TNCS High ZE at origin

[AdamGiguere]

Hi there

Testing at origin of a domestic property, I have found:

L-N: 0.44 Ohms
L-E: 9.89 Ohms

I had the DNO attend clients address, there response was that it was quite high and confirmed readings of 0.44 for ZE. (Pleasee attached document with their response)

My concern is they don't seem to have checked the reading between L-E......

Anyone know what could be the cause of this? MET connections are tight and in good condition....

Many thanks in advance.

Adam

 

[Davisonp]

If there was no link - would I be getting a reading similar to 9 ohms?

I'd expect no reading...?

If so and there is no link - where is the 9 ohms coming from?

It depends on where the 25mm earth goes. If it is not connected to anything at the other end and a Ze reading is taken (disconnect 25mm earth at MET and measure between live and disconnected earth) then the reading will be off the scale. If the 25mm earth is connected to an earth rod or the property pipe work, then you will be getting a TT type reading so 9 Ohms is a good reading if it is a TT system (Attaching to pipe work is acceptable now and I think if that is the case an earth spike would be needed to be fitted to keep the TT system but someone should be able to confirm that) . If it is PME and all cables are good, then you will get almost the same reading as taking a Z reading between live and neutral.

If you are converting from a TT to a PME, then I believe care must be taken about bonding and cross bonding before the conversion takes place.

It also appears from the DNO reply that they have measured between live and neutral at the service head and get 0.44 Ohms, and then again at their transformer on the pole and had a reading of 0.33 Ohms there, so they can and will convert to PME, but it will not meet the 0.35 Ohms max allowable for PME systems, so they require you to calculate if the 0.44 Ohms is good enough.

Out of interest, is the main switch on the CU an RCD? Or, are all the circuits protected by RCD’s.

 

[westward10]

I assume you mean attaching to pipework is NOT acceptable now.

 

[Ian1981]

Unless it's a Private water supply and precautions are taken against its removal it can be considered for use as an earth electrode but in this case I don't think it will apply.
If it's a TT installation at the moment then 9 ohms is a bloody good reading if all parallel paths have been removed during testing.

 

[Davisonp]

I did think this when reading it - supply pole would indicate earth rod...

But if this is the case then they have said 0.33 at the supply pole, I shouldn't be getting a reading of over 9 ohms at the MET should I?

0.33 is the Zs at the DNO pole. From your supply assuming it is TT, the earth cable goes into the ground, then across the high resistance mud to the earth rod at the DNO pole where it joins the 0.33 Ohm DNO earthing system. The resistance between the supply pole earth rod, through the ground and on to the end of the 25mm earth you have at your MET is roughly 8.77 ohms giving you 9 ohms for your Ze.

From their reply, I do not believe the DNO has converted to PME, they have investigated the possibility of converting to PME and their readings are what you WOULD be getting if you were to convert and they require confirmation from you and another visit to convert and give you a PME earth tail.

If you do convert from TT to PME there are things you must be aware of (bonding, cross bonding etcetera).

 

[Ian1981]

Just to be picky if it's TT then it's RA your measuring not ZE.

 

[Davisonp]

Hi Davisonp

This sounds like a good translation of their wording.

I'm pretty sure it's not a TT system/although could be mistaken.

I did think perhaps maybe a TNS due to the 0.44 on the L-N

But have been advised by a colleague who came to site with me originally that it's a TNCS.

Hopefully they have a log of their visit and can advise what system it definitely is

TNS (and PME) should be obvious. If it is TNS, the 25mm earth will be clamped to the outer casing of the supply cable usually just before the main fuse.

 

[Ian1981]

OP are you sure it's a 25mm earth?
Never seen one so big in a domestic property

 

[AdamGiguere]

Hold the phone

Looks like no link from cut out to earth block....

I'll provide the link in next visit, this should definitely rectify.

 

[westward10]

Looks like some seriously bad wiring at that service terminal block, those swa and T&E conductors are not suitably enclosed.

 

[SparkyChick]

How far does the T+E and SWA run?

 

[Ian1981]

Swa live and neutral wires need to have a degree of double insulation or be suitably enclosed as a meter box doesn't constitute as being an enclosure

 

[AdamGiguere]

Thanks for info - could I enclose in some flexible conduit would this suffice?

 

[AdamGiguere]

How far does the T+E and SWA run?

SWA runs to garage/flat at the back of the house.

T&E runs to fuseboard under the stairs but runs into trunking and under the floorboards after exiting the meter cupboard.

 

[SparkyChick]

If it's more than 3m, there is supposed to be protection (switch fuse) for the cable as the DNO won't guarantee disconnection times for the service head fuse.

 

[AdamGiguere]

Ok so do you advise I move the Henley block up in the meter cupboard and install a couple switch fuses isolators/double pole isolators...