Was this a TN supply please?

If the PFC was too low, then presumably the Ze was too high.

R.24 of the ESQCR 2002 makes the DNO and supplier responsible for ensuring that suitable fuses are fitted.

24.—(1) A distributor or meter operator shall ensure that each item of his equipment which is on a consumer’s premises but which is not under the control of the consumer (whether forming part of the consumer’s installation or not) is—

(a) suitable for its purpose;

(b) installed and, so far as is reasonably practicable, maintained so as to prevent danger; and

(c) protected by a suitable fusible cut-out or circuit breaker which is situated as close as is reasonably practicable to the supply terminals.

Was this a TN supply please?

If the PFC was too low, then presumably the Ze was too high.

R.24 of the ESQCR 2002 makes the DNO and supplier responsible for ensuring that suitable fuses are fitted.

24.—(1) A distributor or meter operator shall ensure that each item of his equipment which is on a consumer’s premises but which is not under the control of the consumer (whether forming part of the consumer’s installation or not) is—

(a) suitable for its purpose;

(b) installed and, so far as is reasonably practicable, maintained so as to prevent danger; and

(c) protected by a suitable fusible cut-out or circuit breaker which is situated as close as is reasonably practicable to the supply terminals.

Thanks Chris, and yes, measured Ze around 0.45 ohms in this case, with measured pfc around 430 amps. Increasing load above around 45 amps, can cause the supply voltage to drop below the legal limit too, depending on other customers loads on the supply cable. We are currently involving the DNO and service provider, but so far, the DNO say their cutout fuses work fine, (fogging us off at this stage), and I'm about to challenge that, particularly since I first made them aware of the pfc issue back in 2019 when they were on a single phase supply. As part of converting to 3 phase, they first tried it as a PME system (TNC-S), then seem to have given up and connected as TNS, though at this stage, I'm not convinced that they removed an earth rod, installed when they moved to PME. Currently too, they seem to have a mix of SNE and PME customers fed off the same service cable, but I'm currently making further enquiries as I thought this was not allowed either. As for the fire, the new meter was mostly ash in the only photo that I've seen, and they took all the potentially forensic material away with them before I got involved. 

How do you go from TN-C-S to TN-S?

I must say that my experience with the DNO (SSEN in England) was very good. I reported an elevated Ze at (IIRC) 0.85 Ω and they came round straightaway. They considered down-rating my main fuse, or even pulling it altogether, but just came back later on to change me to TN-C-S.

Well there is nothing stopping you having a local  earth rod on TNS or TNC-s actually in the days of  lead sheathed paper insulated cable the whole cable was one 'earth rod' laying sideways and many hundreds of yards long per substation.

The clear-cut distinction that exists in the wiring regs between TNCs, and TNS is not really evident in DNO land, where after some repairs something more like TNCSCSSSCCS !!   might be a more accurate description, as when repairing or adding to existing TNS systems the practice in some areas is to bond N and E at every junction. even where previously they were separate.  This gives the lowest on load voltage drop, lowest earth loop readings and costs very little to do, if the road is already dug up. And cynically, it may keep old cable going for a few more years.

Hence the more accurate abbreviations SNE and CNE that describe either the cable types, or the presentation of the supply at the service head. For some time now nearly all new work has been 3 core plus armour - so CNE.

Unless you have a private transformer, or know the exact history of your local substation and everything branched from it, it is better to consider all TN supplies as PME-like even if they are not and this is the latest advice.

Now after that slight diversion, to the matter of your low PSSC.

It may not be possible to measure now, but if it is, it would be useful to know how different the L-N loop and L-E loop impedances actually are - it is a far more sure-fire means to identify where the NE bond is.

As regards DNO responsibility, you may find this letter informative, from a well know UK DNO to the customer of another forum member a few years back, where basically the DNO is saying 'our earth has failed on the old lead covered cable, it is not our problem, convert your wiring to TT'.

Now I am not sure if this is still their policy, but certainly at that time they were not that interested in doing anything that looked like maintenance or repair, if they could get the customer to pay to work around it instead. This may appear to you to be rather slopey shouldered - it did to me, but in the end what can you do...
Of course your circumstances may be better.

I am aware of installations that are TNS, or at least present as such, where the only way to meet prompt disconnections is to have a RCD or earth fault relay near the incomer. This may be one of those.

Mike.

mapj1 said:

As regards DNO responsibility, you may find this letter informative, from a well know UK DNO to the customer of another forum member a few years back, where basically the DNO is saying 'our earth has failed on the old lead covered cable, it is not our problem, convert your wiring to TT'.

The problem is understandable as DNOs connect more to an ageing network. Here, a “new” 69KVA supply to a restaurant project we are involved in. I don’t have the paperwork to hand but I believe the earthing was stipulated as TN-S . Photo 3 is the supply to neighbouring property.

When on their then single-phase supply, it was virtually identical, at around 0.45 ohms as I recall, and seems to be much the same now, so above the 0.35 limit for PME, even though it appears to be, so I guess that’s why they reverted to "TNS", or SNE as they put it on the power up certificate, but of course still the same value.

Interesting letter thanks. When we approached them back in 2019, and managed a meeting with them, they suggested a 6 figure sum upgrade, it’s only recently that we learnt of another nearby customer having a PME 3 phase connection (presents itself as such), though I don’t think that has been commissioned just yet, but it was seen by my friend as an opportunity to apply again, thinking they must of uncovered a better quality service cable, but the similar Ze and pfc suggest it’s the same one.

As for fitting an RCD, in this case it was an apparent short circuit; likely within the new meter that appears to have caused it, but the RCD would likely be on the customer side of the meter anyway, so possibly no imbalance to detect. Even an earth fault relay may not have detected it, as no obvious leakage to earth locally.

There’s probably a good case for suitably rated AFDD’s to be fitted at the intake nationally, but I guess there’s too much potential profit in concentrating on the 32 amp loads first!

yes, well if it was a cast iron gas main or a water pipe of that vintage, it would have been renewed as a length back to the TX by now, and any house wiring of that age would be getting the sucking of teeth "needs a rewire missus" treatment - but  DNOs don't think like that.
Mike

well that suggests that most of the 0,45 ohms is not the new short length of 3 core to the property boundary, but the longer length of slightly fatter stuff back to the transformer.

Mike.

OldBoy said:

, though at this stage, I'm not convinced that they removed an earth rod, installed when they moved to PME.

BS 7671 now recommends an earth electrode is connected to MET in TN systems

Thanks for that. and I presume that also connected to neutral too as appeared to be the case in the open trench? Time this "OldBoy" read the reg's again as my involvement mostly stopped at 17th edition! Retired now 15 years, and whilst specialist areas were controls and drives, out of interest, I did take the 2391 exam soon after retirement.

That is a very interesting letter Mike, do you know the date.  Before privatisation many Electricity Boards including as least part of UKPN had a policy to provide an earth terminal even though this was not a statutory requirement.  This sometimes meant that supplies were converted to PME to achieve this along with necessary upgrades to customers installaions.  I dont think this letter is compatible with this situation unless the DNO had notified the customer that it could no longer rely on the earth terminal provided.  Having provided the terminal under conditions / circumstances where it could be relied on it seems quite unreasonable to disclaim responsibility when something goes wrong!  

typos edited