Flat Supply - Dubious earths?

[JohnW2]

I know my employers policy and procedures and what is required on the type of incoming cable shown to supply a PME earth.

I rather assumed all that, but was really 'ahead of you'. I was really asking (basically just out of interest) what aspects of "the type of incoming shown" mean that expensive work would be required to provide a PME earth, and (at least in general terms) what that expensive work would consist of. ... or is that a secret?

Kind Regards, John

 

[securespark]

Tying the neutral down to earth in several places, I'd guess.

This is the reason DNO's often won't PME TT supplies. It'd cost them a small fortune.

 

[mfarrow]

Back to the OP - where do the copper braids go to from the others' cutoffs then? They should go to the DNO earth even if via a conduit connection (which is fine to be used as a protective conductor).

As for the rest of it - the immediate concern is the fabric-covered rubber singles going from the cutoff and meter to upstairs. They will likely crumble internally when touched, but might last several years or decades if not, and pass an insulation resistance test fine.

The difficulty will come though if you, the landlord, or some electrician making a mistake, moves the cables or throws the switch which then breaks. This is not unlikely given the communal setting and the age of the cutoff switch, the mechanism in which will contain ceramic or early plastics. Even replacing the fuse (if it has one) could lead to a shattered fuse holder and an inconveniently expensive call-out charge, during which that old cable will need to be replaced.

So my advice would be, in iteration of others, to get an EICR done and replace the fabric braided old cables as a matter of course, regardless of the outcome from the EICR.

 

[GeeTee]

Back to the OP - where do the copper braids go to from the others' cutoffs then? They should go to the DNO earth even if via a conduit connection (which is fine to be used as a protective conductor).

Yes, they go directly from the cutoffs to the DNO earth block.

As for the rest of it - the immediate concern is the fabric-covered rubber singles going from the cutoff and meter to upstairs. They will likely crumble internally when touched, but might last several years or decades if not, and pass an insulation resistance test fine.

The difficulty will come though if you, the landlord, or some electrician making a mistake, moves the cables or throws the switch which then breaks. This is not unlikely given the communal setting and the age of the cutoff switch, the mechanism in which will contain ceramic or early plastics. Even replacing the fuse (if it has one) could lead to a shattered fuse holder and an inconveniently expensive call-out charge, during which that old cable will need to be replaced.

So my advice would be, in iteration of others, to get an EICR done and replace the fabric braided old cables as a matter of course, regardless of the outcome from the EICR.

That's worrying. I've not so much as touched the cutoff for my flat (it's turned off currently), just the look of it puts me in mind of something in frankenstein's lab! However I'm moving in tomorrow so I suppose I'll have to gingerly throw the switch and hope nothing goes bang

 

[mfarrow]

Yes, they go directly from the cutoffs to the DNO earth block.

Then you need something similar.

 

[securespark]

Back to the OP -

Yup, sorry, OP, guilty as charged....

 

[westie101]

what that expensive work would consist of.

Now this may not apply to every DNO as we all have slightly different procedures! (or are others not following the standards?)

So to convert a TNS to a TNCS we would have to:

Excavate at the main and disconnect the existing PILC service cable, make a new service joint bonding the neutral to the earth and install a length of CNE cable to the existing service

Install a new length of CNE cable into the property jointed to the existing service and bonding N & E there as well

So possibly a days work for a jointing team, possible building work to get the new cable in, possible cost of re-instatement in the highway - should I go on?
All these costs would be the responsibility of the customer requesting a PME earth. And in the situation of a group of flats ensuring the bonding is up to standard in all of them

or

we can fit a new earth wire from the existing lead sheath free and if there is a problem fix it free
(though as discussed bear in mind that the size of the earth from our cable to the MET is not governed by BS7671 so a smaller historic sized cable is still acceptable to us)
You Choice!

 

[GeeTee]

Yes, they go directly from the cutoffs to the DNO earth block.

Then you need something similar.

Thanks, I suspected as much. Is that to prevent the casing of the cutout becoming dangerously live in case of an internal fault on it?

Hopefully the electrician will pick up on it during the EICR - if not I'll specifically ask.

I have a feeling this is going to be an expensive month C'est la vie, better skint than fried by dodgy wiring!

 

[GeeTee]

Back to the OP -

Yup, sorry, OP, guilty as charged....

Don't worry about it - the side discussions are part of what makes this forum such an interesting place. They also serve to keep numpties like me in line by clearly demonstrating how very little knowledge we have and stop us taking on stuff we really shouldn't.

 

[mfarrow]

Thanks, I suspected as much. Is that to prevent the casing of the cutout becoming dangerously live in case of an internal fault on it?

Yes, and anything else which you have in your flat which is attached to the earth of your installation. Furthermore, it's not just limited to faults on the cutoff/cutout; anything connected to earth up in the flat, which develops an internal fault, will, by connection, make everything else dangerously live too including the cutout/cutoff.

So it's pretty important that an earth is in place, and it is effective at:

a) ensuring that in the event of any live to earth fault, that no piece of exposed metalwork becomes dangerously live.
b) that the duration of said event is limited to a short duration to prevent damage to the installation.

It is likely that the only existing 'earth', unless hidden elsewhere, is via the water main. This used to be acceptable but now would not be. I would tighten that up as my first plan of action, as at least you should get a reasonable connection to the DNO earth through that, via the other flats which have the copper braiding attached. It's not 'proper' by any means but the other way would be to attach an earth from the 8-way to the switch or up through to the flat, but I'd sooner not have you messing around near those old rubber cables.

 

[streetlighter]

The cable coming out of the bottom of the grey MEM use unit for the shower looks like either 10/16mm twin and earth is that coming from the silver metal clad Bill switch fuse where the main feed comes into the flat or does it run down back to the meter position ?

 

[JohnW2]

Tying the neutral down to earth in several places, I'd guess. This is the reason DNO's often won't PME TT supplies. It'd cost them a small fortune.

Well, for a start, I don't know how one can tell by looking at what we've seen that the supply in question hasn't already been PMEd. I know that the supply to my house has (in recent years), but I don't have a PME earth - so nothing within my house would offer a clue that it had been PMEd.

Furthermore, as you may have noticed, westie fairly recently disillusioned some of us (certainly me!) by indicating that the 'M' of PME often only means '2' - one N-E connection at the transformer and a second at the far end of the main! Prior to that, I had always assumed that PME implied real 'multiple' (far more than just 'one extra') N-E connections along the length of the main!

Kind Regards, John

 

[Iggifer]

I have TNCS and my Ze is great

I have a TN-S supply and my Ze is great. What's your point?

Well my point was that I ACTUALLY have TN-S and my Ze is great. It appears yesterday was not my day for posting what was in my head!

 

[RF Lighting]

That does make much more sense

 

[Iggifer]

Yea whoops