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??

 

[HappyHippyDad]

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?

Could I ask anyone (perhaps you E54) why someone would do this? If it is ok to extend a PME then why has someone got a TNCS but has not connected the 3rd core of the SWA at load end to utilise it (and get a much lower Zs), rather they have a TT with a higher fault loop impedance?

 

[HandySparks]

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??

OK:

705 Agricultural and Horticutural Premises
705.411 Protective measure: ADS
705.411.1 General

In circuits, whatever the type of earthing system, the following disconnection devices shall be provided:

(i) In final circuits supplying socket-outlets with a rated current not exceeding 32A, an RCD having the characteristics specified in Regulation 415.1.1 [30mA].

(ii) In final circuits supplying socket-outlets with a rated current more than 32A, an RCD with a rated residual operating current not exceeding 100mA.

(ii) In all other circuits, RCDs with a rated residual operating current not exceeding 300mA.

 

[HandySparks]

Could I ask anyone (perhaps you E54) why someone would do this? If it is ok to extend a PME then why has someone got a TNCS but has not connected the 3rd core of the SWA at load end to utilise it (and get a much lower Zs), rather they have a TT with a higher fault loop impedance?

The short answer, is that it goes against the regs:

Agricultural and Horticultural Premises
705.411.4
A TN-C system shall not be used....


I'm sure others can go into the reasons regarding voltage on the neutral under fault conditions, reduced resistance to current in persons and animals in contact with the ground, susceptibility of livestock to electric current, etc.

 

[JD6400]

Hi Happyhippydad , just take a look at section 705 in the regs and it will become a lot clearer .
If you want , you can PM me and I will give you a few pointers out of sight of the preying public !:49:

 

[HappyHippyDad]

Hi Happyhippydad , just take a look at section 705 in the regs and it will become a lot clearer .
If you want , you can PM me and I will give you a few pointers out of sight of the preying public !:49:

Thats very kind of you JD but it really was just asked out of interest. It would be out of my comfort zone and not something I would undertake. I will have a read up on section 705 though just to get the answer in more detail :smile5:

 

[Marvo]

Cattle and other 4 legged livestock are highly susceptible to step voltages which can especially develop in the vicinity of an earth rod. Because of this cattle shed are treated as a special location in the regs.

 

[Knobhead]

Cattle and other 4 legged livestock are highly susceptible to step voltages which can especially develop in the vicinity of an earth rod. Because of this cattle shed are treated as a special location in the regs.

Great when the freezer needs stocking up. So long as the animal is bled straight away the meat is fine.

I’ve attended a few incidents where MV lines have come down and cattle and sheep have been downed.

 

[Engineer54]

OK:

705 Agricultural and Horticutural Premises
705.411 Protective measure: ADS
705.411.1 General

In circuits, whatever the type of earthing system, the following disconnection devices shall be provided:

(i) In final circuits supplying socket-outlets with a rated current not exceeding 32A, an RCD having the characteristics specified in Regulation 415.1.1 [30mA].

(ii) In final circuits supplying socket-outlets with a rated current more than 32A, an RCD with a rated residual operating current not exceeding 100mA.

(ii) In all other circuits, RCDs with a rated residual operating current not exceeding 300mA.

Well anyone that is happy to protect persons against earth faults with 100/300mA RCD's on ''final circuits'' of a TT earthing system is a far more confident man than me!! lol!! Fine using these sort of RCD tripping values on distributing sub-main branch cables.... But NOT on final branch circuits...

I think i'll stick with my way of doing things and disregard that pile of crap to file 13 with the rest of the crap i've found in BS7671!! lol!!

 

[JD6400]

Well anyone that is happy to protect persons against earth faults with 100/300mA RCD's on ''final circuits'' of a TT earthing system is a far more confident man than me!! lol!! Fine using these sort of RCD tripping values on distributing sub-main branch cables.... But NOT on final branch circuits...

I think i'll stick with my way of doing things and disregard that pile of crap to file 13 with the rest of the crap i've found in BS7671!! lol!!

I would not have put it quite as strongly as that , but I have to admit that I always prefer to see a 30mA on final branch circuits .
I would go as far as to admit that we sometimes trade off segregation of supply over having just one 30mA main switch on sub boards , depending on the situation and make of equipment used .


Edit ; It was more I thought that I had been missing something , when you mentioned the need for the 30mA , than anything else .

 

[Engineer54]

I would not have put it quite as strongly as that , but I have to admit that I always prefer to see a 30mA on final branch circuits .
I would go as far as to admit that we sometimes trade off segregation of supply over having just one 30mA main switch on sub boards , depending on the situation and make of equipment used .


Edit ; It was more I thought that I had been missing something , when you mentioned the need for the 30mA , than anything else .

Crap is crap whatever way you want to look at it, ...or come to that, smell it!! lol!!

 

[Deleted member 26818]

The main problem here, is that there are always stray neutral (and to some extent earth-fault) currents present in the ground. The objective is to reduce the risk that these currents pose to livestock such as horses and cattle (also to a lesser extent sheep and goats).
Use of an earth rod where such livestock are present is not advisable. The preferred method would be to install an earthed metallic grid. This would reduce the likely hood of there being 'step voltages' present.

