Earth rod and Zs values in farm. Help needed

[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.