PME and supplier responsibility

[PeeTee2009]

We currently have a 2 wire overhead supply and have been recommended by an electrician that an earth be supplied and the system by PMEd.
I have contacted the distributor (Scottish Power/Manweb) but been told that they are under no obligation to install. Moreover, they have said that there will be thousands of pounds worth of work that I will be responsible for (changing supply to neighbouring properties/digging up roads etc). Is this correct or is there a duty on the distributor to provide an earth for the system?
Distributor suggested that I get the electrician to install an RCD and earth spike. Is this correct or are they just fobbing me off?

 

[rebuke]

You have a TT supply, the distributor is (AFAIK) under no responsiblity to provide you a TN-S / TN-C-S supply (they only have to if one is available). If you don't have an earth spike at the moment, your installation is incredibly dangerous - get this fixed immediately. You should also have an RCD protecting all circuits...

 

[ericmark]

They are correct to use a PME or to use another name for same thing TN-C-S supply ALL properties on that transformer should be on same system so your neighbours would also need to change their supply type.

Although supply authorities should try to supply as a TN-C-S or TN-S where possible there are some notable exceptions. Boats and caravan have to be supplied with a TT supply for example.

And there is nothing really wrong with a TT supply. Before the 17th Edition TT supplies normally used 100ma S type (Delayed) RCD. But with 30ma and 40ms RCD being used now for most circuits in a house then the 100ma RCD has become redundant.

The use of an earth spike (TT supply) is now no real disadvantage over a TN-C-S supply and does have some advantages should you want power into any shed or other out building.

 

[JohnD]

...TN-C-S supply ALL properties on that transformer should be on same system so your neighbours would also need to change their supply type....

for my information, have you a reference for that, or a link I can read?

I am thinking of a street containing some modern houses and some very old ones. My relative does not have PME but some neighbours do.

 

[bernardgreen]

The use of an earth spike (TT supply) is now no real disadvantage over a TN-C-S supply and does have some advantages should you want power into any shed or other out building.

An effective earth spike is going to provide a safety earth at all times.

If the protective "earth" is derived from the incoming neutral then one is dependent on that neutral connductor always being intact and maintained within a few volts of earth potential.

 

[Spark123]

They are correct to use a PME or to use another name for same thing TN-C-S supply ALL properties on that transformer should be on same system so your neighbours would also need to change their supply type.

It isn't uncommon to see a mix of TN-S and TN-CS supplies in neighbouring properties, especially when one has been upgraded.
It can be a bit of a pain when you end up with two supplies entering a building, one being TN-S and the other TN-CS tho.

Although supply authorities should try to supply as a TN-C-S or TN-S where possible there are some notable exceptions. Boats and caravan have to be supplied with a TT supply for example.

Don't think there is anything wrong with using TN-S for those above, bar you may end up with some electrochemical reactions with the boat. Certainly TN-S is allowed for caravan sites.

The use of an earth spike (TT supply) is now no real disadvantage over a TN-C-S supply and does have some advantages should you want power into any shed or other out building.

What is wrong with TN-CS? As long as there are no extraneous conductive parts, or if there are they are correctly bonded then I don't see a problem with it.

 

[RF Lighting]

They are correct to use a PME or to use another name for same thing TN-C-S supply ALL properties on that transformer should be on same system so your neighbours would also need to change their supply type.

Never heard that one before.

It sounds to me that the network is not PME'd, and the DNO want to charge you to upgrade it, which they are entitled to do.

They are under no obligation to provide you with an earth terminal. Their only responsibility is to maintain an existing terminal.

 

[csh]

There is no responsibility for the supply authority to provide an earth. We have several TNS properties with failed earths. When found on test we just fit earth rods . This is done on the authorities advice

 

[Taylortwocities]

For TT there is no responsibility on the DNO to provide an earth. But AFAIK they do for TN-S and TNC-S.

 

[ericmark]

(4) The distributor shall not connect his combined neutral and protective conductor to any metalwork in a caravan or boat.

As to mixing of earth systems. I can’t find the document at this time. It is because with a mixture of earth systems one could have two houses with exposed metalwork for example iron soil pipes which, under fault conditions, could have lethal voltage between them.

