TT vs TN-C-S - preference?

[JohnW2]

All my RCBO's are single pole switching, if using TT really need double pole switching, ....

Why? So you can use the RCBO for BS7671-compliant 'isolation', perhaps?

Given what can happen to the potential of the neutral in a TN-C-S installation, one might understand more if you were saying that double-pole switching was more desirable with TN-C-S !

Kind Regards, John

 

[bernardgreen]

Unless I'm missing something, even if that happened, the situation would be no worse than if the retail unit also had a TN-C-S earth, would it? (and, for better or worse, TN-C-S is 'accepted').

If the retail unit did "up-grade" to a TN-C-S supply then there would be even more reason to ensure the two CPCs and their associated bonded metals could never come in contact with each other.

The services to the cottage and the retail unit come from different street cables. The two Neutrals could be at different potentials. Connecting the two Neutrals to each other (Neutral 1---CPC 1----Bond---Water pipe---Bond----CPC 2---Neutral 2) could result in high currents.

 

[JohnW2]

If the retail unit did "up-grade" to a TN-C-S supply then there would be even more reason to ensure the two CPCs and their associated bonded metals could never come in contact with each other.

My initial reaction was to ask "Why?". However, you go on to say ...

The services to the cottage and the retail unit come from different street cables. The two Neutrals could be at different potentials.

... which obviously moves the goalposts a long way, but that surely is a pretty unusual situation, fairly unique to yourself? Adjacent properties will usually be fed by different phases (all from the same 'street cable') but the neutrals will nearly always come from the same 'street cable', won't they?

Kind Regards, John

 

[bernardgreen]

a pretty unusual situation, fairly unique to yourself?

It is unusual but not unique.

 

[JohnW2]

It is unusual but not unique.

Obviously not literally unique (which is why I wrote "fairly unique") but I would think that it must be extremely rare. Unless there had been a lot of development in a street, it's hard to see why a DNO would run more than one distribution cable down the same bit of the same street (not the least because it has the potential to create the very hazard you mentioned),

As a matter of interest, how do you know that a different 'street cable' supplies the two installations in question? Do they come from different transformers?

Kind Regards, John

 

[bernardgreen]

My cottage is on the corner of the High Street and a country lane.

The retail unit is fed from the service along the High Street. This was originally an open wire over head service with drop cables from posts to premises. It was converted to a new underground cable with spurs from that cable running up the walls to where the over head service cables had landed. This was a convenient way to avoid having to re-new the service cable all the way to the cut outs.

My new cottage supply comes from the cable that that had been installed to supply a number of new properties along the country lane.

how do you know that a different 'street cable' supplies the two installations in question?

The utility maps show them as separate cables to the sub station. No information as to how many transformers are involved.

 

[JohnW2]

My cottage is on the corner of the High Street and a country lane.

Fair enough - I suppose the one situation this is quite likely to happen is on a corner between two streets, since two streets quite probably translates to 'two street cables'

The utility maps show them as separate cables to the sub station. No information as to how many transformers are involved.

If those maps are anything like as 'good' as those for my locality, I would be cautious about relying on them too much

Kind Regards, John