Overhead supply earthing system - help!

[soamesi]

Hi everyone. I am seeking some assistance/advice.

I have almost finished a job on an overhead supply system which has an earth rod. Resistance measures at 62 ohms on the rod so well within guidance. However, there is also an earth from the cut out making it look like a TNCS (PME) system rather than TT. However the Ze measures at 0.39 ohms with the distributor's earth intact which is outside of guidance of 0.35.

My question is two fold:
1) I suspect being an overhead line that the distributors earth cannot always be relied upon and therefore a spike has to be included. In addition, because Ze on distributors' earth is reading just outside of guideline perhaps that was why there is also a spike. Any thoughts? Has anyone come across this before?

2) How should I record this one. Do I annotate as a TNCS but include a note in the comments that a spike is also present and the resistance reading on that? Or do I record as a TT but annotate the other way around?

Any assistance very gratefully received.

Iain :thinking:
Forum virgin!

 

[Richard Burns]

If it is an authorised connection to the cut out (not a DIY addition) then the earthing system sounds like it is TNCS (with an additional rod).
Theoretically as it is outside of the guideline level then you could call the DNO for that area and ask for them to look at it, it may be a loose connection somewhere as it is close to the limit, however they may not be very interested.
Do make sure that you are measuring as close to the incomer as possible to eliminate any loose connections on your side.
But if you do not have any problems with your Zs readings and there are no signs of problems with voltage anywhere and the fault currents are similar then I would say it is OK.
Record as TNCS and note there is an additional rod, there is no particular reason to suspect instability of earthing from an overhead line.

 

[davesparks]

Is the overhead ABC or 4 seperate cables?

The best thing to do is ask the DNO to confirm whether the TNCS connection is kosher or not.
I would suspect that if it is then they will be happy with the 0.39 ohms figure.

 

[soamesi]

Is the overhead ABC or 4 seperate cables?

The best thing to do is ask the DNO to confirm whether the TNCS connection is kosher or not.
I would suspect that if it is then they will be happy with the 0.39 ohms figure.

Thanks for the help here. Not sure what ABC stands for but suspect that is what it is. There is just a single cable coming in (albeit, split at the eves of the property to supply next door too).

TNCS looks kosher but I'll call the DNO and make sure they are happy then proceed as Richard says in his response.

Thanks everyone. Really helpful.

Iain

 

[soamesi]

Thanks Richard. Really helpful.

All looks kosher so I will call the DNO.

Iain

 

[Guitarist]

It's quite common now for many older overhead TT supplies to be converted to PME, and the old rod just gets left in. Nothing wrong with that.
When you measured your Ze, did you do it as a 2-wire high current test or a 3-wire low current? I only ask, as the readings on low current are not as accurate generally.

 

[soamesi]

It's quite common now for many older overhead TT supplies to be converted to PME, and the old rod just gets left in. Nothing wrong with that.
When you measured your Ze, did you do it as a 2-wire high current test or a 3-wire low current? I only ask, as the readings on low current are not as accurate generally.

Thanks Guitarist. Two wire Loop Hi test. It's what I know.

 

[aesmith]

Is your installation overhead 240V right to the premises, or overhead 11kV to a transformer, with an underground service cable to the customer? If the latter, could it not be TNS, as in fact our supply is?

 

[davesparks]

Thanks for the help here. Not sure what ABC stands for but suspect that is what it is. There is just a single cable coming in (albeit, split at the eves of the property to supply next door too).

TNCS looks kosher but I'll call the DNO and make sure they are happy then proceed as Richard says in his response.

Thanks everyone. Really helpful.

Iain

ABC is aerial bundled cable, it looks like four black singles twisted together strung from pole to pole with the drops to each house being normal CNE cable.

I don't know if this applies everywhere but in my area every pole with a PME electrode has a white label with PME on it in red letters so working out if a PME connection is available is just a case of taking a walk along the street.

 

[mechelec]

0.35ohm (now 0.38ohm) is not the max resistance for a PME connection. It depends on the cutout fuse size. The figure is simply to achieve a 5 second disconnection time in the event of a fault.

 

[mhar]

DNO won't be interested unless it is either not kosher or if you can show it has degraded.
In my area the instructions given to linesmen for a new pme connection are if below .35 to connect, between .35 and .8 connect and advise.
Your .39 reading could be anywhere between .32 and .46 (+/- 5% plus 5 digits mft tolerance)