TN-S inadequate earthing conductor

[1Justin]

Hi.

I have come across this ~3mm dia earthing conductor soldered to the PE sheath on a TN_S system. Previous electrician replaced the CU and bonding but left this as is. I'm sure it should not have been left this way.

Incoming cable looks like a lead sheath and I gather it might be paper insulated and dodgy to disturb? (perhaps this is why it was left alone).

I'll be needing to take over the installation, and importantly to move/rotate the CU. (Board needs rotating 90 because a wall disappeared-, but also shifted perhaps 20 cm). Can I safely assuming this inadequate TN-S earthing conductor is the property of DNO and I should get them to fix it before I risk moving anything about?

Before anyone asks, in picture 2: The 10mm cable tied to the distribution cable is main protective bonding (gas/water). It just happens to be cable tied here. The 16mm running towards us is Main Earthing Conductor to CU. The 6mm in the middle is old & disused (which previous electrician didn't remove).

I have made no measurements yet.

Justin

 

[telectrix]

that bare earthing conductor is old DNO equipment and should be upgraded by them. you are right not wanting to touch it. telephone time.unk:

 

[GLENNSPARK]

whats the condition of the cutout here?....any visible damage.....all an argument in your favour.....as tel has said....give the DNO a ring.....ask if PME is available here.......if/when they agree to provide a PME link...just be aware that any outbuilding/s fed from this supply will need to be checked for extraneous-conductive parts as you may need to extend the equepotential zone......

 

[Deleted member 26818]

What is the PEFC and the rating of the cut out, does the CSA of the earthing conductor satisfy the adiabatic equation?

 

[Knobhead]

There’ll never be a better connection to the lead than that. OK it’s undersized by modern standards but it’s so short it redeems it’s self. Get the DNO involved and expect to pay through the nose.

Other’s that know me on here, know what I would do with it. But I’m keeping quiet about constant pressure springs!

 

[1Justin]

Hi Guys.
I agree Tony the impedance will be small, and the huge soldered collar is undoubtedly good. From discussions about paper insulation, I'm not sure if I dare flex it though and the whole board needs to be moved - at least a little bit.

Spinlondon:
Taking you up of the adiabaitc eqn.
.. (I didn't have my vernier at the time, but the cable is ~2mm dia from photos, so say CSA =pi (convenient)

Previous electrician quoted 2100A as PFC (presumably measured to include this short earth conductor)
Supply fuse is 100A (BS88?) So table 3A1. Assuming earth fault at MET, then table 3A1 doesn't go that far.
if I assume worst case (no interpolation) then disconnect time is 0.1 seconds.

K = 159 (For Cu), 58 (for steel) (I think it must be tinned Cu or it wouldn't be soldered). So say 159 .

S = (SQRT(2100^2*0.1)/159 gives 4.2 mmsq for Cu (and I didn't interpolate table 3A1: probable fail if CSA of conductor is 3.14MM^2 (estimate: better measure it)

if I interpolate table 3A1 (is this allowed?) then t =0.02 sec and S = (SQRT(2100^2*0.02)/159 gives 1.8 mmsq (That's very small!).

(BTW are those sensible numbers - regardless of the conclusion?)

It sounds a bit close to me, and anyway I can't verify PFC at least till my tester arrives..(soon!)

DNO tomorrow then.

If I go PME, the I'll need to check that shed. Thanks

 

[ringer]

How will you know you have not induced faults into the installation with the work you are proposing to do? Why are you even considering doing work on this when you do not have a tester? Surely the first step is to take measurements of the existing installation. If you do work on this (or any other) installation and then obtain a tester and then do testing, you are going to run into all sorts of problems. Why do you not have a tester? The first thing the DNO are going to want to know when you call them is your test results.

 

[1Justin]

Hi Ringer.

Please don't jump to conclusions. I appreciate your concern. I want to subscribe to caution here as well which is why I'm on this nice forum!

I am actually looking for ideally some more "introductory" jobs to get signed up for Part P with NICEIC. A local builder friend heard I was looking though and introduced me to this job he's working on. Turns out to be a bit untidy. I am prepared to rise to the challenge - which is why I DO hold a copy of all the existing (previous) measurements and why I will be verifying these before strting to touch anything here.

I need to understand what I'm up against, but wouldn't turn it away without due consideration. This forum is invaluable and so far has echo'd my thoughts. Hey I'm already learning stuff and I've only got my knees dirty so far.

