Earth rod and Zs values in farm. Help needed

[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.