Benefits of a TT system?

[Crosswire]

If an installation is supplied by a two wire system (E.G. a TT system) , what are the benefits of using an earth electrode instead of simply joining earth to neutral at the supply head to create a TNCS system?

Surely using the supplier's neutral is a more effective return path for fault current than relying on the earth electrode and the solid mass of earth?

All replies welcome,

Cheers,
Crosswire.

 

[telectrix]

In a TNCS the fault path uses the neutral (yes I know you know that) so if the neutral breaks and there's a fault inside the installation then you could have every metal part inside it live. In a TT you still have the fault path through the general mass of earth.
TTs are not just used in rural/remote locations, we live near a main road into a fairly busy town and we're on a TT (Ra > 1 ohm)

and that's the reason you can't use TNC-S for caravans ( metal bodied). someone correct me if i'm wrong.

 

[trev]

In a nutshell Tel, as ever you're right.

 

[Crosswire]

In a TNCS the fault path uses the neutral (yes I know you know that) so if the neutral breaks and there's a fault inside the installation then you could have every metal part inside it live. In a TT you still have the fault path through the general mass of earth.
TTs are not just used in rural/remote locations, we live near a main road into a fairly busy town and we're on a TT (Ra > 1 ohm)

I see. So would it be a good idea to use a TNCS system ( which is good if it is working correctly), backed up by an earth electrode to provide an alternative path, should the neutral fail?

 

[trev]

I see. So would it be a good idea to use a TNCS system ( which is good if it is working correctly), backed up by an earth electrode to provide an alternative path, should the neutral fail?

Although I'm assured by my learned friends here that it does happen I've not seen a failed neutral on a TN in my 34 years in the trade. I've seen whole cables fail but not just the N.
Some countries do exactly what you're talking about so maybe it would be a good idea if it were adopted here. Would I do it? Probably not

 

[telectrix]

been wrong before and no doubt i'll be wrong again.

 

[SPARTYKUS]

Crosswire that is effectively what IS done, the bonds to the gas and water, although no one will admit it, act as back up TT rods (obviously only if metal).

Almost every time I have found a half decent TT reading, it is due to the parallel paths to these services, not the electrode itself.

 

[trev]

Crosswire that is effectively what IS done, the bonds to the gas and water, although no one will admit it, act as back up TT rods (obviously only if metal).

Almost every time I have found a half decent TT reading, it is due to the parallel paths to these services, not the electrode itself.

Fancy a cuppa at wor hoose? Bring ya tester

 

[SPARTYKUS]

Sorry Trev, i forgot to exclude your gaff! With your earth rod farm or summat.......

 

[SPARTYKUS]

I have an earth rod job to do soon. Ze is 55 ohms. I will see what I can do. At mo it looks like one 3/8ths twig.... That will be ignored!!

 

[trev]

Sorry Trev, i forgot to exclude your gaff! With your earth rod farm or summat.......

Only 2 or 3 rods under our floor mate. Not the ideal place for them but it was either that or hacking my way through a lot of concrete.
It was never going to happen really was it?

 

[Crosswire]

Although I'm assured by my learned friends here that it does happen I've not seen a failed neutral on a TN in my 34 years in the trade. I've seen whole cables fail but not just the N.
Some countries do exactly what you're talking about so maybe it would be a good idea if it were adopted here. Would I do it? Probably not

Maybe a burnout at the supply head caused by a high resistance connection on the neutral might do this, but I also have never seen it personally.

I also have never driven earth electrodes to back up the TNCS earthing, I always relied on the supplier's earth ( which they are apparantly "not obliged to provide") once the Ze has been confirmed.

Anyhow, all good food for thought, as I find this subject fascinating.

 

[trev]

Maybe a burnout at the supply head caused by a high resistance connection on the neutral might do this, but I also have never seen it personally.

I also have never driven earth electrodes to back up the TNCS earthing, I always relied on the supplier's earth ( which they are apparantly "not obliged to provide") once the Ze has been confirmed.

Anyhow, all good food for thought, as I find this subject fascinating.

Not obliged to provide but are obliged to maintain if they do

 

[SPARTYKUS]

Trev I do fancy a cuppa, one day. Bit of a drive though. 8 hour round trip, £90 in petrol, would have to be one hell of a cuppa

 

[trev]

Trev I do fancy a cuppa, one day. Bit of a drive though. 8 hour round trip, £90 in petrol, would have to be one hell of a cuppa

Mrs Trev makes a fantastic bacon sarnie

 

[telectrix]

had a call-out the other week. elderly disabled gent. i'd done him a CU upgrade some months ago. his daughter phoned me---no power.

thinking possible RCD trip, called round. at the incomer found L-N 0V. L-E 245V, N-E 180V. called DNO who called @ 6.15pm. they confirmed it was a supply fault. after digging up the road, they found a faile N connection where he was tapped off the underground feeder.