

If the PEN is open circuit then the Neutral and hence the "Earth" ( MET and CPC ) will no longer be connected to the Neutral at the sub station. Any load between Neutral on the consumers side of the break will drag the potential on the disconnected Neutral towards the Live potential. Hence if there is nore than a volt or two difference between true Ground and the MEY / CPC then it is possible the PEN is open circuit. Anything more than 50 volts difference then it is 99% certain the PEN is disconnected.

 

[JohnW2]

Very dangerous, you could be connecting stray voltages to the house that were previously carried away by the unbroken PEN.

How much current can you get through your voltmeter - a microamp, perhaps?

Kind Regards, John

 

[winston1]

How much current can you get through your voltmeter - a microamp, perhaps?

Kind Regards, John

Rather depends on the multimeter. An Avo7 for instance could be a milliamp.

 

[mwatsonxx]

If the PEN is open circuit

The question was about the PEN being broken, not an open circuit.

 

[JohnW2]

Rather depends on the multimeter. An Avo7 for instance could be a milliamp.

It could, but very few people still use ancient moving coil voltmeters! Even the Avo 8 (I still use mine occasionally) has a movement of only 37.5 μA.

In any event, whether it's 1 μA, 37.5 μA or 1 mA, I still don't see what would be "very dangerous" about using a meter to measure it!

Kind Regards, John

 

[John D v2.0]

Best way to find out quickly is go outside and stand in a puddle, then touch the outside tap.

In more seriousness, if your lights are going very dim or your stuff is exploding or the bonding conductors are glowing or melted, there's a good chance is your neutral. If so, go and switch off your main switch without touching anything metal.
If your cu is metal, fit a separate main switch.
Then don't touch anything metal until the supplier has fixed it.

 

[ban-all-sheds]

The question was about the PEN being broken, not an open circuit.

So with a broken conductor the circuit is still closed?

 

[dobby1]

Looks like there is some ambiguity, surprised no one has come out with a device to mitigate such situations, however rare they are. Maybe a device that monitors L-N-E and trips if E > 50v?

Also, something else that has been playing on my mind which I can't find an answer for elsewhere: in TN-C-S, the earth is derived from the neutral, so why does it only carry current when the neutral is cut? shouldn't current flow through both earth and neutral (thinking parallel circuitry) ?

 

[EFLImpudence]

It's not that.

It is the fault of the DNOs for not satisfactorily maintaining 'their' TN-S networks and simply connecting your CPCs(earth wires) to the neutral.


In normal circumstances a small current is flowing in the bonding conductors and extraneous-conductive-parts (if any) in the inverse proportion of the resistance of the neutral and ground - so not much.
The CPCs are, of course, all dead ends that don't go anywhere.

When the neutral supply is broken it all flows through the bonding and extraneous-c-ps - any extraneous-c-ps within the premises and exposed-c-ps become live waiting to be touched.