Dead short readings on TNS

[Questforknowledge]

Evening all

A few weeks ago we were doing an Eicr and were getting dead shorts on every circuit between neutral and earth. In the end it was believed that the fault might actually stem from the incoming earth. I understand you will get a short if it was tncs but not usually tns. However the matter was further investigated by an electricity provider and they said that although it is tns they often make supplies tncs further down the line and so you will get these readings unless you disconnect the neutral bar etc. My question is for example you tested a commercial premises in the same situation and are unable to isolate an entire board at once, how would you ever get readings between neutral and cpc for any circuit.

(im still training currently and it's just something that's been on my mind)

 

[Ian1981]

As the neutral is referenced to earth at the supply end on a TNS arrangement then yes the same thing will happen in a TNS and a TNCS system if the neutral is not isolated at the consumers part of the installation

 

[Pete999]

As the neutral is referenced to earth at the supply end on a TNS arrangement then yes the same thing will happen in a TNS and a TNCS system if the neutral is not isolated at the consumers part of the installation

Simples

 

[Charlie_]

how would you ever get readings between neutral and cpc for any circuit.

Disconnect the respective neutral and cpc for the circuit you are testing

 

[westward10]

Only the neutral.

 

[Charlie_]

I prefer to disconnect both, in order to verify its coordination with that circuit.

 

[Lucien Nunes]

The take-away fact here is that neutral is only neutral (at least as far as a single-phase installation is concerned) because it is connected to earth at least once. The connection can be only at the substation (TN-S), somewhere else (TN-C-S) or all over the shop as with PME but, by definition, any TN supply should have a low resistance between N & E.

On TN-S it will not be so low as on TN-C-S as the circuit goes via the distribution cable back to the substation but it will be similar to Zs. It's difficult to measure that resistance with normal instruments because the voltage drop in the neutral is impressed upon the test circuit. From the origin the circuit between N & E looks more like a low impedance AC supply of a few volts than a resistance.

On TN-C-S it is indeed a dead short (well, unless you define a 'short' as a circuit completed short of its intended destination, which means an actual fault).

Whilst I note you're a trainee, what puzzles me here is that someone who was doing an EICR was not aware of this. It's pretty fundamental to how electrical supplies work.

It's also illogical to test like that - even if N & E were not linked together deliberately, a moment's thought would reveal that when both connected to the supply they must be connected to everyone else's installation too, so a fault in any one would show up in all the others. This is a key reason for public supplies using TN: A fault on one installation doesn't affect the safety of another.

 

[Ian1981]

I prefer to disconnect both, in order to verify its coordination with that circuit.

That’s fair enough but for the purpose of insulation resistance testing you are required to test between live conductors and live conductors and the cpc with the the cpc always connected to the source of earthing for the installation as required in part 6 of bs7671