Discuss Dead short readings on TNS in the Electrical Forum area at ElectriciansForums.net

Welcome to ElectriciansForums.net - The American Electrical Advice Forum
Head straight to the main forums to chat by click here:   American Electrical Advice Forum

Reaction score
16
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)
 
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
 
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
 
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.
 
Last edited:
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
 

Reply to Dead short readings on TNS in the Electrical Forum area at ElectriciansForums.net

OFFICIAL SPONSORS

Electrical Goods - Electrical Tools - Brand Names Electrician Courses Green Electrical Goods PCB Way Electrical Goods - Electrical Tools - Brand Names Pushfit Wire Connectors Electric Underfloor Heating Electrician Courses
These Official Forum Sponsors May Provide Discounts to Regular Forum Members - If you would like to sponsor us then CLICK HERE and post a thread with who you are, and we'll send you some stats etc

Electrical Forum

Welcome to the Electrical Forum at ElectriciansForums.net. The friendliest electrical forum online. General electrical questions and answers can be found in the electrical forum.
This website was designed, optimised and is hosted by Untold Media. Operating under the name Untold Media since 2001.
Back
Top
AdBlock Detected

We get it, advertisements are annoying!

Sure, ad-blocking software does a great job at blocking ads, but it also blocks useful features of our website. For the best site experience please disable your AdBlocker.

I've Disabled AdBlock