External PMR to horticulture, - back-up earth electrode or not?

1Justin · Nov 14, 2013

I have been asked to do some maintenance for a small co-operative horticulture set-up. They operate on a shoe-string, and have a couple of hectares of land in a low-lying flat area.

The DNO has given them a PMR system with a service head and meter in nearby woodland. (Ze 0.23). I find a 30mA RCD main switch and 16A breaker, from where the supply runs in (probably 2.5mm) SWA (which is flopped onto the surface, becoming less visible over time..) to an earth-floored shed some ~ 100M away. At shed there are a couple of sockets for kettle/fridge (Zs 1.75).

In addition to advising them to bury the cable safely, I could:
A) Leave the PMR as it is.
B) Convert to TT (not sensible it seems to me, since they have decent PMR at source, - although a long way off.
C) To add back-up earth electrode(s) at the shed, (@ < 200 Ohm), I guess effectively extending the PMR. Not much fault current would go that way versus 1.75 Ohms, but as a back-up?

Given this operation will continue with or without my intervention, should safety favour A, B or C?

Engineer54 · Nov 14, 2013

Think you mean PME not PMR!! lol!!

You don't say if there is any extraneous metal water pipe, or if the shed type structure is metal or not. If either of these exist then you will need to run a 10mm earth conductor from DNO supply to the shed/building to bond said pipe and/or structure.

If i were to drive in a reinforcing earth rod it would be at point of supply, as a local N-E PME point and it wouldn't be 200 ohm Ra rod either. If 200 ohms is your idea of a TT system, then leave the installations PME earth as is, you'll just be wasting both you're your time and money. A local PME earth rod on a PME system needs to be a max of 10 ohms or less!!

DPG · Nov 14, 2013

Here we go again. I think it's Glenn's turn next

Daz

1Justin · Nov 14, 2013

Hi Engineer 54, Doh! PMR "Polymyalgia rheumatica" I blame QWERTY

There is no extraneous pipe (only plastic).

The shed DOES have some metal frame parts which are extraneous and not bonded.

Yes I wasn't serious about the 200R.

But re. "A local PME earth rod on a PME system needs to be a max of 10 ohms or less!!" That's from where?

If a rod were driven at the point of supply (We have a supply point as reliable as any local domestic PME), then what would be the point? I was considering it at the far end where we have exported PME 100M away, and soil floor might be at a different potential under fault conditions.

1Justin · Nov 14, 2013

I meant to add some more: re "then you will need to run a 10mm earth conductor from DNO supply to the shed/building to bond said pipe and/or structure".

But what about using to using SWA sheath to bond?
1000 A PFC at source
16 A type B breaker 3a4 gives break in <= 30mS
K = 51 (54.4)
Adiabatic gives minimum allowable CSA of an earthing conductor 3.4mm^2

So 544.1.1 would allow CSA for bonding to go down to 6mm^2. The CSA of SWA in 2.5mm SWA is 19mm steel according to charts, equivalent to 6.1 in Cu. So that's tight, but it looks like I could bond the shed extraneous with the SWA sheath.

Have I cocked up or is that plausible?

telectrix · Nov 14, 2013

you can't calculate bonding conductor size from 50% of required main earthing conductor on a PME. it's min 10mm end of.

1Justin · Nov 14, 2013

Hi Telectrix.

Ah, I just saw 5.44.1.1 does read "Except" where PME conditions apply. Yes of course.

Just trying to keep their costs down. So that's a non starter, (or replace the metal bits of the shed with wood..)

Engineer54 · Nov 14, 2013

1Justin said:
Hi Engineer 54, Doh! PMR "Polymyalgia rheumatica" I blame QWERTY

There is no extraneous pipe (only plastic).

The shed DOES have some metal frame parts which are extraneous and not bonded. Then you would need to run a 10mm earthing bonding conductor from supplies MET to the extraneous part. Have you tested the frame to confirm being extraneous or not?? To be honest a 2.5mm SWA cable run over 100 metres doesn't make much sense to me from the outset. So think about replacing the existing cable, ...hell, volt drop must be getting a little bit strained too...lol!!

Yes I wasn't serious about the 200R. Glad to hear it!!

But re. "A local PME earth rod on a PME system needs to be a max of 10 ohms or less!!" That's from where? DNO's max Ra on a PME systems single earth rod. In reality generally much less.

If a rod were driven at the point of supply (We have a supply point as reliable as any local domestic PME), then what would be the point? I was considering it at the far end where we have exported PME 100M away, and soil floor might be at a different potential under fault conditions.

You don't export you extend!! The soil floor will be at a different potential to any provided earthing point at all times, fault conditions or not... Which is why extraneous earths need bonding. The point would be to disapate the effects of an upstream broken neutral. [/QUOTE]

Sorry can't help much further tonight, as i need to get some of my own work done. I'm sure others here will point you in the right direction though....

1Justin · Nov 14, 2013

Hi Engineer 54. Your red text sure stands out eh !

I have not tested whether the the frame is extraneous, and will look at that (~ 7K ohms limit according to NIC, more conventional figure being 23k)
I have made only a quick visit, and the person who installed all of this is "no longer available."

"DNO's max Ra on PME systems single earth rod" - Thanks I hadn't come across that figure.

"You don't export you extend" : No that's the point of all this. I find it HAS BEEN already "extended".

This is handily re-kindling stuff hovering under a few months of memory neglect so to speak.

I would not like to wash my hands of it, but the customer is extremely cost sensitive, and will continue to work regardless.

I will recommend the bonding if necessary, but 16mm (unprotected cable) and getting on for 100M is quite a bit of copper. Or I will recommend TT.

Rgds

jamie-spark · Nov 14, 2013

1Justin said:
I meant to add some more: re "then you will need to run a 10mm earth conductor from DNO supply to the shed/building to bond said pipe and/or structure".

But what about using to using SWA sheath to bond?
1000 A PFC at source
16 A type B breaker 3a4 gives break in <= 30mS
K = 51 (54.4)
Adiabatic gives minimum allowable CSA of an earthing conductor 3.4mm^2

So 544.1.1 would allow CSA for bonding to go down to 6mm^2. The CSA of SWA in 2.5mm SWA is 19mm steel according to charts, equivalent to 6.1 in Cu. So that's tight, but it looks like I could bond the shed extraneous with the SWA sheath.

Have I cocked up or is that plausible?