TN-S installation not earthed!

[homespark]

Hi- I was round at a friend's house, checking the boiler in their basement while they were away and noticed a big old ugly main incomer feeding their consumer unit. The incomer's covered in a kind of hardened cloth tape and below that, where the MET clamp is connected, it seems to be have a spiral wrapping of steel.
Just by chance, as I had my trusty meter with me, I happened to check the EFLI and found it to be around 14 ohms. This was without isolating the system (didn't want to trigger the alarm system or anything) and without disconnecting the main equipotential bonding.
Concerned by this, I then went to various sockets in the house and checked the Zs. It read over 14 ohms on each one, with a PFC of only 16A. Not good, clearly!

Anyway, to cut to the chase, they are now back from holiday and a couple of days ago I went back to make a proper EFLI check, this time switching off at the main switch and disconnecting the main earth (and connecting my meter green lead to this). I got a reading of >2000 ohms on the meter (in fact, it couldn't carry out the test), i.e. no earth at all! When I reconnected the earth to the bar, I got the 14 ohm reading again.

My conclusion is that the only earthing they have is coming via the gas/water pipework and that the earth on the supply cable is not actually connected to earth at the transformer. It has been this way for years, it seems. Of course, I impressed upon them the need to call the LEB immediately, and hopefully they'll have that fixed by now.

It is shocking (!) that whoever made the connection from the CU to the supply cable never actually tested to see if it was indeed an earth. Luckily, they have had had no problems (no-one has died and house hasn't burned down). But I wonder how many homes in the land are similarly unprotected?

Finally, as this house is on the same street as mine, where there is PME, should it be a straight-forward job to change their system to PME/TN-C-S by simply making a link at the neutral to the earth bar? Obviously, this is a job for the LEB, but I am curious.

 

[JohnD]

I know someone with a similar old London house, their "earth" was a pipe-clamp to the lead sheathing inside the spiral steel armour; EDF agreeed to sweat on a new earth terminal to the lead sheath for £60 but wanted an electrician to inspect the installation and certify it as safe first. They wouldn't provide PME, I don't understand why. The service head had been changed from cast iron to a plastic one within the last ten years.

You can se it (not very clear) here. The spiral armour is stripped back a bit.

 

[slippyr4]

Or if they're not feeling helpful, mightn't they just redesignate the supply as TT?

 

[JohnD]

As it's in a built-up area, in a street where the newer houses are PME, I can't see why that wouldn't be the easiest solution.

 

[plugwash]

i thought once they had supplied an earth once they had an obligation to maintain it if they can safely do so.

 

[slippyr4]

As it's in a built-up area, in a street where the newer houses are PME, I can't see why that wouldn't be the easiest solution.

It'd be easiest for them - they'd have to do nothing.

 

[Lectrician]

They should find the fault on the service cable. It isn't that it was never connected, it is that a joint has has failed or the cable sheath is rotting. It could be that some muppet jointer didn't earth the old sheath when replacing a length of cable of providing a new service to a house or something.

They could easily PME it, but in my area (WPD) they prefer to sort the problem on the TN-S rather than PME the supply.

In any case, the sheath needs to be earthed to provide protection to the cable - otherwise that cable could become live under fault conditions on the network. They may just PME the head, and connect this to the sheath.....but it stinks of cowboys. I am glad I work in an area where the engineers and managers at the DNO take pride in their work.

 

[homespark]

Thanks for those replies, gents. I'll be seeing my friend tonight and will ask what's been done.

Even a TT system shouldn't be as bad as that. I believe 8 ohms is the max Ze?

There is, inside his basement, a huge joint that has been made (what do you call these things, torpedos?) some time in the past: perhaps that's where the fault is, or else further upstream.

John D, the supply cable you mention sounds v similar. If so, is the outer spiral steel wrap meant to be the earth connection, or is there an inner lead sheath that serves this purpose? Mind you, if they're touching then they'll both be at the same potential...

But I agree, some folks have no pride in their work, yee haw

 

[JohnD]

On this one, there is an inner lead sheath, about the size of a smallish finger. The steel spiral is for armouring not earthing and perhaps an inch across. I have an idea there might have been tar between them.

 

[homespark]

So what would be the standard method of making an earth connection to this type of cable? (not that I'll be doing that, thankfully!)

 

[Lectrician]

Remove oil impregnated cloth from outer.

Cut steel band armour and remove a section of it.

Remove tar with blow torch.

Sweat a braid to the sheath with blow torch and lead solder.

Re-cover with tar usually.



If not in the WPD area, they seem to simply wack a BS951 clamp onto the steel amour.

 

[dingbat]

Here in Leeds the absolute lack of a principal means of earthing is the norm, rather than the exception. (At least 80% of the jobs I see)


The only feedback the DNO (YEDL) gets is via people like me, reporting the absence of earthing on every job. Makes you wonder how many untested jobs remain totally unprotected by EEBAD?

 

[securespark]

Remove oil impregnated cloth from outer.

Cut steel band armour and remove a section of it.

Remove tar with blow torch.

Sweat a braid to the sheath with blow torch and lead solder.

Re-cover with tar usually.



If not in the WPD area, they seem to simply wack a BS951 clamp onto the steel amour.

BUT DON'T DO THIS AT HOME, CHILDREN!

Clamps have been used in the past and are accepted, but new earth connection to TNS sheath should not be clamped any more.

There are many properties in Staffs with TNS but no earth from sheath, They are designated TT, and have rods fitted (mostly!).

You should "AT RISK" that installation, as the earth connection is coming from the gas or water pipes and is not allowable. Gas, never, Water since '66.

TT has max of 200 Ohm, or 100 if NIC.

But strictly speaking, an RCD which you should have on a TT anyway will disconnect safely right up to 1666 Ohm.

 

[plugwash]

But strictly speaking, an RCD which you should have on a TT anyway will disconnect safely right up to 1666 Ohm.

that depends on the RCD,

iirc the formula is R=V/I where V is the maximum shock voltage that may be allowed to remain (iirc 50 normally and 25 on agricultural installs) and I is the trip point of the rcd.

50/0.03=1666 ohms but 30ma incomers are frowned upon (and i belive disallowed on installs with mains smoke alarms).

repeating for other usual sizes of RCD gives us the following maximums

50/0.1=500 ohms
50/0.3=166 ohms
50/0.5=100 ohms

however the 200 ohm (100 if niceic) reccomendation (i think its a reccomendation rather than a requirement) comes from the fact that earth connections above that resistance are considered potentially unreliable.

 

[homespark]

You should "AT RISK" that installation, as the earth connection is coming from the gas or water pipes and is not allowable. Gas, never, Water since '66.

I believe my friend has been in contact with the DNO and they're going to look into it and probably charge him something for whatever they do. Will find out more later. As I was not doing electrical work or carrying out a PIR, I will not be reporting the matter myself (not yet DI certified anyway, but hopefully next month ).

However, is the danger not from the low PFC? At 14 ohms Zs on his ring, the PFC is 16A, which means (correct me if I'm wrong) that in the event of an earth fault (exposed metalic part becoming live or human in direct contact), the maximum fault current would be 16A which would not trip his 32A protection device for that circuit, causing potential fire or electrocution. So how does that work for a TT installation with even higher resistance?