TNS

[mrloy99]

Should I be getting a dead short at DB on ohms scale when connecting leads of ohm meter across neutral bar and earth bar or would that only apply to TNCS??

 

[westward10]

Well a dead short is little or no resistance.

 

[mrloy99]

Well a dead short is little or no resistance.

Yes,and?
What I'm trying to ask is SHOULD I or should I not be getting a dead short on TNS

 

[Wilko]

If you are using a regular multimeter it may not reliably tell the difference between TNCS and TNS values - is there a problem?
Here's a description of the earthing types that may help
Types Of Earthing Systems - Electrical Engineering Centre - https://www.electricneutron.com/earthing-system/types-of-earthing-systems/

 

[westward10]

What reading are you getting and with what are you measuring it with.

 

[mrloy99]

What reading are you getting and with what are you measuring it with.

Meggar 1720 mft...
My "issue" is I'm trying to figure out if I should or should not be getting a dead short.
I have a DB with dual supplies(A+B)

On A supply on ohms scale I am getting a dead short(0.03)ohms
On B supply I am showing open circuit.

They are fed from 2 different substations(both containing H.V transformers)

I'm thinking maybe I shouldn't be getting a short.And I maybe have a neutral earth fault,but due to only having a 3 pole switch I cant isolate the neutral unless I can get access to LV switchboards to remove neutral link on mccb
I was just trying to figure out generally if I should be expecting to see dead short.The cable run is just about 30M.

 

[Wilko]

Ok, with TNS I would not expect to see 0.03 Ohms N-E. What size cables? And when you say 30m run - from where to where please?

 

[PEG]

Hi,bud,Could do with less of the dead short,as myself and six of my pals,are feeling the lack of diversity treatment

 

[mrloy99]

Ok, first question is what is the installation means of earthing?

Thanks for your reply but I thought I made that pretty clear in the title

 

[mrloy99]

Hi,bud,Could do with less of the dead short,as myself and six of my pals,are feeling the lack of diversity treatment

Sorry Peg(or porg in a morgue if you prefer)

 

[Wilko]

Thanks for your reply but I thought I made that pretty clear in the title

Yep good for you. Best wishes.

 

[PEG]

Hi,was there a reported fault,which instigated your testing?

 

[mrloy99]

Ok, with TNS I would not expect to see 0.03 Ohms N-E. What size cables? And when you say 30m run - from where to where please?

From DB to HV/LV substation 70mm cable (my understanding of tns is the neutral is derived from the earthed star point on the secondary windings .I'm wondering has the earth been omitted to the neutral.)

 

[mrloy99]

Hi,was there a reported fault,which instigated your testing?

Theres a fault on a circuit yes.Its a new installation.About to be handed over

 

[PEG]

What is the nature,of the fault?

 

[westward10]

Have you carried out a Ze at the boards.

 

[mrloy99]

Have you carried out a Ze at the boards.

A couple of months ago yes when it was first powered up..As far as I remember it was around 0.07..(quite big job over a hundred DBs so not sure)

 

[mrloy99]

What is the nature,of the fault?

No continuity on an accessory fault traced to defective modular kit.faulty circuit disconnected (I think)unless labled wrongly.not confirmed yet.
Bed time for me.Maybe I'll have another look tpmorrow

 

[Andy-1960]

Are both A & B supplies used at the same time or is it a duty / standby situation?

 

[Wilko]

Assuming 70mm conductor size for both N, at 20C the resistance for 30m is about 0.024Ohms. And a 37m run would be 0.03Ohms. Unless I've made a no-coffee-yet mistake.