Protection against a lost neutral in TN-C-S

[La Poste]

I'm just having a little think to myself (I know it's dangerous) and have a quick question.

Say for instance you lived in another country with overhead power lines, just Line and Neutral and you wanted to make a safe electrical system for your home.

One method could be to put a nice big rod at the origin of your supply where you could bond the Earth and Neutral.
After the Earth Neutral bond you could then wire your house in the same way as the UK with a split load dual RCD consumer unit.

The only thing you would have to worry about would be a lost supply Neutral as this would make all the metal work in your property live.
Is there a device like a large RCD you could put on the supply cables (just Line and Neutral) before the rod and before the E-N bond?
If you had a RCD placed in this way then if the supply Neutral was lost and your metal work became live the leakage current through the metalwork would be enough to create an imbalance between supply Line and Neutral and the RCD would switch off the supply to the whole property.

I understand that due to bonding of gas and water pipes we may get leakage current to real earth through these pipes and the supply Line current would never be 100% equal to the supply Neutral current but surely a protective device could be calibrated to account for this leakage.

Has anyone ever come across a protective device that is used to protect against a lost supply neutral in a TN-C-S system?

Thanks.

 

[StuSpiers]

Not used in uk but in other countries yes they are used.

I posted a thread about this in the arms recently I'll get the info for you....

 

[StuSpiers]

This is a direct cut and paste from another forum I visit, it is not a UK forum so there are slight differences in terminology and methods but I thought it was a real interesting post.
​

Please note, this thread is intended as information for those with some electrical knowledge who may not be aware of the simple technique described. As always please take care when dealing with anything related to your power supply.​

The problem
Losing the neutral in a conventional TT supply system (just a ground stake, no MEN link between N and E) simply results in all your lights going out with little or no serious issues.
However with an increasing number of expat homes requiring 3-phase supply and the requirement for new installations in Thailand to be MEN connected the loss of the neutral can lead to significant and potentially lethal consequences.
Losing the neutral of a 3-phase system results in the most lightly loaded phase voltage increasing, potentially to over 400V with the associated effects on your delicate technology none of which will be particularly impressed.
Loss of the neutral in a MEN system can result in all the ‘earthed’ metalwork in your home becoming live, potentially at 220V, an even more unpleasant situation, particularly with small children about.

The solution
So, it would be nice if we could detect a lost neutral and open the supply in order to protect ourselves, our family and our technology. There are commercial supply monitors that will provide this facility (along with over/under voltage and other detections) but these are expensive and can be difficult to find in sunny Thailand, they also require installing by someone who knows what he’s doing.
All is not lost, whilst researching something else I came across a method whereby a regular RCD (ELCB) can be used as a lost-neutral detector, as an aside it also detects a phase-neutral reversal (another potentially damaging and worryingly common occurrence) . This technique was developed in South Africa where copper theft is endemic and the neutral is usually the bottom of the wires on the pole.
All you need is an additional RCD (4-pole if you have a 3-phase supply) and a second ground stake (you can do away with this if your system is MEN wired).
Examine the diagram below, all you need to do is install the new RCD before your distribution board and add a separate ground stake to your neutral as shown, the new RCD can also act as an incoming supply isolator.


How it works.
Consider normal operation (top diagram), the neutral is at or near ground potential so no current flows down the ground stake and the RCD remains in balance. Open the neutral (second diagram), now there is no current in the neutral leg of the RCD and it opens protecting your supply. Reverse phase and neutral (third diagram) and we have a similar effect, again the RCD is imbalanced and trips.

IMPORTANT NOTES. This RCD is only there to detect an open neutral, it will NOT that’s NOT protect you from shock, you need another conventionally connected RCD in your distribution board to provide shock protection. Also if your supply is IT (neutral is not earthed at all) you gain nothing from adding this system.

 

[Philpot]

Thanks a lot, this is a really interesting post. I remember that " a long time ago" in Molesy High Street there was a lost neutral in an underground cable joint. This didn't cause loss of life but it was very scary and expensive incident. some , but not all, of the shops lost most of their lighting and various other bits of equipment. It was pre-computer systems otherwise it would have been a lot more costly.

 

[StuSpiers]

Of course this is unsuited to uk tncs.

We do not have men links in the fuseboard we have them in the cutout.

EDIT:

should still work though......

Would need the spike.

 

[Philpot]

When I was in the Merchant Navy it was the electricians job to check for earth fault every morning. In those days both the DC and any AC system they may have were non earthed "floating systems". They had two lamps in series between L an N with the centre point earthed. Each lamp had an on/off switch. With no earth faults.....Switching each lamp on in turn they should not light. With both switches on the lights should be half brilliance( in series).
Are you still awake!!!!:confused5:

If the Live light came on there was a fault on the Neutral, and vis-versa. Tracing the faults was a nightmare but unless it was done you would get multiple earth's on both L and N with very strange things happening. Poorly maintained electrical systems were the cause of many fires on board ship.

So it's a lot easier with and earthed Neutral system.

 

[thebigcheese]

How it works.
Consider normal operation (top diagram), the neutral is at or near ground potential so no current flows down the ground stake and the RCD remains in balance.
​

Surely you then have a parallel path for the neutral current to flow down and this would cause the RCD to trip?