TT SYSTEM

[Farmelectrics]

Took a swa cable to an outside metal shed, earthed at house end but insulated at shed end. Installed earth rods and got reading of 58ohm . all good. However glanded cable shed end for experiment and got 0.24ohms. Question what are the dangers and why cant you have both earthing systems in place. For the record the cable has been insulated shed end and is a TT system

 

[LastManOnline]

Took a swa cable to an outside metal shed, earthed at house end but insulated at shed end. Installed earth rods and got reading of 58ohm . all good. However glanded cable shed end for experiment and got 0.24ohms. Question what are the dangers and why cant you have both earthing systems in place.

If the DNO supply is TNCS it would under certain fault conditions cause a dangerous situation for the TT system.

 

[pc1966]

You can have both the supply earth (TN) as well as local earth rods. In many cases in the past that is what you would have once metal service pipes were bonded in any case. Of if any local lightning protection system (where Ra generally would be less than 10 ohms)

The issue for an outside metal building being linked to the supply CPC (i.e. extending TN system) comes when you have a TN-C-S supply and the risk of an open PEN causing the supply earth to try and return the live current via any metalwork. That has two serious implications:

• The bonding of extraneous parts has to be, by the regs, at least 10mm copper equivalent due to the prospect of a high and sustained fault current. For many SWA relaying on the armour that is not met (for 2-core it is met for 70mm and above, for 4 core it is more-or-less met at 35mm as the steel CPC is 78mm CSA so approx 9.75mm Cu) so you might have the expense of an additional copper CPC or going up a core-count in the chosen SWA.
• If the extraneous arts can be touched by a human or animal while in contact with the true Earth there is a risk of a shock under open PME conditions. Recently this has come to the fore with EV charging and the risk of folk washing the car while on charge. For a farm where animals are larger, don't have rubber boots, and more sensitive to shock, that presents all sorts of problems which is why TT has often been preferred.

 

[Farmelectrics]

ok thanks pc1966, wasn't sure you could have both.

 

[pc1966]

I think, but would have to check, that if you have local backup generation it has to have its own means of earthing (i.e. you cannot assume the supply earth will be present if the supply has failed). So then you would need some earth rod(s) for the generator and that would be linked to the supply earth as well (and generator to have output RCD appropriate for the rods alone).

 

[loz2754]

extraneous arts

Also, with such large animals around, it would be worth considering bonding extraneous farts...?

 

[Rockingit]

It's worthwhile considering here that if the proposed ammendment 2 sections all get enacted, then every new dwelling is about to become just that - TN-C-S also connected to basically TT. All that happens when we introduce a 'new' rod/tape/whatever onto a system is that we've just added yet another reference point onto the PME network so I've never understood the fascination for divorcing otherwise perfectly good TNCS systems.

 

[timhoward]

Also, with such large animals around, it would be worth considering bonding extraneous farts...?

I remember when the sign that originally read “Coventry Centre for the Performing Arts” inevitably got modified….

 

[telectrix]

Also, with such large animals around, it would be worth considering bonding extraneous farts...?

problem there is how do you bond your arse, a static strip trailing on the ground? my extraneous farts make the dogs think it's thunder and they hide in their bed.

 

[Marti]

Took a swa cable to an outside metal shed, earthed at house end but insulated at shed end. Installed earth rods and got reading of 58ohm . all good. However glanded cable shed end for experiment and got 0.24ohms. Question what are the dangers and why cant you have both earthing systems in place. For the record the cable has been insulated shed end and is a TT system

This is something bothering from a slightly different angle; I have an installation (using SWA) where the land is very free draining and a bit sandy. The Ze to rod(s) can be highly variable to say the least and so I 'm opting for a type C at the main board and and type B's at the wooden outhouse. Reliable earthing back to the main building TN and no earth rods. Risk assessed and mitigated. Am I missing anything and is there a recognised RA form to document this?