High Ze - Low Zs

[Big_Spark]

Spark, I see how your reading it, but I think you missed something, unless the supply had been upgraded to PME, then the sheath of the cable would simply be exposed conductive metalwork, and MUST be connected to Earth. The rod should not be connected directly to it, but there should be a main bond from the main earthing terminal to it.

 

[Spark123]

Yes, it would be exposed metalwork but it is also DNO property. The problem I can forsee with connecting it to the MET would be that in the event of a L-ECP fault on the DNO cable it will pull the voltage on the MET up. All RCDs are downstream hence are useless. The only thing protecting the supply is likely going to be a large DNO fuse upstream.

 

[Big_Spark]

Yes, it would be exposed metalwork but it is also DNO property. The problem I can forsee with connecting it to the MET would be that in the event of a L-ECP fault on the DNO cable it will pull the voltage on the MET up. All RCDs are downstream hence are useless. The only thing protecting the supply is likely going to be a large DNO fuse upstream.

You missing something crucial here.

The exposed sheath of the cable should be connected to the Main Earth terminal, yes by the DNO. As this is a TT system, the Tx on the Poles will have a fused isolator mounted under it for each service coming from it, this will operate as the current drawn would cause an overload, this would immediately alert all that a problem existed.

The supply would be disconnected and it would be safe as a result.

If it were not connected to Earth then the overload would not occur unless it was phase to neutral, therefore the real danger would be the cable sheath remains live until some poor sod connects it to earth the hard way, and is seriously injured or killed as a result.

 

[plugwash]

The exposed sheath of the cable should be connected to the Main Earth terminal, yes by the DNO. As this is a TT system, the Tx on the Poles will have a fused isolator mounted under it for each service coming from it, this will operate as the current drawn would cause an overload, this would immediately alert all that a problem existed.

a rod earth is not going to cause an overload sufficiant to disconnect the DNOs fuse.

service pipes may provide an earth capable of disconnecting thier fuse but you should never design a system that relies on earths from service pipes because they could be replaced with plastic at any time!

 

[Big_Spark]

In extreme circumstances that is correct Plug, but generally the combined effect of the rod and other sources of Earthing combine to ensure automatic disconnection will occur. And I agree, no installation should rely on secondary earthing for protection, but they do supplement the main bond.

 

[plugwash]

i'd hardly calls plastic water and gas pipes and extreme situation, even if a house isn't like that now it could very easilly become that way.

that just leaves a rod earth which is unlikely to be less than 10 ohms. 10 ohms means 24A at 240V. A 60A fuse feeding a lightly loaded house isn't going to get blown by an extra 24A.

 

[securespark]

As this is a TT system, the Tx on the Poles will have a fused isolator mounted under it for each service coming from it, this will operate as the current drawn would cause an overload, this would immediately alert all that a problem existed.

What happens in the case of subterranean TT?

 

[owls]

Thanks a lot for the replies. I'll be contacted the distributor and find out what the earthing system "should" be and then take it from there.

cheers

 

[Big_Spark]

What happens in the case of subterranean TT?

Even that will originate at a local Tx that will have a service feed fused Isolator. However in my experience these are the ones they first replace with PME installations..but obviously that will vary from DNO to DNO..

 

[Big_Spark]

i'd hardly calls plastic water and gas pipes and extreme situation, even if a house isn't like that now it could very easilly become that way.

that just leaves a rod earth which is unlikely to be less than 10 ohms. 10 ohms means 24A at 240V. A 60A fuse feeding a lightly loaded house isn't going to get blown by an extra 24A.

Plug, if the house has a TT supply, then I would be very surprised to find it had plastic water and gas mains..HOWEVER, if it does, then the electrical installation should be so connected as to protect the user from the risk of electric shock.

In all likelyhood the TT supply will be overhead and as such not be in an armoured cable. As per BS7671 it should have a Type S RCD installed, and further, it is easily possible to drop the Rod resistance below 10 Ohms..that is why they can be joined together...

However, the earthing connection of an armoured TT feed is the sole responsibility of the DNO anyway, so there is little point in us keep going over the same ground..both of us could come up with "what if" scenarios almost constantly, and it will serve niether of us...

 

[securespark]

But, to avoid confusion, earthing arrangements on TT are the responsibility of the consumer, not the DNO.

NOPE, the law was changed Simon, all main earthing arrangements are now the sole responsibility of the DNO, that is the main reason many TT's get changed to PME.

It was all changed when they changed the Electricity Supply Act to the new one (name escapes me off the top of head)

Have now downloaded the ESQCR 2002 doc and trawled through it. I can't find anything about responsibilities towards TT earthing connections. This morning, I rang the ENA.

They told me that the DNO does not have any responsibility towards the provision or upkeep of an earthing terminal on a TT supply.

They don't even have any responsibility to provide one on other supplies, assuming there was not one already provided. They only have a responsibility to maintain one already in service.

They will, where possible, upgrade supplies from TT to PME, but reserve the right to charge for doing so.

 

[Big_Spark]

Simon, did youmiss this earlier post of mine?

It is the responsibility of the DNO to provide an earth connection when upgrading or renewing a supply or even exchanging the head, that is regardless of the supply type..this is why most change the supply to PME free of charge at the moment.

Simon, from Section II Protection and Earthing of ESQR 2002

(3) A generator or distributor shall, in respect of any low voltage network which he owns or operates, ensure that -



(a) the outer conductor of any electric line which has concentric conductors is connected with earth;

(b) every supply neutral conductor is connected with earth at, or as near as is reasonably practicable to, the source of voltage except that where there is only one point in a network at which consumer's installations are connected to a single source of voltage, that connection may be made at that point, or at another point nearer to the source of voltage; and

(c) no impedance is inserted in any connection with earth of a low voltage network other than that required for the operation of switching devices or of instruments or equipment for control, telemetry or metering.


And from Part VII SUPPLIES TO INSTALLATIONS AND TO OTHER NETWORKS

(4) Unless he can reasonably conclude that it is inappropriate for reasons of safety, a distributor shall, when providing a new connection at low voltage, make available his supply neutral conductor or, if appropriate, the protective conductor of his network for connection to the protective conductor of the consumer's installation.

(5) In this regulation the expression "new connection" means the first electric line, or the replacement of an existing electric line, to one or more consumer's installations.


Note part 5, whilst not saying so, it has been widely accepted that the replacement of the head of an installation, or upgrading of the service head also requires the installation, if none present, of a connection to earth.

 

[Chris.J]

Seems to me Plugwash is right, the sheath should not be connected to a TT rod. The logic follows that as the sheath is the DNO's, they should either upgrade the supply to PME (if they can), or provide insulation around the exposed sheath so it's no longer exposed?

 

[Big_Spark]

Chris, Plug was argueing that the sheath should not be connected to Earth as it introduced a hazard, we have all agreed that it should be the DNO that is responsible for connecting to Earth.

The discussion was actually about the merits of it being connected but it has got slightly off the original track.

 

[Chris.J]

Chris, Plug was argueing that the sheath should not be connected to Earth as it introduced a hazard, we have all agreed that it should be the DNO that is responsible for connecting to Earth.

The discussion was actually about the merits of it being connected but it has got slightly off the original track.

It has'nt got off track, I'me saying it should be connected to the MET if the supply is PME. If it's not PME then it should not be connected to a TT rod.
Whether it's PME or TT will depend upon what the DNO decide to provide.
This is in agreement with Plugwash.

I'me also saying that if the DNO can't supply a PME earth then I think they should insulate it, as it then becomes unexposed, and hence need not be connected to the house MET.