TN-S Main Earth

[stummish]

Hi,

I have just completed an EICR on a house my In-laws have just bought (I actually get on very well with them, so I'd like them to be safe)

The EICR was an Unsatisfactory for various other reasons, some of which I will be solving with a CU swap. my main problem is with the Main Earth.

I'm assuming the cut-out is a 63 or 80 amp bs88, I'll call them tomorrow to confirm.

It is a TN-S supply, 0.21 Ze, so clearly not a TT that has had a clamp put on the sheath.

The Main Earth is what looks like 2.5mm² uninsulated stranded steel(?) that is clamped twice (not a bs951, the dno type) onto the outer sheath and another on an inner sheath (unsure of how supply cables are made, but this is the cloth and tar wrapped type, not a modern swa style).

Line and neutral are 16mm² to the meter, 25mm² to the CU (smart meter fitted <5 years I'd guess).

I plan on having an isolator fitted so I can do the CU swap.

So my questions are;

The adiabatic equation tells me it should be a 10mm² minimum. Being that it is a TN-S supply it is their duty to maintain an existing connection IIRC. Should the DNO upgrade this earth for this reason alone?

Who should pay if they upgrade the existing TN-S? Northern Powergrid or customer?

If at all possible to do, would they charge for conversion to TN-C-S and thus provide an earth anyway?

And does anyone have any idea on Northern Powergrid's charge for an isolator to be fitted?

I'm not sure how I've never come across this problem before, I've worked on plenty of CUs with TN-S and TT supplies, but never one in this state.

Thanks for any help

 

[Darkwood]

Welcome to the forum

My overall question is why you want to change the supply from a TNS to a TNCS ????

 

[stummish]

Thanks for the reply darkwood, and the welcome.

I wasn't clear on that point, I'm not specifically after a conversion to tn-c-s. The only time I've ever had the dno out to do work on the service head was when I found an old cast iron cutout and no met, even though the dno said it was a tns and I'd did have an earth. They came out next day and cut the tns cable short and swapped it to a tncs. In the process they upgraded the tails and obviously supplied a MET with an appropriately sized main earth.

Even if someone said it's not the dno to provide an earth in this situation I'll be happy to do a TT. I'm not after a specific supply.

 

[Octopus]

There are countless home with a "small" cable from the TNS head to the board. As you have a decent reading I would fit a MET close to the head, then run a 16mm cable from the MET to the CU.

Tin hat firmly on.

 

[YoungScud]

I had a look at NP's website and can't find any mention of charges for fitting an isolator. It's probably the MOP who will install it.

 

[stummish]

Thanks murdoch. Would you contact the dno about the size of their main earth? Obviously the ze is excellent, my concern is with the pfc and the (my opinion) undersized earth.

 

[amlu]

had a pipe clamp type on one of the jobs, quite a loose one too.
called for DNO, they came after couple of days and replaced it with their own device.
Free of charge.

 

[Richard Burns]

Maintaining an existing earth is the responsibility of the DNO and they should not charge to maintain this.
Modifications or upgrades to the earthing system may or may not be charged depending on the network situation.

For an installation with an 80A BS88-3 and a Ze of 0.21Ω the fault current would be 1095A causing disconnection in about 0.12s so on an open cable the the csa would need to be 2.66mm² copper equivalent, this is fairly close to the value you guess of 2.5mm² which may mean that the earth is sufficient for purpose. If this is the case then the DNO could well not change it.

An isolator cost is very variable but I would expect less than £70 but this is only based on my area which is not northern power grid, the last one I did they provided free of charge because it was asked for, however they told me that unless it was requested they would not fit one. however I think other areas say they will not fit one at all. So in that case you provide an isolator from them to terminate the tails into.