Earth rod and Zs values in farm. Help needed

[t01474]

Evening all.

Am currently doing my first EICR on a farm and have come across the following.

Supply is TNC-S.
There are many stables on the farm and various outbuildings.
At some of the stables / outbuildings a 3-core SWA has been installed but the CPC is not connected. Instead a separate CPC has been installed via an earth rod.
An example of the Zs reading is as follows -
Zs - 26ohms
PFC - 9.0A

But Zs between Neutral and Phase -
Zs - 0.08
PSCC - 2.9kA.

RCD's are installed upstream,ie, at the 'supply' side of the circuit in question.

So,

Am I right in thinking that an acceptable value of Phase -Earth is 200ohms as there is an RCD to protect against Earth fault, and that as long as the Zs for a Neutral - Phase fault (short circuit) complies with BS7671 then the results are fine or have I completely misinterpreted the idea?

Also, how close to a stable block / livestock transit path should an Earth rod be installed?

Any help /advise will be greatly appreciated. Sometimes I wish I was a painter. If the hardest decision I had to make each day was how to open the tin i'd be a happier man!!

 

[Marvo]

Did they gland and earth the armour of the SWA cable at the supply side?

 

[netblindpaul]

I'm going to break the form rules now, again.
As far as I was aware, it is a requirement for personnel undertaking EICR's to be fully familiar with the installation methods in use, so that they can make competent and professional engineering judgements as to their suitability, along with the regulations in force when the installation was originally deaigned and commissioned.
Thus they require experience and competence above that of a normal installer.
I don't feel that this is so here.
OP, I trust that you are insured for this work, and that your insurance company is totally aware of your levels of qualifications and experience for them to cover you for this work?

 

[Clive P]

Hi,

As NBP says you need to look at what you are saying. Re-read your post and then resubmit it without the glaring mistakes. I won't give you any hints, just look at it again.

Regards.

 

[t01474]

Glanded and terminated so the armoring and core are earthed.

I am both competent and fully insured to carry out an EICR. Also a member of a governing body.

I have used this forum before as a source of advice and guidance and was under the impression that it is a good way to ask questions for which we are not sure of answers. I feel that you are being unduly harsh on a person trying to do their best to comply with all legislation both statutory and non-stat and just tryuing to better themselves and improve their lot.

In my opinion the previous results are fine as the upstream system is classed as TT and therefore providing the test results are compliant with the regs then all is fine, and that the suppliers earthing facility is not being 'exported'. The post was merely asking for other peoples opinion, not a how to. I was asking what the thoughts of the community are regarding the safe / practical distance to livestock that an earth rod should be placed. I think that >5m is fine but im not an expert in the field like you.

The sort of reply that you posted will simply make people stand outside the tent and p##s in, rather than be inside the metaphorical tent and p##s out. What would you rather?

 

[netblindpaul]

Unbelieveable!
If it were not true.
People I speak to ask me why I am moving away from the trade.
To be competent to undertake the EICR you MUST be an expert in the installation you are inspecting and testing, that is the whole idea of the EICR.

 

[HappyHippyDad]

1074.. I think you may be in for a rough ride here so hold on tight! :smile5:

I'm purely interested in a couple of points you've made and this is not something I would take on:

1. Why is the TNCS not utilised?
2. 200ohms is what the regulations state as 'stable' for an earth rod (but most here will be looking to get a much lower figure) but if this is TNCS then???
3. Not sure what you mean by Zs between neutral and phase? Zs is earth fault?

 

[ImpededLoop]

1. Why is the TNCS not utilised?

Special Location.

2. 200ohms is what the regulations state as 'stable' for an earth rod (but most here will be looking to get a much lower figure) but if this is TNCS then???

Loads about TT and earth rod stability on this forum. Use Google to search for it

3. Not sure what you mean by Zs between neutral and phase? Zs is earth fault?

This is probably why the OP got adverse comments earlier. Why would you measure the phase-neutral loop impedance?

 

[Engineer54]

Glanded and terminated so the armoring and core are earthed.

So how has the PME/TNC-S been isolated from the secondary TT system(s) ??

I am both competent and fully insured to carry out an EICR. Also a member of a governing body.
What governing body?? I hope you are not referring to these Part P providers, because they are NOT, none of them govern ''Anything'' , though one or two like to think/dream that they are and do!! lol!!

I have used this forum before as a source of advice and guidance and was under the impression that it is a good way to ask questions for which we are not sure of answers. I feel that you are being unduly harsh on a person trying to do their best to comply with all legislation both statutory and non-stat and just tryuing to better themselves and improve their lot.

In my opinion the previous results are fine as the upstream system is classed as TT and therefore providing the test results are compliant with the regs then all is fine, and that the suppliers earthing facility is not being 'exported'.
Well first things first, ..no such thing as exporting a PME earth, you are actually just extending a PME equipotential zone, and there is nothing wrong in doing that in the vast majority of cases.
The upstream system is PME/TNC-S, it's parts of the downstream system that on a TT system


The post was merely asking for other peoples opinion, not a how to. I was asking what the thoughts of the community are regarding the safe / practical distance to livestock that an earth rod should be placed. I think that >5m is fine but im not an expert in the field like you.
Why do you think you need 5m distance from a building housing livestock, to place/position an earth electrode for a TT system??

