Installing a TT system and marconite

Graham Hyde · Mar 31, 2017

I will be helping on a install next week where we will be installing a TT system for a summer house in a garden, the customer has gone and bought a hot tub thing costing the same as a good van!. ( he should have waited Aldi are doing them for £399 in week or two)
The house is a TNC-S set up and i know the whole exporting earth thing is a contentious one when PME is concerned! the DNO western power were a bit non committal about there views or opinion on the issue.
Given the summer house has a small kitchen area with stainless sink, copper piping etc.. the guy i am helping is of the view that we can't run a earth cable back to the main earth point at the supply. we are installing one or more rods to get a good reading and will set the summer house CU up as a TT system and bonding for the pipes will link to this rather than the houses earth.

Back when i started my apprenticeship we would start a small hole with a spade (this step also helps with the inspection pit) then use a fence post borer thing to go down a good depth before hammering the rod in to the ground, we would then, if i recall correctly use a conductive type of sand i think was called marconite, we would mix this with cement to then back fill the bored hole. this would increase the conductivity or the rod and improve earthing. i can not recall what ratio of marconite and cement we would use though.

Does anybody else have experience of this method or know if this complied with the regs as they were then or now?
Also where do you buy this marconite from? as i never asked that all them years ago!

thanks

Leesparkykent · Mar 31, 2017

Graham Hyde said:
I will be helping on a install next week where we will be installing a TT system for a summer house in a garden, the customer has gone and bought a hot tub thing costing the same as a good van!. ( he should have waited Aldi are doing them for £399 in week or two)
The house is a TNC-S set up and i know the whole exporting earth thing is a contentious one when PME is concerned! the DNO western power were a bit non committal about there views or opinion on the issue.
Given the summer house has a small kitchen area with stainless sink, copper piping etc.. the guy i am helping is of the view that we can't run a earth cable back to the main earth point at the supply. we are installing one or more rods to get a good reading and will set the summer house CU up as a TT system and bonding for the pipes will link to this rather than the houses earth.

Back when i started my apprenticeship we would start a small hole with a spade (this step also helps with the inspection pit) then use a fence post borer thing to go down a good depth before hammering the rod in to the ground, we would then, if i recall correctly use a conductive type of sand i think was called marconite, we would mix this with cement to then back fill the bored hole. this would increase the conductivity or the rod and improve earthing. i can not recall what ratio of marconite and cement we would use though.

Does anybody else have experience of this method or know if this complied with the regs as they were then or now?
Also where do you buy this marconite from? as i never asked that all them years ago!

thanks

http://www.kingsmillearthing.co.uk/products/soil-conditioning-agent/

Midwest · Apr 1, 2017

Re the 'exporting earth thing is a contentious one when PME is concerned', you could run a suitably sized cable, that would enable bonding of the extraneous conductive parts.

As regards the hot tub; never done one myself, but read on this forum conflicting views. But majority (I think) seem to go for extending/exporting the TN-C-S supply, with an additional earth rod for the tub area, if the tub is sat on terra firma, as opposed to being inside the summer house (some people have too much money).

Leyland30 · Apr 2, 2017

IET produced guidance 'electrical matters' on this subject in 2005 in relation to a detached garage arrangement, try google, wether this is still valid?! it would appear so in relation to TT arrangement.

I've also read conflicting views relating to the provision of earthrods to an otherwise PME supply, mostly relating to the consiquences of a failed supplier DNO earth bonding in the street / transformer and the resulting route for any stray network fault currents I.e. Your new out house earth rod. Presumably for this reason; I believe the guidance is to ensure you only provide L and N to the out house. Any SWA from a supply cable must not connect to the out house earth.

The iet document I referred too also suggests providing a supply for the out-house without the need for a dedicated earth rod. Im not convinced this is best practice given this particular application.

Midwest · Apr 2, 2017

This has been discussed at great length here and various other forums etc. There is no reason not to consider exporting/extending PME to an outbuilding, whether it has extraneous conductive parts or not, as long as it is designed correctly.

Here's a forum resource with some guidance; Exporting Pme - http://www.electriciansforums.co.uk/resources/exporting-pme.101/

The issue here, is the design for the supply to the hot tub, e.g. if the hut tub is sat on the lawn of the property.