.54 ohm tncs impedemce

[David Thomas]

So, it has been flagged that I have a external earth loop impedance of .54 with TNCS earthing and a 100 amp fuse. Advice is to speak to DNO. UK power networks publish that the max is .35.

What are the chances that if I call UK Power Networks to tell them about this they will just bung a 60 amp fuse in there and call it a job well done (as opposed to actually some how lowering the impedence)? That would be sub-optimal, especially given I have RCDs an induction hob and will one day get an electric car and possibly electric boiler!

 

[buzzlightyear]

o, it has been flagged that I have a external earth loop impedance of .54 with TNCS earthing and a 100 amp fuse. Advice is to speak to DNO. UK power networks publish that the max is .35

who tested it your self .

 

[Rockingit]

That shouldn't happen as the overall demand / CPD of the installation isn't changing and won't make a bit of difference to youe ELI anyway.

It's probably nothing worse than a loose connection - either way, it's their fault to resolve and is potentially dangerous left as it is.

 

[David Thomas]

who tested it your self .

It’s written on the cert for the consumer unit I got installed a couple of years ago. Electrician flagged it when I got a quote to get an SPD retrofitted. I’m just a householder.

 

[David Thomas]

That shouldn't happen as the overall demand / CPD of the installation isn't changing and won't make a bit of difference to youe ELI anyway.

It's probably nothing worse than a loose connection - either way, it's their fault to resolve and is potentially dangerous left as it is.

Thanks - my understanding was that the high impedence coud prevent enough current flowing in an LE fault to cause the 100a fuse to blow in a reasonable period of time. My worry is just that the most cost effective option from their perspective is to say ‘ok- we will derate your fuse’…

 

[timhoward]

It’s written on the cert for the consumer unit I got installed a couple of years ago. Electrician flagged it when I got a quote to get an SPD retrofitted. I’m just a householder.

Was it re-tested by the electrician?

 

[David Thomas]

Was it re-tested by the electrician?

No- he just looked at the paperwork: he didn’t want to touch it unless I got written confirmation from UKPN that .54 is fine or that they have lowered the impendence.

 

[buzzlightyear]

No- he just looked at the paperwork: he didn’t want to touch it unless I got written confirmation from UKPN that .54 is fine or that they have lowered the impendence

then the DNO should be informed of the high reading.

 

[David Thomas]

then the DNO should be informed of the high reading.

Correct - what I’m wondering is what action they are likely to take.

 

[buzzlightyear]

Correct - what I’m wondering is what action they are likely to take

yes ring them up they will send a engineer to test it and make assumption. i had a job where the value exceed the cut out .

 

[Baddegg]

Correct - what I’m wondering is what action they are likely to take.

They’ll repair it, as mentioned above putting a smaller fuse in won’t reduce the Ze reading…

 

[David Thomas]

They’ll repair it, as mentioned above putting a smaller fuse in won’t reduce the Ze reading…

Thanks - my worry was that they had a higher acceptable impedence where there is a smaller fuse (which would be consistent with BS7671 (even though that technically doesn’t apply to them). Repair would be great, except if it involves pulling up my floorboards and digging up my drive to run a new supply cable!

 

[Baddegg]

Thanks - my worry was that they had a higher acceptable impedence where there is a smaller fuse (which would be consistent with BS7671 (even though that technically doesn’t apply to them). Repair would be great, except if it involves pulling up my floorboards and digging up my drive to run a new supply cable!

Sounds like they problem it’s they equipment

 

[buzzlightyear]

Thanks - my worry was that they had a higher acceptable impedence where there is a smaller fuse (which would be consistent with BS7671 (even though that technically doesn’t apply to them). Repair would be great, except if it involves pulling up my floorboards and digging up my drive to run a new supply cable

its either sorting it out or a fuse down road jobby .

 

[timhoward]

Personally I'd have tested "my end" of it, and also compared it to the L-N reading to give some sense of whether the issue is in the cut-out or not.
There is also an outside chance someone ticked the wrong earthing type box two years ago. I presume you / the sparks is sure it's TNCS.

Legally speaking if UKPN supply an earth it has so be "suitable" and they have to maintain it. From what I understand there is no 0.35 hard limit for the DNO to comply with according to their own policies.
So they may
a) disagree with the reading
b) declare "it's fine"
c) find a loose connection in the cut-out and fix it
d) find an issue further away and fix it
e) fit an up front RCD (it's been known)
f) try and make it your problem and run away

Let us know the outcome, it will be interesting.

 

[David Thomas]

Personally I'd have tested "my end" of it, and also compared it to the L-N reading to give some sense of whether the issue is in the cut-out or not.
There is also an outside chance someone ticked the wrong earthing type box two years ago. I presume you / the sparks is sure it's TNCS.

Legally speaking if UKPN supply an earth it has so be "suitable" and they have to maintain it. From what I understand there is no 0.35 hard limit for the DNO to comply with according to their own policies.
So they may
a) disagree with the reading
b) declare "it's fine"
c) find a loose connection in the cut-out and fix it
d) find an issue further away and fix it
e) fit an up front RCD (it's been known)
f) try and make it your problem and run away

Let us know the outcome, it will be interesting.

Thanks - that’s helpful. Will update after it’s been sorted.