High Ze, should I argue with DNO

[timhoward]

On an EICR I found a 200 amp 3ph supply, TNS earthing (which looks original) and Ze of 19 ohms. No rods. No RCDs.
L1 -> N was about 0.5 ohms. Other phases were similar.
So I reported this Ze to DNO.

According to customers, they came out the next day, confirmed my reading, but said there was nothing wrong with it and no action would be taken.
This is a first for me. I was out of my usual patch and it was UKPN. I've mainly experienced SP who have always taken anything like this very seriously and acted swiftly.

My only argument seems to be that they provided an earth once that must have been deemed acceptable, and therefore this earth should be maintained.
That said, as far as I'm aware DNO's are not obliged to meet any particular value, the Energy Networks Association P23 guidance document seems to just contain typical values that a certain percentage of supplies will meet. It feels as though an upstream cable repair has left the lead sheath floating.

So the question is, do I argue and persist, or get busy designing RCD-based fault protection at the customers expense? (It's a multi board installation with a bus-bar chamber at the origin and it's not going to be trivial.)

 

[OLDBOY]

May be worth asking if the existing head can be converted to TNCS.

 

[timhoward]

May be worth asking if the existing head can be converted to TNCS.

I like the idea of at least getting a quote for PME.
The Line to neutral impedance may be deemed too high.
I’d also have some issues with distribution circuits sizing and bonding as there are some 6 sq mm SWA runs.
But it could be a way forward, thanks.

 

[pc1966]

I like the idea of at least getting a quote for PME.
The Line to neutral impedance may be deemed too high.

If it really is 0.5 ohm (PSCC around 0.5kA) then you are looking at 10% VD for around 46 amps, way short of a usable 200A supply!

That alone, and ESQCR limits, merits another look.

I’d also have some issues with distribution circuits sizing and bonding as there are some 6 sq mm SWA runs.
But it could be a way forward, thanks.

The only other alternative really is to TT it.

I can't see a 19 ohm return path surviving much of a high fault current anyway without something, somewhere, becoming very hot and either clearing some tarnishing on a poor bolted joint, or more likely opening some corroded wire(s) completely.

But to go down the TT route then obviously its a lot more than just a rod & delay 300mA (or whatever) RCD at the supply point, as multiple downstream DB need to be looking at RCD protection as well for selectivity.

 

[pc1966]

Out of interest, could you also do the PSCC from L-L on your MFT to see if the issue is a high impedance neutral (and L are all good), or if something is flawed with all live conductors?

 

[timhoward]

Thanks @pc1966 some good food for thought there.

 

[westward10]

If this is a metal sheath service cable then I would have thought they had a responsibility to ensure fault protection is sufficient.

 

[Froggie]

The information available seems to be inconsistent. If you look on the National Grid supply details they list maximum typical Ze in line with those given in BS7671, but go on to state that these may be exceeded in certain scenarios (such as long overhead supplies).
My experience with SP Energy Networks has been different, however. Where they have provided an earth path previously, they will ensure that this meets those given in BS7671. I have witnessed SPEN restore a domestic TNS connection that was reading 5 Ohms and also dig a 30M trench adjacent to a pole mounted transfomer in order to restore a TNCS connection, both of which were at no cost to the customer. So it seems to be a bit of a lottery postcode.

 

[timhoward]

If you look on the National Grid supply details they list maximum typical Ze in line with those given in BS7671, but go on to state that these may be exceeded in certain scenarios (such as long overhead supplies).

Agreed. This IET article was quite interesting Earth fault loop impedance revision of ENA Engineering Recommendation P23 - http://tinyurl.com/yc5rdhks

My experience with SP Energy Networks has been different

I have had exactly the same experiences many times over as SPEN is my 'home area' DNO. I'm now feeling rather lucky!
This job was a one off in London in cahoots with another sparks there that I know, and UKPN seemed to have a totally different take on things.

It seems a little chicken and egg. The metal service sheath is behaving more like an an extraneous part than an earth. So it must be bonded to an acceptable earth, and there isn't one except the exact same thing! (braid still intact and connected).
There is no possible argument that it wasn't a supplier provided earthing arrangement.

I'm having some strong coffee and will try talking to them again. Hopefully a different engineer will attend!

 

[Vortigern]

Definitely argue the case and speak to supervisor. I have had this with WPD and raised a complaint in the end. This had a great effect they revised their on line commitment to ensuring the earth provided met standards for us. Under section 24/25 ESQCR National Grid as it is now called, are legally obliged to maintain the earth at nominal quoted ohms at a minimum. It may only involve a phosphor wound spring on an earth braid so not a big deal.
I'm (was) a dog with a bone on this one, you should be as well!

 

[Vortigern]

From: Jones, Gwyn T. (Distribution Manager) <[email protected]>
Sent: 20 July 2018 08:46
To: '[email protected]'
Subject:Domestic and Commercial Earthing Practices

Dear Mr Francis – Hammond

Since our conversation about this topic I’m concerned that you may be somewhat anxious that the subject matter has been forgotten, this is not the case.

Indeed, a colleague within our policy team is currently working on an information/fact sheet that will be put onto our web site to obviate further related issues.

The concern you raised has been helpful in focusing light on the subject of both domestic and commercial earthing practices and will hopefully enable us to produce a helpful guide.

In advance of this “official” guide, I’m happy to confirm the following:

· ESQCR reg 24(4) and 24(5) refers to making an earth terminal available for new/replacement LV connection in all cases unless unsafe to do so.

· ESQCR reg 24(1) refers to maintaining equipment i.e. an earth terminal.

So Western Power Distribution should be observing the above two fundamental regulations.

Yours sincerely

Gwyn Jones
Distribution Manager, Bristol


From: [email protected] [mailto:[email protected]]
Sent: 28 June 2018 15:45
To: Taylor, Tony S. (Management Information)
I include this to show that the requirements are real!

 

[brianmoooore]

I can't see a 19 ohm return path surviving much of a high fault current anyway without something, somewhere, becoming very hot and either clearing some tarnishing on a poor bolted joint, or more likely opening some corroded wire(s) completely.

I think I might be tempted to test this theory, if it can be done without causing danger to anyone.