Farm earth? TT OR TNS

edexlab · Mar 4, 2011

Went to a farm this week to look at a possible job
had a good look at the earthing arrangement ,
as I always thought that farms were always TT
( and I'm sure I've read a reg in the BRB stating all farms should be TT )
but this one is unlike any I've noticed before

Has a Met with one earth going to a steel band type clamp on cable sheath with a fine braided steel wire(flat) going to another clamp which has a larger twisted unsheathed wire which follows the main cable into the ground
can't see this wire at transformer (25mtrs away) as all the cabling is capped

So is this likely to be TT and this twisted wire the earth electrode ?
Or could it be a TN-S type arrangement with the twisted wire instead of using cable sheaf
Any help much appreciated

nickblake · Mar 5, 2011

very difficult to tel some times one way of identifying is to do a Ze test max values are :

TNCS = 0.35
TNS = 0.8
TT = 200

Deleted member 26818 · Mar 5, 2011

There's no restrictions on residential buildings being TN-C-S.
It's just areas for livestock where it is not recommended, unless there is a metal grid in place beneath the floor.

Deleted member 9648 · Mar 5, 2011

The only system that is not permitted in agricultural installations is a TNC earthing system...705.411.4.
A TNCS systen is permitted but not recommended in areas used for livestock unless a bonded metal grid is laid in the floor....705.415.2.1.

There are also specific requirements for RCD protection regardless of earthing arrangements....705.411

See Electrics · Jun 12, 2012

I am taking a supply to a farm shed used for sheering / lambing / parking tractor. The supply will be coming from a TN-C-S system via a SWA. I plan on terminating the SWA at the meter point into it's own M/C switch fuse, and feeding this with 25mm tails, no earth. Then putting a rod in at the shed end, and connecting this in at the shed M/C consumer along with the SWA. If I put a warning notice at the source end stating not to be coonected to the MET would this suffice?

spark 68 · Jun 12, 2012

The armoured cables armouring needs to be earthed at the TNC-S supply end, but insulated and isolated at the TT end, and must not be connected to the remote DB's earthing system at all, your earth rod needs to be connected to the earthing system at the remote DB.

Engineer54 · Jun 13, 2012

edexlab said:
Went to a farm this week to look at a possible job
had a good look at the earthing arrangement ,
as I always thought that farms were always TT
( and I'm sure I've read a reg in the BRB stating all farms should be TT )
but this one is unlike any I've noticed before

Has a Met with one earth going to a steel band type clamp on cable sheath with a fine braided steel wire(flat) going to another clamp which has a larger twisted unsheathed wire which follows the main cable into the ground
can't see this wire at transformer (25mtrs away) as all the cabling is capped

So is this likely to be TT and this twisted wire the earth electrode ?
Or could it be a TN-S type arrangement with the twisted wire instead of using cable sheaf
Any help much appreciated

From your description it sounds like a TN-S earthing system with possibly an added Rod thrown in for good measure!! lol!! If there is no connection to the neutral conductor, it can't be a PME/TNC-S earthing system.

Dam, ....just noticed this is yet another old thread that's been resurrected for no good reason!!!!

See Electrics · Jun 13, 2012

spark 68 said:
The armoured cables armouring needs to be earthed at the TNC-S supply end, but insulated and isolated at the TT end, and must not be connected to the remote DB's earthing system at all, your earth rod needs to be connected to the earthing system at the remote DB.

Thanks for that, I thought as much, I'll have to isolate shed end, as it's a M/C consumer, so can change this to a switch fuse first or something. any ways tried putting in a rod at that end today, and doing loop test got 327 ohms. farm is atop 1500 foot hill in sunny scotland. wet but not much soil.