TN-C-S to TN-S !!!

Maybe you ought to do a bit of reading up on electrical supplies and earthing systems...

I'll let him off if he has read that book off amazon, even though he is completely wrong!

So how far from the star point can you go before splitting N & E for it to be classed as TN-S?

So how far from the star point can you go before splitting N & E for it to be classed as TN-S?

Given a typical ground-mounted substation the N/E connection will be generally within the LV distribution equipment, rather than at the transformer.
If this is a transformer mounted cabinet it is directly on the outgoing neutral/starpoint. If cable connected between the transformer and the LV distribution equipment it could be up to 10m away. In either case it will be before the first point of protection on the LV system. The connections as classed by us to be part of the HV zone as thet is the point where they could be isolated for work.

This location also gives us the option of connecting the LV earth to the HV metalwork earth depending on the various impedances.

For overhead it would generally be at the transformer pole but can be on the first LV pole away in an overhead system to provide separation from the HV earth.

This method will apply to a TN-S or a TN -C-S (PME) earthing system on the LV network. The only difference is the additional earth connections that need to be applied on a TN-C-S system

Though to fully answer the question we take TN-S supplies direct off this equipment

This has been the subject of many a heated talk. Reading BS7671 it would seem a TN-S can become a TN-C-S further down line but not visa versa.

If the earth and neutral have been combined as a single cable, I don't mean just earthing the neutral, then it's TN-C or TN-C-S.

The problem is where TN-C-S is not permitted be it a caravan, boat, or petrol station the only people who can say it's TN-S are the DNO. We as sparks have no idea what has been done with the supply before the head.

With a house the ELI changing from 0.8 to 0.35 ohms is really the only difference. And in the main we are not really that worried all we need to know is if TT or TN although many a EICR form asks for the information. Unless we can see it's a TN-C-S or TT really all we can enter is TT or TN as we really don't know what type of TN unless the DNO has put a label on the supply.

So unless there is a DNO label may be we should just enter TN. But many a form has tick boxes and not only for supply we are often asked what cable is use. Again unless marked on cables to work out is thermal setting or thermal plastic is really going to be no more than a guess.

I see the whole idea of guessing flawed as this guess could be repeated on forms for 50 years, may be well after one is dead and gone. To me if we don't know then N/A or ----- or any other no committal entry should be made.

This has been the subject of many a heated talk. Reading BS7671 it would seem a TN-S can become a TN-C-S further down line but not visa versa.

Wellllllllllll!

Under certain conditions we can joint a section of new cable to a TN-S system, the new cable being combined N/E, but if these conditions are met any existing TN-S cable past this point stays as TN-S.

Certainly any extensions to existing systems will normally be TN-C-S, but we can obtain a modern TN-S cable design with separate N & E

I look at fig 2.2 in the old 17th Edition which shows the combining and splitting of earth and neutral but most of the drawing refers to the DNO's system which we as the user have no control of and also does not come under BS7671 anyway. We as users are not in the main allowed to use TN-C so really it is not something we are concerned with.

However when we need to know system type some times we can see plainly it is TT or TN-C-S but we can't unless the DNO has posted a notice saying what type of supply it is ever be certain that it's a TN-S supply.

So if I need a TN-S supply then I want a letter from the DNO saying it is a TN-S supply. Be it a petrol station, caravan park, or marina I would never rely on my inspection of the head to confirm a TN-S supply I want something in writing from the DNO. I will not phone them up as I want to put the reply on the records.

Marinas are a nightmare with earths so much damage can be caused although often diodes are put in the earth supply two diodes only give 1.2 volt threshold and aluminium to steel is getting close. With marinas I use steel earth rods not copper coated and large lumps of steel so they will not rust away too quick. With steel, Zinc, and Aluminium already being used on the hull to add copper as well seems silly.

The question I have asked is even with a TN-S supply could one be certain that the voltage is within 1.2 volts of the waters potential? I think not so TT would seem to be answer. As to how many TT supplies that is also something talked about. I would TT each wharf so one could direct all Aluminium hulls to same wharf.

As to lamp posts I don't know how quickly the metal would be eaten away, and to use a non TT supply I would want to inspect the first lamp regular to see if there is any erosion.

Under certain conditions we can joint a section of new cable to a TN-S system, the new cable being combined N/E,
When you do that how do you join things? do you connect the neutral and earth from the old cable together?

When you do that how do you join things? do you connect the neutral and earth from the old cable together

Yes

Wellllllllllll!

Under certain conditions we can joint a section of new cable to a TN-S system, the new cable being combined N/E, but if these conditions are met any existing TN-S cable past this point stays as TN-S.

Certainly any extensions to existing systems will normally be TN-C-S, but we can obtain a modern TN-S cable design with separate N & E

Down this end, the only mains cable we have is CNE....I've never heard of a fault repair using 4 core. SNE service cable is hidden away in a corner and hardly ever used

So it's quite common in this area for a 'TNS' main to become 'TN-C-S' for a short run where there has been an overlay or fault

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