Not happy that the SWA and CPC are earthed at the supply end.
To my mind, this means the TNC-S earth has been exported along the length the cable. Depending upon how and where the cable is run, there is a possibility that difference in potential could be introduced between the TNC-S earth and the earthing for whatever the cable supplies.

A 100mA or 300mA upfront RCD, is quite acceptable, though as has been already stated circuits that supply socket-outlets would require 30mA RCD protection.
It may also be advisable to provide 30mA RCD protection for other circuits.

 

[davesparks]

The armour of the SWA has to be connected to the earthing system appropriate to the OCPD of the circuit, that is to say it must be connected to the earthing system at its origin, not its destination.

 

[Deleted member 26818]

Perhaps I should have phrased that differently.
I'm not happy that the CPC and SWA are connected to the TNC-S earth.
Doing such is effectively exporting the TNC-S earth into the area where it should not be.
An earth rod at the origin of the circuit would appear to be the solution.
However this would be an additional point which could introduce a difference in the earth potential.

I'm also wondering why there is a core being described as a CPC.
What circuit is it protecting, and how is it providing said protection?

 

[davesparks]

It isn't exporting it anywhere, that would require sending it to another country!

There is nothing wrong with using the TNCS earth for the submain, the armour should be isolated from the sub-DB it is feeding and insulated to prevent any contact.

The use of a stuffing gland and a bit of heatshrink over the cut end of the armour is fine.

 

[Deleted member 26818]

I suggest you consider the purpose of the SWA.
Then consider where the earth would be if at any time the SWA fulfilled it's purpose.

Still want to know what this CPC is doing?

 

[davesparks]

The purpose of the SWA is to ensure that any metal object penetrating the vale becomes reliably connected to the earthing of the supply to ensure that the protective device operates when it subsequently hits the live core.

This is why it is required to be connected to the earthing system associated with the installation that the OCPD is part of.

 

[davesparks]

And the extra core is completely pointless, and a waste of copper. But it is generally recommended that unused cores of cables should either be insulated to prevent contact with live parts or else connected to earth.

We can only assume that the installer chose to earth the spare core at source rather than insulate it.

 

[Deleted member 26818]

Not a bad effort.
The purpose of the SWA is to provide an earth-fault path if the cable is penetrated by a metallic object.
Yes I'm glad that you agree, the SWA and 'CPC' should be connected to the earthing system for the installation that the OCPD is part of.
ie. it should not be connected to the supply TNC-S earthing but rather the TT earthing used for the installation.

 

[Deleted member 26818]

The extra core may be pointless then again it may originally have been provided to reduce the resistance of the earth-fault path due to the earthing point being at the other end of the cable from where the OCPD is positioned.

 

[davesparks]

Not a bad effort.
The purpose of the SWA is to provide an earth-fault path if the cable is penetrated by a metallic object.
Yes I'm glad that you agree, the SWA and 'CPC' should be connected to the earthing system for the installation that the OCPD is part of.
ie. it should not be connected to the supply TNC-S earthing but rather the TT earthing used for the installation.

If the TNCS earth is being used for the installation at the origin then it should be used for the SWA. as previously stated the SWA should be insulated at the load end of the cable and so prevented from introducing the TNCS earth potential in to the new earthing systems equipotential zone.

 

[Deleted member 26818]

Sorry but where is the requirement that the SWA be insulated at the load end to be found?
Could you also explain how insulating at the load end would prevent the TNC-S earthing being exported to any point where the cable could be penetrated by a metallic object?

 

[ImpededLoop]

It's very thought provoking and good to think outside of the norm.

I think that 542.1.3.3 covers the earthing arrangement for the swa with TT at the load end.

Interesting debate.

 

[davesparks]

Sorry but where is the requirement that the SWA be insulated at the load end to be found?
Could you also explain how insulating at the load end would prevent the TNC-S earthing being exported to any point where the cable could be penetrated by a metallic object?

It'll be in guidance note 8 if it isn't in the BGB.

It won't be exported whatever you do to it, it is impossible to export an earthing system, unless you've found a way of shipping it to another country?
It can either be extended in to another part if the installation, or it could introduce an unwanted earth potential into part of the installation.

As we have already discussed if the SWA is penetrated by a metal object then the OCPD operates and cuts of the supply. This is a fault condition and not normal operating conditions for the installation.

What danger are you seeing from this metal object penetrating the cable?

 

[Deleted member 26818]

I think it is rather pointless continuing this discussion.
You are detailing the exact reasons why such a cable should not be connected to the TNC-S earthing, but seem unable to understand why those reasons make the practice unsafe.

I also find this practice of referring to non existing Regulations rather tedious.

 

[ImpededLoop]

I think it is rather pointless continuing this discussion.
You are detailing the exact reasons why such a cable should not be connected to the TNC-S earthing, but seem unable to understand why those reasons make the practice unsafe.

I also find this practice of referring to non existing Regulations rather tedious.

Hi Spin,

Just to help my understanding, why doesn't regulation 542.1.3.3 apply here?

I can understand your reasoning for thinking that the swa should be connected to the TT earthing system, but the above regulation clearly states that the swa is connected at the supply end of the cable.