0.37 TNC-S Not Available

[Steve3948]

During a refurb job, it was required to re-site the existing meter position.
The original installation was connected TT with a spike in ground reading 402 Ohms.

As they were going to renew the overheads we asked for TNC-S and they said should not be a problem.
The meter is on one side of the room and we have new 17th Fuseboard on the other side of the room about 4M away.

Thinking we were getting TNC-S we installed a 63A Double Pole Mcb adjacent to meter fed with 25mm tails and 16mm Earth, and outgoing 16mm 6242Y to 17th FB.

On going back to connect we found note saying 0.37 TNC-S Not Available.

So now the 16mm Sub-main has no Rcd protection, being just over 0.35 you would have thought they would have connected.

I did a test and inspection some time back to a house on a small site that had around 12 dwellings where the reading was 0.40 assume all connected as PME, the installations were installed by the same electrical provider.

Comments anyone.

 

[JohnW2]

So now the 16mm Sub-main has no Rcd protection, being just over 0.35 you would have thought they would have connected.

As I would say to someone moaning because they had only fractionally missed the pass mark of an exam, they obviously have to draw a line somewhere (essentially arbitrarily); whether they've drawn it in the right place is obviously a matter for debate - and, to some extent, personal opinion.

Kind Regards, John.

 

[EngStudent]

So now the 16mm Sub-main has no Rcd protection, being just over 0.35 you would have thought they would have connected.

As I would say to someone moaning because they had only fractionally missed the pass mark of an exam, they obviously have to draw a line somewhere (essentially arbitrarily); whether they've drawn it in the right place is obviously a matter for debate - and, to some extent, personal opinion.

Kind Regards, John.

That and the person in charge of the job is responsible. If s/he's been told 'no TN-C-S above 0.35' then s/he's getting into trouble if management ever find out.

As in all walks of life some will bend the rules a little, some will do everything exactly by the book. At the end of the day, it's the engineer's job and we know how strict these companies can be.

 

[Steve3948]

So now the 16mm Sub-main has no Rcd protection, being just over 0.35 you would have thought they would have connected.

As I would say to someone moaning because they had only fractionally missed the pass mark of an exam, they obviously have to draw a line somewhere (essentially arbitrarily); whether they've drawn it in the right place is obviously a matter for debate - and, to some extent, personal opinion.

Kind Regards, John.

John I know what you mean, bit like being just over the drink/drive limit and everything else we have rules for like 1 Hour Parking not 1Hr 2Min.

Suppose I need to fit 100mA Rcd now at my own cost

 

[westie101]

TNC-S is quite acceptable up to 0.8, as that is the minimum value to operate the 100A cut-out fuse for a fault before the CU, 0.35 was not really a calculated figure but decided upon as it would be less than 0.8 owing to the design of a PME system.
The decision will be based on the Code of Practice of the DNO, we are allowed to use the higher figure

 

[Steve3948]

TNC-S is quite acceptable up to 0.8, as that is the minimum value to operate the 100A cut-out fuse for a fault before the CU, 0.35 was not really a calculated figure but decided upon as it would be less than 0.8 owing to the design of a PME system.
The decision will be based on the Code of Practice of the DNO, we are allowed to use the higher figure

Westie,
You saying I should connect then, they did leave the knockout open in the side which I though odd.

 

[westie101]

Were you within my DNO area yes I would authorise a connection.
Sorry can't do that elsewhere other than suggest there are no issues that I know of!

I'm assuming you have RCD protection at the CU so that is a plus (if we come across a high ELI the first thing we do is fit an RCD).

 

[JohnW2]

John I know what you mean, bit like being just over the drink/drive limit and everything else we have rules for like 1 Hour Parking not 1Hr 2Min.

The problem, of course, is the 'endless creep'. If it becomes known that someone was allowed to get away with 1hr 2min, then the next one who stayed for 1hr 3min will try to argue that's only slightly more than 1hr 2min, and if (s)he is allowed to get away with that, then the next one comes along with 1hr 4min (citing the 1hr 3 min case) ... and so on, ad infinitum!

Suppose I need to fit 100mA Rcd now at my own cost

Maybe, and I guess you'll subsequently find a use for the MCB, although I can't see anyone but you (least of all the DNO,who would say you should have waited for the TN-C-S before buying and installing the MCB) paying for your additional time! Mind you, I see that others are suggesting other possible courses.

Kind Regards, John.

 

[westie101]

The other question would be who connected it.
If the supplier it is more than likely the meter operative was not aware if the DNO has a policy so a check with them might be worthwhile. As it is a modified supply the DNO is. in my view, obliged to provide an earth so needs to be consulted.

 

[westie101]

This is the policy I work to in these situations.

2.1 ENA Engineering Recommendation P23/1 gives Guidance relating to Consumers Earth Fault Protection and advice required from Public Electricity Supply Companies.

2.2 Within this document are published suggested maximum values of Earth Loop Impedance (ELI), primarily for use as a guide to designers of private installations for consideration of their protective devices. The ELI values should allow the service termination fuse to operate within 5s
in line with BS7671 and CP331.

2.3 In line with these suggested guidance notes *** has adopted the following figures as acceptable values of ELI to be measured at the incoming service termination where an earth terminal has been provided

Non PME System = 0.8 ohm

PME System = 0.35 ohm (Higher values can be accepted up to 0.8 ohm)
(The value for a PME system being lower as to be expected with the combined neutral / earth configuration)

2.4 The following actions shall be applied when high values of ELI are recorded, on both PME and non-PME systems, at the supply termination, with the customers earthed metalwork disconnected.

CAT1 When the ELI is less than or equal to 0.8 ohm leave supply on. Nevertheless, when the
ELI exceeds 0.54 ohm, the rating of the cut-out fuse should not exceed 80A.

It then goes on to give various other solutions for higher values.