Supply type identification and opinion on earth loop reading

[Muzza80]

Hi all,
Thanks for taking the time.

I'm looking to supply power to a shed/workshop I've just built a few metres from the house, and from there down a 25m run to a small BBQ cabin.

So I've had an electrician out to quote for the job this morning left concerned that the supply was not earthed sufficiently/at all. I believe I have a TT system, perhaps someone could confirm (if that's possible from just a picture?).

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• Muzza80
• 11 Nov 2017

The electrician wanted to confirm earth as none is visible from the front as you can see. He removed the front of the CU and the first earth cable on the earth bar is large gauge 16mm2, great, we've found the main earth bond cable. He's cut the supply, removed this 16mm2 cable, setup his Fluke MFT, connected one cable to this and then one to the main switch live and it's come back with a reading of exactly 200ohm, which I now understand to be the loop test Ze result. He confirmed the connection at the earth cable and tested again, again 200ohm.
He then tested the next cable along, a 6mm2 and that gave a reading of 30ohm, we believe that to be the bonding to the gas meter and mains water pipes.

Essentially he's now advised that he cannot carry out any work until this is addressed as effectively the supply is not earthed & that reading is effectively open circuit.

Now, the property underwent a full re-wire and renovation including an extension about 5 years ago (before we bought it) the CU is a new metal type and all the wiring, including this 16mm2 earth all looks new, I just find it hard to understand how this could be so serious after going through a full re-wire quite recently.

I've been doing a little reading and from what I can tell 200ohm is the limit of what is acceptable, not ideal but still acceptable and not in line with the advice that the house is unearthed and actually dangerous.

Can someone verify? Thanks!

 

[Adam_151]

the IET state that an earth electrode should be <200 ohms as above that is unstable . Did his meter indicate 200 ohms or >200 ohms. The NICEIC advise their members that >100 ohms is not ideal. Either way I wouldn't be happy with it. Has the earth electrode itself been located?

You might find that the supply authority may be able to do a PME upgrade, their cable looks like it should be undersized too!

Can we have a close up picture of the CU? TT with a metal consumer unit can be a bit of an issue, there is a recommended way round it, but I bet its not been done here

 

[ban-all-sheds]

So I've had an electrician out to quote for the job this morning left concerned that the supply was not earthed sufficiently/at all. I believe I have a TT system, perhaps someone could confirm (if that's possible from just a picture?).

Well there's certainly no supply earth visible in it.

There are L & N conductors for the incoming supply which look undersized and of the wrong type though.

A photo of where the supply enters the house would be useful.

The electrician wanted to confirm earth as none is visible from the front as you can see. He removed the front of the CU and the first earth cable on the earth bar is large gauge 16mm2, great, we've found the main earth bond cable.

Did he try to trace it?

Did he look at where the supply enters?

200Ωis a bit high, particularly if there's no drought. What is your soil like, and how wet has it been of late?

Now, the property underwent a full re-wire and renovation including an extension about 5 years ago (before we bought it)

So what does the EIC say about the supply type and fault loop impedance?

the CU is a new metal type

A metal CU and a TT supply.

I wonder how that was done? Can you show us where the tails enter the CU?

 

[Muzza80]

the IET state that an earth electrode should be <200 ohms as above that is unstable . Did his meter indicate 200 ohms or >200 ohms. The NICEIC advise their members that >100 ohms is not ideal. Either way I wouldn't be happy with it. Has the earth electrode itself been located?

You might find that the supply authority may be able to do a PME upgrade, their cable looks like it should be undersized too!

Can we have a close up picture of the CU? TT with a metal consumer unit can be a bit of an issue, there is a recommended way round it, but I bet its not been done here

Thanks for the reply.

Now that I don't know, it may well be >200 ohms, perhaps the MFT displays that as an upper limit, I didn't ask?

I have no way to know where the electrode is, it's a 1950's end terrace ex council, certainly no sign of one outside that I can see (and I've had a good look), an electrician friend from home (not close to us) has suggested that the electrode(s) will most likely be in the ground under the house, there's no easy access under the floor (the downstairs is fitted with laminate throughout, including the cupboard with the CU) so I don't even know where the access hatch is or if there is one.

I'm at work just now but I'll take the cover of the CU tomorrow morning and take a snap, here's the front.





Cheers.

 

[Muzza80]

Well there's certainly no supply earth visible in it.

There are L & N conductors for the incoming supply which look undersized and of the wrong type though.

On an old house like this (1950's) would you expect this to have been upgraded as a matter of course by the DNO? Out of interest, what should it be?

A photo of where the supply enters the house would be useful.

I've no idea where that might be, looks to me like the wall has been filled in below the CU so perhaps it;s behind there and access was needed then filled? As mentioned above there's laminate throughout so no access to any hatch, might have to get that taken up I guess!

Did he try to trace it? No, only took the cover off the CU

Did he look at where the supply enters? See above, no idea where that is?

200Ωis a bit high, particularly if there's no drought. What is your soil like, and how wet has it been of late? Very wet this last few months but soil has no clay.

So what does the EIC say about the supply type and fault loop impedance? I have no such document sadly!

A metal CU and a TT supply.

I wonder how that was done? Can you show us where the tails enter the CU? Absoloutely, i'll take a cover off pic of the CU tomorrow morning.

Thanks for this.

Replies above.

P.S, we're only assuming this is a TT supply based on what we can see, it may not be the case but the loop reading would seem to indicate it he says.

 

[EFLImpudence]

The CU doesn't look like metal (at least the door isn't) and there's no reason why it would have been five years ago.

 

[Muzza80]

The CU doesn't look like metal (at least the door isn't) and there's no reason why it would have been five years ago.

It's this one Wylex NHRS6606, metal for sure.

http://4841-nhrs6606.aboutsemi.com/

 

[EFLImpudence]

Oh ok.

 

[JohnW2]

The CU doesn't look like metal (at least the door isn't) and there's no reason why it would have been five years ago.

It is metal bodiesd but, with that plastic front cover, I presume it would not be Amd3 compliant (which, of course, it would not have needed to be 5 years ago).

Kind Regards, John
Edit: typo corrected

 

[John D v2.0]

If it is TT and the rod is no good, there's nothing stopping the electrician putting in a new one or two externally.
The tails do look a bit odd though, I wonder if this is a submain?

 

[333rocky333]

The cables coming out the hole look like Twin and earth or swa, i wonder if theres an earth connection behind that wooden boarding

 

[ban-all-sheds]

the downstairs is fitted with laminate throughout, including the cupboard with the CU

Always a mistake.

 

[Muzza80]

Always a mistake.

Can't take credit for that one, we only moved in a year or so ago and it was already like that.


So.... here's some pics of the CU open, and one pic of one of the "mains" tails next to a core from a 10mm 3-core SWA (for size, looks like 16mm to me?).

Cheers

 

[Muzza80]

Ok, so a little update.

I've spoken to the previous owner and they've forwarded me the two certificates of installation and it seems that actually the supply is TN-C-S, confirmed on both.

The house was fully re-wired in 2012 and this electrician tested it all and fitted a new CU, here is the test report (the 1st page) and the second is after a garden room extension was put on the back of the house and the lights and sockets rings were extended into there.

So I guess behind that board (the CU is on) then, somewhere, there should be another connection to the suppliers neutral in order to get the earth is that correct?

 

[John D v2.0]

That cert is for another install! It says 25mm tails and tn-c-s neither of which match your photos.