Diy TN-C-S

[La Poste]

Greetings all.

I have been working with my electrical Boss this week and he was telling me about this guy he knew who had a habit of just joining the earth and neutral on any system that had no earth, thankfully this was quite a long time ago and the gentleman has since retired.

So for instance if this guy in question went to a property and found no earth and just connected the earth to neutral what would the potential dangers be?

If the supply was TN-S but for some reason there was no earth or if the supply was overhead just line and neutral what could happen if the earth were just simply connected to the neutral at the main intake and then left like this?

I understand a lost supply neutral would be dangerous but is there anything else?

Thanks.

 

[Knobhead]

Jesus, give me time to do some drawings. There’s so many possibilities!

The drawings are so I can get things right in my own head. I find it the easiest way to work things out. I don’t go from regulations but physics.

 

[La Poste]

Thanks Tony.

I'm looking forward to seeing them.

 

[Engineer54]

Greetings all.

I have been working with my electrical Boss this week and he was telling me about this guy he knew who had a habit of just joining the earth and neutral on any system that had no earth, thankfully this was quite a long time ago and the gentleman has since retired.

So for instance if this guy in question went to a property and found no earth and just connected the earth to neutral what would the potential dangers be?

If the supply was TN-S but for some reason there was no earth or if the supply was overhead just line and neutral what could happen if the earth were just simply connected to the neutral at the main intake and then left like this?

I understand a lost supply neutral would be dangerous but is there anything else?

Thanks.

Just think about it for a few minutes, a TN-S neutral is only solidly bonded to earth at the supplying sub-station, whereas a PME neutral is earthed at several locations along the main supply cables length, ideally at each joint in a buried cable and at every 3rd pole on an overhead supply... And don't forget, also at every premises MET via services bonding... People often forget that PME is in essence an ''artificial'' earth!!

So what do you think the problems are, that could arise from using a neutral as your main form of earth in such circumstances??

 

[brightspark2010]

I dont actually know but please enlighten me

 

[Welchyboy]

when he couldnt TT a system my old boss too used to do a loop test between L-N at head, if it was lower than 0.35 he assued it was PME and earthed the neutral

But having said that when i have called EDF out before to ask if a house could be PME'd the lady said the engineer would have to perform a loop test to know if it was available, i thought they would have records? surely if the house was relatively close to the local transformer the reading could easily be that low without being PME??

 

[La Poste]

So what do you think the problems are, that could arise from using a neutral as your main form of earth in such circumstances??

Well excuse my limited knowledge but two things spring to mind so I will have a punt.

Firstly a PME system is closely monitored. The neutral is monitored for the rare event of it breaking and if the neutral does break the relays should switch the system off.
I doubt a TN-S system would be so closely monitored and hence a broken neutral would not be picked up so quickly and could present a very real danger if all the metal work at the consumer end were bonded to it.

The other and this is a long shot is volt drop in the neutral.
If the neutral is only grounded at the supply transformer there could be a substantial volt drop in the neutral from the consumers premises all the way back to the star point hence the neutral could be above zero volts at the consumers end.
If you then connect all your metal work to this neutral that is floating above zero volts this could present a danger.
The more current that flows down the neutral the more volt drop and so if the neutral in heavily loaded the voltage at the far end IE the consumer end of this neutral could be quite substantial and so would the voltage appearing on the metal work that has been bonded to it.

Engineer I am not really sure what you mean by this, could you explain more?

People often forget that PME is in essence an ''artificial'' earth!!

I am genuinely interested.

Thanks.

 

[trev]

when he couldnt TT a system

If an earth is not present, under what circumstances could you not drive rods to provide a TT?

 

[Knobhead]

Firstly a PME system is closely monitored. The neutral is monitored for the rare event of it breaking and if the neutral does break the relays should switch the system off.

What relays? The DNO’s don’t fit them!

 

[La Poste]

What relays? The DNO’s don’t fit them!

I really have no idea about safety systems on the network, that was a wild guess.

There must be some sort of monitoring device on a TN-C-S system that will switch the power off in the event of a neutral fault I'm just not sure what it is called.

So anyway, are my two theories correct?

 

[Welchyboy]

If an earth is not present, under what circumstances could you not drive rods to provide a TT?

Normally a screeded floor dwelling with paving/driveways and /or adjoining houses preventing you from providing a possible siting for the rod

 

[trev]

Normally a screeded floor dwelling with paving/driveways and /or adjoining houses preventing you from providing a possible siting for the rod

Sorry mate but if an earth was not present there is nothing in the world that would stop me driving rods. The scenario you suggest is fairly simple to get round, take an appropriately sized cable, route it through the ceiling void to a conduit box. Down the wall in pipe, lift paving etc install a housing, sink rods and connect cable, replace anything you've lifted, jobs a goodun

 

[Welchyboy]

Well I was really referring to the attitude of my last guvnor, rather than goin through all that he would just pme the supply himself

 

[Knobhead]

I really have no idea about safety systems on the network, that was a wild guess.

There must be some sort of monitoring device on a TN-C-S system that will switch the power off in the event of a neutral fault I'm just not sure what it is called.

So anyway, are my two theories correct?

Even on industrial systems you will only find O/L and REF (Restricted Earth Fault) on the L/V side. You won’t find earth/neutral monitoring.

On DNO transformers the monitoring is about as subtle as a brick. O/L (fuses) at the M/V side only.

An industrial set up
View attachment 11070
DNO set up
View attachment 11071

 

[UKMeterman]

Tony,
Your LV neutral, is it supposed to be connected to one of the phases or should it be a delta star transformer?