But it is a new connection, as they moved from single phase to 3 phase, with the joint box several metres from the property boundary, 3 core and armour, I guess intended for PME, and they ran the protective conductor separately, after they had tried it as PME.

I guess intended for PME, and they ran the protective conductor separately, after they had tried it as PME.

The extra G/Y might be an additional electrode (one of the Ms in PME) - sometimes connected at the customer's end rather than out in the street where the service cable is long or it's just more convenient that way.

   - Andy.

I have a photo of the open trench, the G/Y exits separately from the joint box, and also tees off to the new earth rod; interestingly though, they have called it TNS ( to get around the PMS 0.35ohm limit I guess, which has left me a bit confused, and implies that it's now TNS in name only).

to get around the PMS 0.35ohm limit

The old 0.35Ω for PME and 0.8Ω for TN-S limits no longer apply (since P23/1 has been replaced by P23/2) - they now only say that "90% of premises will have an EFLI below 0.34Ω" - so if you're in the unfortunate other 10% it can be higher.

https://electrical.theiet.org/wiring-matters/years/2018/72-september-2018/earth-fault-loop-impedance-revision-of-ena-engineering-recommendation-p23/

  - Andy.

Ha A picture tells a thousand words!
Apart from the non standard colours of all the cables now being carbon black there are a few more serious observations. 

1) The DNO cut-out looks fine, there is no evidence of the incoming cable overheating or even being damage by this experience. Amusing it is thinner than the SWA leaving on the customer side. Probably 35mm2 for 100A supply.

2) That's a "bog standard" TNCs head (Probably the 3 phase model in the series 8  it certainly has the rounded lower part and the modular look., ) and it looks very much like it has  an electrode wire.

3) The customer box of fuses (and RCD ?) and outbound cable show no signs of distress. Apart from being externally covered in soot of course..

It's a clear case of a meter fire and to be honest, if there was a high resistance joint in or at the connection to the  meter that could have happened just as easily at 5 amps as it could happen at 50 - you only need a few tens of watts to start a decent fire, there may well have been no overload of the supply at all to cause that, just a few extra volts drop at a few tens of amps.

I presume the building lights flickered a bit, and then smoke was noticed ?
Unless some one else had touched it, it should be squarely the responsibility of the metering company and or their installers.
Not sure that any AFD would be guaranteed to  help unless it was upstream of the meter.

Mike
Edit with a 400A PSSC, a dead short, L-N or LE would probably get the power off in about ten seconds, so not that long.

Yes, lights flickered for quite some time before the power failed, perhaps in excess of 15 mins apparently. Two of the 100 amp fuses blew, though so far, I don't know which one survived. Interestingly, the neutral out of the meter to me, shows signs of excessive overheating, though I'm still thinking about that one as beyond the isolator, it looks quite healthy, but the connection may well have been loose as it seems it could have blown away from the meter terminal, so yes, I agree, metering company or their installers, and we are waiting for them to respond. It could be that the house circuit was on the surviving fuse, and powered the lights well after the other two fuses blew, until the neutral blew clear from the meter terminal. As for the fuse characteristics, in my haste, I must have read the logarithmic scale wrongly. Here's a photo of the cubicle before they reverted to "TNS":

That first "isolator" really gets up my nose! 

I do not see the purpose of the fuses in the switch-fuse (assuming that there are not solid links), but you have a perfectly decent big boy's means of isolation so why fit another one? What you have is 8 more potentially loose connections.

I had exactly this, but refused point blank to let them fit one. Once my isolator is off, it stays off and can be fitted with a proper padlock and not just one of those flimsy dolly things.

Why has the armour of the SWA been reinforced with the green and yellow please?

arguably on a system with inadequate ability to clear a line to earth fault, it is the big metal clad switch that is out of proportion and more dangerous under a fault condition due to its metal case being at half mains voltage.
Certainly if the company fuses and the ones in the metal box are the same rating, but only the DNO ones failed, then the problem is in the meter, but the fuse current is only exceeded probably  right at the end of the event -  well in the last ten seconds - when two phases were fairly solidly bridged, either as the live metal moved through the plastic like a hot wire cutter, or because the build up of conductive char formed a phase to phase bridge.

Phase to phase we may expect a rather higher PSSC of course - perhaps more like 600A, or more like 900 on an all 3  phases fault. Either could stop after blowing 2/3 fuses.

Mike.

Thanks Chris,I've only been on site once, just before recent holiday, long enough to take a few more photos to aid me knocking up a block schematic of the load side, for when I got home (last week). I've not yet seen inside the new cabinet (or the interlocked old one), but I asked him to fit the extra fuses (63amp BS88-3's) before I went away, really as a temporary safety measure, to minimise risk of a further fire whilst on holiday. So at the time of fire, it relied on the company fuses, and I agree, the phase to phase is apparently nearer likely nearer 800 amps as you say. I also hate the interlocked isolator, my pet hate when in industry, especially inside a locked cabinet. I'm still trying to find out what company fuse survived the fire, particularly as the neutral appears open circuit at that stage, and may well have livened up the isolator, but hopefully, the specialists on site post fire checked all that before touching it. For my only visit, so far, the installation was up and running again, but yet another interlocked door (plastic isolator this time), so I've not seen inside it yet as did not want to power off the household while they were offsite. I hope to get back later this week. Company fuses are 100 Amp.

Thanks Chris,I've only been on site once, just before recent holiday, long enough to take a few more photos to aid me knocking up a block schematic of the load side, for when I got home (last week). I've not yet seen inside the new cabinet (or the interlocked old one), but I asked him to fit the extra fuses (63amp BS88-3's) before I went away, really as a temporary safety measure, to minimise risk of a further fire whilst on holiday. So at the time of fire, it relied on the company fuses, and I agree, the phase to phase is apparently nearer likely nearer 800 amps as you say. I also hate the interlocked isolator, my pet hate when in industry, especially inside a locked cabinet. I'm still trying to find out what company fuse survived the fire, particularly as the neutral appears open circuit at that stage, and may well have livened up the isolator, but hopefully, the specialists on site post fire checked all that before touching it. For my only visit, so far, the installation was up and running again, but yet another interlocked door (plastic isolator this time), so I've not seen inside it yet as did not want to power off the household while they were offsite. I hope to get back later this week. Company fuses are 100 Amp.