Oh BTW, ...PME can be used within a livestock barn/building if the concrete floor incorporates an earthed metal rebar grid that's connected to the MET/EMT of that buildings supply....

The sort of reply that you posted will simply make people stand outside the tent and p##s in, rather than be inside the metaphorical tent and p##s out. What would you rather?

Please don't run away with the idea that the 200 ohm Ra mentioned in BS7671 should be read as the maximum Ra value for a TT system, it is nothing of the sort, it's a remark on the stability of an electrode. The only way the vast majority of UK TT installations comply with BS7671 is via 30mA RCD protection, unfortunately RCD devices are not the most reliable of protective devices out there. Ra values should be as low as the electrician can possibly achieve. To be honest on a farm installation you really shouldn't have too much trouble at all, achieving near TN values. If and when each farm building supply has it's own earth rod(s) (as it certainly should do). Connecting all those earth rods/electrodes together with a suitably sized conductor, will drop Ra values like a stone.... Provided of course that Real earth rods are used and NOT those imitation short thin 3/8'' twigs that some fools here have the cheek to call ''earth rods''!! ...lol!!

As you are only conducting an EICR, and not actually installing anything, you need to follow the special requirements of BS7671 relating to farm installations etc...

 

[t01474]

Fair enough. Used the wrong terminology. Obviously not a Zs value between L and N but a line impedance value which is used to derive the PSSC.

On a three core SWA the CPC core and the armouring are connected at the 'supply' side of the circuit but not at the 'load' end. Therefore they are deemed 'earthed'. At the 'load' end there is a 16mm2 CPC to an earth rod. Most Zs results at the CU at the 'load' end of the circuit have been around 12 - 14ohms with a PFC of about 14A.
Although under earth fault conditions the 14A would be enough to operate the 100mA RCD, it would not operate the disconnection device at the 'supply' end, ie, 40A MCB in time / at all.
However, the measured line impedance of say 0.39ohms with a PSSC of 0.594kA would.
The question I was trying to ask in a very roundabout way is that although I can see in theory that this would all stand up to scrutiny (I thought), is it correct in your opinions?

 

[Engineer54]

Fair enough. Used the wrong terminology. Obviously not a Zs value between L and N but a line impedance value which is used to derive the PSSC.

On a three core SWA the CPC core and the armouring are connected at the 'supply' side of the circuit but not at the 'load' end. Therefore they are deemed 'earthed'. At the 'load' end there is a 16mm2 CPC to an earth rod. Most Zs results at the CU at the 'load' end of the circuit have been around 12 - 14ohms with a PFC of about 14A.
Although under earth fault conditions the 14A would be enough to operate the 100mA RCD, it would not operate the disconnection device at the 'supply' end, ie, 40A MCB in time / at all.
However, the measured line impedance of say 0.39ohms with a PSSC of 0.594kA would.
The question I was trying to ask in a very roundabout way is that although I can see in theory that this would all stand up to scrutiny (I thought), is it correct in your opinions?

The RCD should be rated at 30mA not 100mA, As an ''added'' safety measure it is prudent to incorporate a 100mA/300mA S type front end RCD device, ... as i stated before, typical 30mA RCD devices especially those protecting several final circuits are not the most reliable of protective devices!!

 

[Fox]

Ha, I was waiting for E54 to come in on this thread! Luckily OP, it seems he got up on the correct side of bed this morning!

 

[JD6400]

The RCD should be rated at 30mA not 100mA, As an ''added'' safety measure it is prudent to incorporate a 100mA/300mA S type front end RCD device, ... as i stated before, typical 30mA RCD devices especially those protecting several final circuits are not the most reliable of protective devices!!

Why would it need to be 30mA and not 100mA ?

 

[Engineer54]

Why would it need to be 30mA and not 100mA ?

Do i REALLY need to answer that question JD??

 

[JD6400]

I am rather hesitant to say yes !?
I am assuming that the OP was talking about a 40 A circuit and not one supplying a socket outlet of 32 A or less .

 

[Engineer54]

I am rather hesitant to say yes !?
I am assuming that the OP was talking about a 40 A circuit and not one supplying a socket outlet of 32 A or less .

JD, were talking about providing protection on a farm TT system, so final circuit protection should be 30mA or less.

 

[HandySparks]

JD, were talking about providing protection on a farm TT system, so final circuit protection should be 30mA or less.

It was a reasonable question. Take a look at 705.411.1

 

[JD6400]

705.411.1 ?

 

[JD6400]

It was a reasonable question. Take a look at 705.411.1

Thanks Handysparks , I am the worlds slowest typer !

 

[Engineer54]

It was a reasonable question. Take a look at 705.411.1

lol!! I would if i could, but alas it's after 8 pm here i'm at home and the BGB is at the office. So how about enlightening me as to what 705.411.1 is saying??