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.

 

[Richard Burns]

For the Neutral Earth loop on the installation you describe the 0.03Ω sounds about right (as Wilko has just said!)
So the A installation sounds correct but the B does not.

 

[Wilko]

Great diag RB, and kind as always.
I'd made a slip but by luck ended up still in the ballpark

 

[cprfenom]

Can i ask how you calculated the 0.024? At risk of looking dumb

 

[Wilko]

Hi - RB diagram in post #21 - 30m of the 2 cables added up

 

[cprfenom]

I knew it would be obvious when pointed out.

Thank you

 

[mrloy99]

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

One or the other...manual changeover.

For the Neutral Earth loop on the installation you describe the 0.03Ω sounds about right (as Wilko has just said!)
So the A installation sounds correct but the B does not.

View attachment 39394

Thank you...

 

[Richard Burns]

One or the other...manual changeover.

Thank you...

Therefore the neutral of the non operational supply (B) is probably not connected to your live supply as this would mix the two systems of the supply transformers.

 

[mrloy99]

Therefore the neutral of the non operational supply (B) is probably not connected to your live supply as this would mix the two systems of the supply transformers.

Both supplies are operational..voltage to earth is 240 on both systems..There is some info about earthing neutrals through impedences(fig 44.1)and section 442 .not too clear to me..
Incidently when I measure on megohm scale I am showing 0.00

 

[PEG]

Hi,what is the set-up,regarding the two supply transformers,and what is feeding them?

 

[mrloy99]

Hi,what is the set-up,regarding the two supply transformers,and what is feeding them?

Hi.I cant really comment.I will need to investigate further.The complex has 5 sets of HV/LV Transformer rooms(Subs) (Times 2 :A+B) I believe there are two independant 11,000volt ring mains.They wont let us doe_heads in to the substations.
So I could still be looking at either a neutral earth fault or a missing earth link on neutral...I've been trying to read up but it appears the HV earth &LV earthed neutral can either be separate or common or through an impedence..

 

[PEG]

Hi,it may be beneficial,to find the spec on the supply characteristics,which should be available to yourself.

Maybe not in this case,but some installations can have dual intakes,TN-S and PME,and there are very specific requirements,regarding separation distances,bonding,removable links,etc.

 

[mrloy99]

Hi,it may be beneficial,to find the spec on the supply characteristics,which should be available to yourself.

Maybe not in this case,but some installations can have dual intakes,TN-S and PME,and there are very specific requirements,regarding separation distances,bonding,removable links,etc.

Ok thanks..Substations arn't really my game.,but I'll try and get more info