At the moment, I have two consumer units - one for the usual domestic supply and
the other for off-peak heating with each having its own ELCB and earth rod.

About ten years ago, the electricity board fitted a new off-peak timer and
unwrapped a perforated strip which was connected [and still is] to the sheathing
on the incoming cable, screwed an earthing block to it, and earthed the new
timer to that block. I'm a bit hazy on this, but I seem to recall the fitter
saying at the time that it was a PME system, but it's connected as TN-S.

I'd like to fit new CUs - one split load with a 30mA RCD, and the other without
RCD for the off-peak heating - junk the ELCBs and TT system, and earth to the
block that the fitter put in.

A couple of years ago, my neighbour had storage heaters put in by the board and
they installed a new CU for him. A couple of weeks later, a different crew
turned up to remove his ELCB and re-earth to the incoming cable sheathing.

Is the process as straightforward as that or is there some work that the board
needs to do behind the scenes?

Ken Sparky wrote:
At the moment, I have two consumer units - one for the usual domestic supply and
the other for off-peak heating with each having its own ELCB and earth rod.

About ten years ago, the electricity board fitted a new off-peak timer and
unwrapped a perforated strip which was connected [and still is] to the sheathing
on the incoming cable, screwed an earthing block to it, and earthed the new
timer to that block. I'm a bit hazy on this, but I seem to recall the fitter
saying at the time that it was a PME system, but it's connected as TN-S.

I'd like to fit new CUs - one split load with a 30mA RCD, and the other without
RCD for the off-peak heating - junk the ELCBs and TT system, and earth to the
block that the fitter put in.

A couple of years ago, my neighbour had storage heaters put in by the board and
they installed a new CU for him. A couple of weeks later, a different crew
turned up to remove his ELCB and re-earth to the incoming cable sheathing.

Is the process as straightforward as that or is there some work that the board
needs to do behind the scenes?

Changing the actual earth connection from your local earth rod to the
company supplied earth is probably "as straightforward as that".
However there *may* be issues with the sizing of your earth bonding
connections as for a PME system the earth bonding requirements are
more demanding.

By the way PME is TN-C-S if I remember correctly, not TN-S. It
depends whether the earth that the 'electricity company' have supplied
is actually their neutral or separate.

TN-C-S 'Common' company earth, i.e. the earth block is connected
to their neutral, 'separate' house earth wiring. Supply feed wire
will be a single conductor with screen/armour used as neutral/earth.

TN-S 'Separate' company earth and house earth wiring. Supply
feed wire will be two core (live and neutral) with screen/armour
being just the earth.

--
Chris Green

wrote in message ...
Ken Sparky wrote:
At the moment, I have two consumer units - one for the usual domestic
supply and
the other for off-peak heating with each having its own ELCB and earth
rod.

About ten years ago, the electricity board fitted a new off-peak timer
and
unwrapped a perforated strip which was connected [and still is] to the
sheathing
on the incoming cable, screwed an earthing block to it, and earthed the
new
timer to that block. I'm a bit hazy on this, but I seem to recall the
fitter
saying at the time that it was a PME system, but it's connected as TN-S.

I'd like to fit new CUs - one split load with a 30mA RCD, and the other
without
RCD for the off-peak heating - junk the ELCBs and TT system, and earth
to the
block that the fitter put in.

A couple of years ago, my neighbour had storage heaters put in by the
board and
they installed a new CU for him. A couple of weeks later, a different
crew
turned up to remove his ELCB and re-earth to the incoming cable
sheathing.

Is the process as straightforward as that or is there some work that the
board
needs to do behind the scenes?

Changing the actual earth connection from your local earth rod to the
company supplied earth is probably "as straightforward as that".
However there *may* be issues with the sizing of your earth bonding
connections as for a PME system the earth bonding requirements are
more demanding.

By the way PME is TN-C-S if I remember correctly, not TN-S. It
depends whether the earth that the 'electricity company' have supplied
is actually their neutral or separate.

TN-C-S 'Common' company earth, i.e. the earth block is connected
to their neutral, 'separate' house earth wiring. Supply feed wire
will be a single conductor with screen/armour used as neutral/earth.

TN-S 'Separate' company earth and house earth wiring. Supply
feed wire will be two core (live and neutral) with screen/armour
being just the earth.

And if it's TN-C-S do you really want to change to it. Ours runs at up to
40volts away from the potential of the ground outside, which I found to my
shock unfortunately includes the soil below the stone slabs in much of my
house.

On Tue, 14 Dec 2004 22:16:01 -0000, "Mike" wrote:


wrote in message ...
Ken Sparky wrote:
At the moment, I have two consumer units - one for the usual domestic
supply and

snipped
TN-C-S 'Common' company earth, i.e. the earth block is connected
to their neutral, 'separate' house earth wiring. Supply feed wire
will be a single conductor with screen/armour used as neutral/earth.

TN-S 'Separate' company earth and house earth wiring. Supply
feed wire will be two core (live and neutral) with screen/armour
being just the earth.

And if it's TN-C-S do you really want to change to it. Ours runs at up to
40volts away from the potential of the ground outside, which I found to my
shock unfortunately includes the soil below the stone slabs in much of my
house.


Any response from the supplier about this?

I'd be happy with TN-S because I was trying to avoid having to install a 100mA
time-delayed RCD which is required with TT earthing.

On 14 Dec 2004 15:06:06 GMT, wrote:

Ken Sparky wrote:
At the moment, I have two consumer units - one for the usual domestic supply and
the other for off-peak heating with each having its own ELCB and earth rod.

snipped

By the way PME is TN-C-S if I remember correctly, not TN-S.
Agreed. What I was trying to say was that although the fitter said it was PME, I
think it is connected as TN-S. I thought it was TN-S because the earth block is
wired to the sheath [but maybe it's not TN-S in view of your definition of
TN-C-S using the sheath as the neutral conductor].

It
depends whether the earth that the 'electricity company' have supplied
is actually their neutral or separate.Would this be the case

TN-C-S 'Common' company earth, i.e. the earth block is connected
to their neutral, 'separate' house earth wiring. Supply feed wire
will be a single conductor with screen/armour used as neutral/earth.

TN-S 'Separate' company earth and house earth wiring. Supply
feed wire will be two core (live and neutral) with screen/armour
being just the earth.

Would the sheathing also be likely to be the neutral conductor in a supply cable
installed 45 years ago?

How do I find out if the sheath is earth only or earth/neutral combined?

On Wed, 15 Dec 2004 13:20:43 GMT, (Ken Sparky)
strung together this:

Would the sheathing also be likely to be the neutral conductor in a supply cable
installed 45 years ago?

Yes and no. If the supplier alters things on the network then a
previously TT only supply can become PME.

How do I find out if the sheath is earth only or earth/neutral combined?

Ask the supplier. Some TT systems can look as if they might be PME
from tests carried out, but actually aren't.
--

SJW
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