My 1720 arrives probably tomorrow (something of a delay as I expected it last week..), so I'll not be without a tester!

Rgds

 

[HandySparks]

Welcome to the forum 1Justin. Nice to see clear photos to illustrate the question. I always have a decent camera with me, rather than relying on the grotty one in my phone, and use it a lot. (You'll probably tell me now that they were taken with your phone!)

 

[Deleted member 26818]

A couple of things you should be aware of:
It is very very rare to find a TN-S supply with a 100A fuse. The carrier is rated up to 100A, but it is most likely to be 60A or 63A. Occasionally you may find an 80A fuse. This is because the Ze is generally too high to allow a disconnection time of 5s.
PFC is the highest value of either PSCC or PEFC. With PEFC being the required value, and with TN-S systems PEFC is invariably lower than PSCC.
Lastly, you are using 159 for k, with this type of conductor 143 is the norm. Have you corrected for a lower temperature?

 

[1Justin]

Hi HandySparks.
No those are from my old Canon. Like you my phone isn't up to much (but some do have optical focus and macro - worth looking out for!).

Hi Spinlondon.
I assumed PEFC because this is the earth conductor, surely PSCC would not onvolve this particular conductor anyway?
And I pulled K159 off table 54.6 which is specified for bare conductors (and no risk of damage). (Was that right?). Seems sound to me. Doesn't 143 value come from 54.2 for insulated cable?

In fact it's probably rather academic because unless I were to interpolate table 3A1 it's all too close (or a definite fail) and I guess they stop table 3A1 at 1KA because fuses aren't characterized beyond so interpolation is not on. Either way it's interesting to know in principle if this is the right theoretical approach, even though it looks like we'll be getting this fixed properly.

Ok, and a big box from megger arrived today.

 

[Deleted member 26818]

Yes PEFC, but you stated the previous electrician had recorded PFC.
PFC is the the highest of either PEFC or PSCC.
As such using the value recorded for PFC when sizing an earth conductor, would with TN-S produce an inflated CSA.
Yes you are correct, a bare conductor, not in contact with any other conductor, would be 159 for k.
When using the table s in Appendix 3, you can use the quoted values, or the graphs, which is what I take your iterpolation to mean.
Also if using the quoted values, you should not mix and match.
If you have a PEFC value that falls between the quoted values, you should use the lower of the two, with the corresponding time.
Using the measured or calculated value for PEFC with a time that is slower than would actually be achieved, would again produce an inflated CSA.
Another thing, I just realised, is that you are using a table for a fuse standard that was introduced in 2007. It is unlikely that a TN-S installation uses such a modern fuse.

 

[1Justin]

Hi Spinlondon,

Yes PFC was recorded at 2.1KA. So it's highly likely the figure is PSCC. So I agree this would be wrong value to assume when working out CSA and would provide inflated value for the required CSA.

I was interpolating the graphs up to 2.1KA (not using the table in the corner of the plot), and I was not sure that was safe. But it's irellevant since 2.1 KA (by the point you made) is most likely invalid anyway for PSCC.

PSCC (if measured) might fall within the range of the plot or table.

And now it's also clear that if I had an accurate measurement of PSCC, then I'd be relying (as you say) on a table which might not be valid for the old fuse anyway.

So that's food for thought. I have not been back to the place yet, but urgently need to get going. I can measure the Ze now I have the Megger, so that will be a priority ASAP. I am pretty sure the previous elecrician quoted the standard value from DNO (0.12R Ze is stated in the report front page under "supply characteristics" and I believe this is by enquiry).

This is very useful discussion for me.

Rgds

 

[Deleted member 26818]

The standard value of Ze for TN-S is 0.8Ω.
If the actual value is 1.2Ω, then you are looking at a PEFC of around 190 - 210A.
There is no way a DNO would allow a 100A supply to this installation, unless earth fault protection was provided by an RCD.
Without RCD protection, you'd be lucky to get 40A.

 

[ringer]

The standard value of Ze for TN-S is 0.8Ω.
If the actual value is 1.2Ω, then you are looking at a PEFC of around 190 - 210A.
There is no way a DNO would allow a 100A supply to this installation, unless earth fault protection was provided by an RCD.
Without RCD protection, you'd be lucky to get 40A.

Hey, Spin, he quoted 0.12ohms, which would be theoretical PEFC of 1.9kA, giving disconnect time of well under 0.1s