Mixing TT and TNCS supply systems

[LastManOnline]

Was informed of this incident last week. A row of period houses in an older part of the city. All on TT earthing. One gets rewired. TNCS is obligatory for all new installations. DNO suffers a neutral break close to the trafo. All 23 houses now divert their neutral current back through the neutralised houses main (and supplementary) bonding. As it was an older part of town the metallic services were all intact and provided a low resistance path back to the trafo. It was estimated this home had up to 700 amps travelling through it at one point. Though no fire occurred there was massive smoke damage resulting in huge damage.

Two lessons for me.
.1) Check the neighbours supply system when doing rewires.
2) Consider electrically isolating any main metallic services still in use

 

[Mikegh]

Well that's interesting

Do the DNO not have any rules on mixing supply types

You would think it would be down to them to ensure this doesn't happen

 

[catdog1121]

Took me a few mins to work that one out but yeah I can see how the other houses have no path back as the earth is completely separate, however one house has neutral an earth combined so all the current tries to go back via that earth connection..

Is it obligatory to change from TT to TNCS?

I have found that the metallic services provide a lower resistance to earth than the earth rod although would that not have made it worse as the earth rod would still be the only path back to the transformer?

I wonder if we will ever get to a point where it will be mandatory to fit a device that will detect a PEN fault, we now have them, why not install one at incomer, probably a better safety feature than an SPD?

 

[Mikegh]

Interested but don't know much about this


For the 23 houses would that not be fed from a 3P trafo and the neutral current would be the vector sum so 700 seems high ?

Had a few beers could be talking rubbish


How does the pen fault detector work on an incommer then ?

 

[LastManOnline]

Is it obligatory to change from TT to TNCS?

Yes. Currently we have no other options

I wonder if we will ever get to a point where it will be mandatory to fit a device that will detect a PEN fault,

My thinking is that it's more a question of when than "if"

 

[LastManOnline]

How does the pen fault detector work on an incommer then ?

There is, nt one. The current protection stratagy is to rely on the very low incidence of open PEN faults and the lower again number of these incidents that result in damage to people, property or livestock

 

[bill01803]

Who says it has to be tncs, it would make sense to stick with tt.

 

[pc1966]

Who says it has to be tncs, it would make sense to stick with tt.

I think that is the ROI regulations on the supplier (DNO) side

 

[pc1966]

I thought the ROI now mandated double-pole MCB as the main incomer switch, so I presume the 'neutralising link' is at the supply head, and not in the CU?

I think the Americans have a N-E link in the CU but I don't know if it is before or after the incoming isolator. If it were on the isolated side a DP MCB would have cut out the fault fairly fast!

 

[marcuswareham]

PEN fault, we now have them, why not install one at incomer

We only have a reliable solution if all 3 phases are present

the current signle phase options are not really a solution

I think the DNOs should implement PEN fault disconnection as they have access to all phases easily, but that would require more cost