Unsure About An Old Earthing Arrangement

[Mattja]

Testing a 50+ year old installation I found a Ze of 0.56 ohms. What I the main earth wire (which is a 6mm^2 wire but hey it's 50 years old, and it's using the earth bar in the fuse box as the MET just fyi) disappears into the wall cavity behind the fuse box and my original suspicion was that it went to an earth spike somewhere (based on the knowledge that another house of a similar age down the street is using TT). However I'm surprised to see just 0.56 ohms on a 50 year old TT system, I was expecting a lot more than that. The whole area around the house is concreted over plus paving over most of the concrete, so my fruitless search for an earth spike doesn't mean there isn't one hidden under there.

Could it possibly be a TN-S with a high reading? If so would the DNO have record of this and would they do anything about it? It's not PME, and it's in the EDF area.

Or should I forget about it and suggest sticking an RCD (in a separate insulated unit) in before the fuse box just to be sure?

Or if there's any other ideas please have a say.

 

[Midwest]

Normally you could start deducing the earthing facility by looking at the service head, have you done that?

 

[Megavolt]

Have you looked at the cut out

 

[telectrix]

did you diss. the bonding before measuring the Ze? if not, the low reading could be from the gas and water pipes.

 

[Midwest]

Have you looked at the cut out

Have you looked at the previous post?

 

[Mattja]

The supply to the house has an earth connected to the side of the house about 4m roughly above the meter, however no earth is present at the DNO's fuse/meter. Therefore I am unsure of where the earth goes from the fuse box without tearing the wall apart and searching through the cavity (a rather drastic and silly idea I think you'd agree).

And yes I did disconnect the cable before measuring.

I suppose it's technically possible that there is a separate MET hidden somewhere in the house, and that the earth wire from the fuse box goes there and then bonding from that MET is messing up my reading. However there is no separate MET anywhere for a few meters in any direction from the fuse box, there's no record of anything left from anyone who's previously done any work on the property, the owner isn't aware of another MET (not surprising), and so if there is another MET hiding somewhere my chances of finding it with the house still furnished... Well I'll take the needle in a haystack any day.

 

[Leesparkykent]

Was there bonding to gas and water? or could of this 6mm been connected to the water pipe? it wouldn't be uncommon on an installation of that age.

 

[trix]

It sounds like you're still getting a parallel path via boiler cpc or the shower cpc.
Or have you dissed the suspected tt.
If there is no met you need to inform dno
50 year is a little young for tt

 

[Mattja]

There was bonding to the gas and water if my continuity tests are to be believed, although I didn't manage to locate where the pipes are bonded. I suppose the 6mm could be going to gas/water pipes and that's all the house is earthed by. Yikes!

There's no power to the shower, it's an old hot-tank and gravity job. The boiler wiring steers well clear of any lighting or power cables. I did disconnect the suspected earth cable from the earth bar in the fuse box to test it.

I think they must've used the earth bar in the fuse box as the MET, perhaps I'm wrong here but I thought that was sometimes done in old installations and was nothing to be concerned about (besides it being a pain in the --- to access in those tiny crowded old fuseboxes).

 

[telectrix]

you're right in that it was common for the earth bar in the fuse board to be used as a MET up till 16th ed ( 1991).

 

[Wilko]

Hi - TN-S of that value is is within the normal range for SSE (happen to know). But I would not stop till I found the cutout and eyeballed the tails and earth strap. Happy hunting

 

[GLENNSPARK]

The supply to the house has an earth connected to the side of the house about 4m roughly above the meter, however no earth is present at the DNO's fuse/meter. Therefore I am unsure of where the earth goes from the fuse box without tearing the wall apart and searching through the cavity (a rather drastic and silly idea I think you'd agree).

And yes I did disconnect the cable before measuring.

I suppose it's technically possible that there is a separate MET hidden somewhere in the house, and that the earth wire from the fuse box goes there and then bonding from that MET is messing up my reading. However there is no separate MET anywhere for a few meters in any direction from the fuse box, there's no record of anything left from anyone who's previously done any work on the property, the owner isn't aware of another MET (not surprising), and so if there is another MET hiding somewhere my chances of finding it with the house still furnished... Well I'll take the needle in a haystack any day.

So what your saying is that there's a G/Y jointed onto the supply cable before it enters the property?

 

[GLENNSPARK]

Just check that it's not a shared supply...looped from this to next door....or vice versa

If it is then you can count out the supply cable being a CNE

 

[GLENNSPARK]

Is it a cast iron cutout...coming of a paper-lead?

If so then the main earthing could be picked off the cast cutout with a short stab of earth to the MET

 

[westward10]

you're right in that it was common for the earth bar in the fuse board to be used as a MET up till 16th ed ( 1991).

Can't the earth bar in a CU be used as the MET?

 

[Midwest]

I do lots of time.

 

[westward10]

I do lots of time.

That's what I thought.

 

[Deleted member 26818]

0.56 ohms is ok for TN-S.

 

[westward10]

0.56 ohms is ok for TN-S.

Good question DNOs will quote typical values of 0.80 for TN-S and 0.35 for TN-C-S but these are not hard and fast and often quoted for new builds. Older networks may often exceed this.

 

[Deleted member 26818]

Wasn't a question.
Was a statement.