tn-s system

[clarke24]

I am about to re-wire a property but the property has an incoming earth that appears to be soldered to the armouring of the supply cable, the problem is it is only approx. 2.5mm^2, can I cut this cable back to the amour and then add a new cable connected to an earth clamp clamped to the incoming supply armour.
I have to contact the electricity supplier before i start work anyway because the meter tails need to be upgraded and the client would like the meter to be moved to fit the new consumer unit into the existing cupboard, but I just thought i'd make sure I could definatley do what I propose to do before contacting them.

 

[ban-all-sheds]

AFAIK you aren't supposed to do that as the clamp can crush the cable and damage it.

Also - check if the DNO has any special requirements for main bonding conductors in case they convert it to PME - I'm sure I read here that they are doing this in some places because the buried armour is deteriorating to the point where its integrity as a conductor can no longer be relied on.

 

[plugwash]

id say do the main bonding in 16mm (which some recs reccomend for PME) and then ask the rec to do the earth upgrade at the same time as the tails (they will likely wan't to do a pme rather than replaceing the cable to the lead sheath)

 

[Paul_C]

id say do the main bonding in 16mm (which some recs reccomend for PME) and then ask the rec to do the earth upgrade at the same time as the tails

Agreed. Run your new main earth and let the PoCo connect when they do the changeover.

The PoCo can sometimes be very unsure of what system will be used themselves though. Last year I did a substantial rewire in a High St. shop, which involved the removal of a partition wall and the relocation of the service and meter to a front wall. As it was an old TN-S system with an armored cable such as you have, I inquired what the new system would be, and was told "Definitely PME." After wiring accordingly, the conversion was completed and connected back as TN-S.

Still, better than using the minimum sizes expecting TN-S and then having to change to suit PME, I suppose!

 

[clarke24]

Thanks for the advice didnt know about the earth clamp problem though I must admit, been to lots of properties with that kind of arrangment, the reason not to do it does make sense though, thanks again.

 

[ban-all-sheds]

id say do the main bonding in 16mm (which some recs reccomend for PME) and then ask the rec to do the earth upgrade at the same time as the tails (they will likely wan't to do a pme rather than replaceing the cable to the lead sheath)

Since the main bonding is downstream of the meter, and therefore not the DNO's property, and since BS7671, and therefore the Building Regulations don't demand 16mm², only 10mm², can they do more than "recommend"? Can they demand, refuse to connect unless?

 

[plugwash]

im not entirely sure what power they have legally but im not entirely sure that thier people on the ground are sure of that either from reports i have seen.

you have 3 choices

1: keep the rec happy.
2: live without grid power.
3: go to court

since you probablly don't want the hassle of 3. 1 starts to look like the best option.

 

[ban-all-sheds]

It's just that my main bonding for the water pipe, technically, doesn't run from where it should - not close enough to the point of entry. If I start it at the correct place, the only route for the cable is up the wall to the first floor, under the bathroom floor, the landing floor and then down into t'cupboard under t'stairs.

Before the bathroom floor goes down, I was going to leave a coil of cable under there that could be dropped down the wall in next year's kitchen refurb. I've got a reel of 10mm² but not 16...

Ah well - guess I'll have to use the Edward Kennedy method of decision making.....

 

[BruisedThumb]

AFAIK you aren't supposed to do that as the clamp can crush the cable and damage it.

The NICEIC 'Snags and Solutions' book on Earthing and Bonding deals with this, it's snag #8. It says that 'Under no circumstances should an earthing clamp (complying with BS 951 or any other type) be attached to the lead sheath ... ' (my emphasis). The illustration shows the sort of clamp you use for bonding a water pipe where the operating bolt imposes a high point load through the tape on to the pipe.

There seems to be some difference between what this book says and the hardware out there in the field.

The main earth connection in my house is made via a clamp on the steel tape armour of the supply cable, it's positioned immediately under the Company fuse.

The clamp looked a bit tired so I queried it with the supply company, expecting that they'd change it. Their bloke said 'That's OK, that's our standard type of clamp.' or words to that effect.

The thing is that this clamp isn't the same sort illustrated in 'Snags and Solutions'. Mine comprises two saddle straps, two shaped brass components and two tension screws. The cable(s) are held against the armour by the brass bits and the saddles bear evenly on the armour all round the cable. (Afraid I don't have a photo at the moment but I could get one if folks are interested.)

Neither is 'my' cable lead-sheathed.

The facing page of 'Snags and Solutions' talks about lead sheathed cables and steel wire armoured cables but not about steel tape armour.

Should I conclude that my supply company knows more about the cable that feeds my house than does the NICEIC?

 

[plugwash]

if its steel and plastic then as long as the clamp is tight it should be ok

the problem with clamps on lead and paper is they can crush the relatively soft cable and the fort metal also tends to move which can make a bad contact

having said that clamps on lead and paper cable are extremely common. And you would think that if they really caused problems you would hear a lot more about it.