Curious DNO MET on TT System

JB1983 · Nov 29, 2021

Just completed an EICR which had an Unsatisfactory Report done earlier in the year by another Spark, i have a copy of the report and he's put the earthing system down as a TN-S and a ZE of 0.21. However as you can see the main earthing conductor (16mm) doesn't go to what appears to be a DNO MET, I presume it goes to an earth rod hence the Ze reading I got of 28.2 Ohms and a Zdb reading of 16.2 Ohms. All of my ELIT readings are around the 19 Ohm mark though. I did try a Ze from the DNO's block and got 30 odd Ohms so not sure what it's there for? Any Thoughts? I'm unable to lift the tiled floor in the porch to confirm if an earth rod is installed however from previous experience similar age houses in the same are are TT'd.

[ElectriciansForums.net] Curious DNO MET on TT System

telectrix · Nov 29, 2021

sounds like a TT . never get reading that high on a TN unless there's a problem.

timhoward · Nov 29, 2021

I can't seem to expand the 2nd picture which is the important one.
Also, just wondering, is there an up front RCD or RCD main switch on the consumer unit? It looks like either a dual RCD / split load board. Some people have been known to get twitchy about single insulated link cables prior to the RCD protection on a TT setup, rightly or wrongly.

Mikegh · Nov 29, 2021

Not too well up on the UK stuff

So doesn't look like TN anyhow from the pics , dunno why the external fault loop is 30ohm there on the DNO ? MET

As above I suppose , is there upfront RCD on the board ?

JB1983 · Nov 29, 2021

timhoward said:
I can't seem to expand the 2nd picture which is the important one.
Also, just wondering, is there an up front RCD or RCD main switch on the consumer unit? It looks like either a dual RCD / split load board. Some people have been known to get twitchy about single insulated link cables prior to the RCD protection on a TT setup, rightly or wrongly.

The board was set up as a split load board with 2 Non-RCD protected lighting circuits, No Up-Front RCD, I've moved the two lighting circuits to the RCD protected side.

(The abandoned cables are for cooker and shower which are not being utilised and must be coiled or JB'd somewhere. Also the smoke detector circuit which had the CPC cut out of it for some strange reason, battery ones now installed).

The worrying this is the landlord seems to think it was rewired 12ish years ago meaning the two lighting circuits have had no RCD protection on a TT system for that length of time, the PEFC was 9 amps meaning a direct earth fault would take around half an hour to trip.

JB1983 · Nov 29, 2021

timhoward · Nov 29, 2021

I see what you mean. It looks as though it had an effective TNS setup once.
You could take a length of 2nd hand nicely faded 16mm earth, connect it in addition between that block and the CU and then call the DNO reporting a high Ze. You have evidence they supplied an earth once, and evidence it was once effective (the EICR paperwork).
Chances are it will find itself being changed to TNCS before you can say 'earth rod'.

buzzlightyear · Nov 29, 2021

Mikegh said:
So doesn't look like TN anyhow from the pics , dunno why the external fault loop is 30ohm there on the DNO ? MET

As above I suppose , is there upfront RCD on the board

there you go mikegh .

RCDs, metal consumer units and TT installations | NAPIT

NAPIT's Bill Allan answers another common contractor query. Contractors sometimes ask whether it's necessary to install an RCD in a TT installation before a metal consumer unit (i.e. a 'front end' RCD

professional-electrician.com

UNG · Nov 30, 2021

I'd call the DNO it's not that uncommon to find what you have found these days because an electrician CBA to call the DNO and thinks it's easier to knock an earth rod in

Strima · Nov 30, 2021

I'd try a Ze from the block below the meter, looks like a TN just never had the earth connected.

timhoward · Nov 30, 2021

Strima said:
I'd try a Ze from the block below the meter, looks like a TN just never had the earth connected.

I think he did that and that is what he says is giving 30 ohms! (Can't quote top post for some reason.)

I also think the consumer unit as it stands presents an interesting conundrum regarding regs. As far as I can see it wouldn't have complied with 17th or Amendment 3 without an up front RCD due to the single insulated internal links before the RCD input terminals. Strangely it may comply with 18th in that regard as the revised reg regarding internal links specifically applies to metal consumer units. But other factors make it obvious it wasn't installed to 18th. My point being I can't see a moment in time when it was ok to install exactly that setup.
I'm weighing up whether I'd just move the tails to the top of the RCD and dispense with the main switch completely as to my thinking that would have complied once.

As above, I'd be inclined to contact the DNO saying the TNS earthing is giving a Ze of 30 ohms and the house has no visible means of providing a main earth.