I did queried this at the time of reading as for example where a caravan is parked next to the house and supplied with power to ensure batteries etc are kept topped up you could if supplying it with a TT supply as required with regulations get the same problem.

Which brings me to next point Yes I was in error in saying has to be TT and yes 704.411.3.1 says must not use TN-C-S and TN-S is permitted and the supply from house to caravan would be TN-S.

However with a boat ref fig 709.3 Unless you have an isolating transformer
fitted on board to isolate the electrical system on your craft from the shore supply
system, corrosion through electrolysis could damage your craft or surrounding craft. So with a boat a TN-S supply would not normally be used. I still feel the whole idea of connecting the earth through diodes does not seem safe but it is a common method with boat supplies and at £300 for an isolation transformer I can see why.

I have also questioned in the past of the problems where one house on a PME supply has for some reason a good earth independent of the boards earth and under fault conditions could end up being the earth for a whole group of houses and have quite a high current travelling through the earth cables. I should think this is why 10mm cable is required. I have seen the result of using smaller cable.

A radio ham employed an electrician to put a supply into his shack. The electrician did not use the armour of the SWA for earth but ran a 4mm cable under the SWA (2.5mm) with earth and bonded all metal work in the shed. The radio ham also connected his earth to all metal work and his earth was 4 sets of rods in each corner of the garden with earth tape between them.

Workmen down the road hit the supply cable. The radio ham called me when the plastic started to drip off his earth wire in spite of turning off the main supply switch.
By time I got there the earth cable had fused. Removing the cover off the CU to measure I found voltage floating between 10 and 390 volts. His was only house not to have electrical equipment blow up that day.

After that we changed his house to a TT supply but were told that we should not be doing this by the supply company but under the circumstances they turned a blind eye. Hence why I know it is officially up to the supply company to say what supply you have. I was shown the regulation at the time but didn’t have any way to make a copy.

Maybe the rules have now changed this was back in 1994 or there about.

 

[rebuke]

If I'm reading your post right, then you mean the workmen went through the neutral, but left the live conductor intact, so the return current from lots of houses ended up using the radio ham's decent earth as the best return to the transformer, because with TN-C-S the neutral and earth are tied together.

Surely for that to happen it would mean the distributor hadn't properly PME'd their network - as I thought the whole point of PME was that they would tie neutral to earth at multiple places (i.e. extra spikes at various points on the cable), such that a loss of neutral in one place wouldn't cause this to happen?

 

[Spark123]

It might have been that there were DNO bonds to earth, however as this was a parallel path the current used it as well.
Good reason to follow the minimum sized specified for main protective equipotential bonds specified by the DNO.

 

[hairyben]

If I'm reading your post right, then you mean the workmen went through the neutral, but left the live conductor intact, so the return current from lots of houses ended up using the radio ham's decent earth as the best return to the transformer, because with TN-C-S the neutral and earth are tied together.

Surely for that to happen it would mean the distributor hadn't properly PME'd their network - as I thought the whole point of PME was that they would tie neutral to earth at multiple places (i.e. extra spikes at various points on the cable), such that a loss of neutral in one place wouldn't cause this to happen?

Hence, protective multiple earth.

I can envisage a scenario where the earth bonding in multiple properties- if they were all on a common metallic water/gas system- could easily provide a better return path than the PME spikes.

 

[stardanny]

I used to have TT overhead cables from poles to my house and a earth rod under my paving slab, 10mm to earth block next to fuseboard ( as part of upgrade for electric shower in 1995 ) a few years before the electricity Board remove all poles and rerun new cables under ground in my street. I had a pole in my back garden and they used to pay a small fee for siting their pole in my garden, now stopped.

They ran it from road to my house wall and up to connect to existing cables.

I still have a old rewireable board as earlier post. ( I still haven't start yet as I have been advised to call Building Control first )

Do this count as a TT system?

 

[securespark]

If the protective "earth" is derived from the incoming neutral then one is dependent on that neutral connductor always being intact and maintained within a few volts of earth potential.

Which is why it is protective multiple earthing. It is tied down to earth in several places to ensure exactly that.