Lost 11 kV neutral?

• 11 KV circuits in the UK are almost always 3 phase, 3 wire with no neutral. 2 wire 11 kv circuits are generally any two phases from a 3 wire system. At the origin of the 11 kv system, where this is derived from a higher voltage, there is generally a protection relay that trips in case of an earth fault. Not sensitive.
• Any downed or with reach 11 kv line should be treated with great suspicion. Keep well clear even if it is believed to be dead. 

I suggest the right thing is to call your DNO 

https://speed.energybrokers.co.uk/wp-content/uploads/2020/05/electricity_distribution_networks_-_march_2018-coloured.png.webp

as you describe  also suggests that the primary current is returning via earth - I'd expect the earth fault relay to pop  off, but presumably it has not reached the threshold yet..  11kV line to line and about 6.6kV from either line to ground is about right with the 130volts.

In faraway lands HV is indeed sometimes distributed on a single wire with earth return, but such systems cannot easily detect a line down. We d not do that here for that reason..

Mike.

Presumably by private you mean it only feeds you.  Just to check the obvious, you have reported it to the DNO?

Thanks folks!

2-phase makes sense. I know next to nothing about HV, but it did seem likely that a couple of hundred milliamps could pass from a wet cable in the grass to the general mass of the earth.

Would a relay trip? There wasn't really an earth fault with the supply side hanging in the breeze.

This all happened to a chum of mine, but it was fixed pretty promptly by the DNO. I still have at the back of my mind a lost LV neutral in a PME supply. I think in these circumstances, switch off at the CU, call the DNO, and go down the pub.

I am reminded of the 1987 storm. A colleague lost his electrics, but in order to get down his drive he had to move a cable which, he said, sparked when it hit the ground. 

Would a relay trip? There wasn't really an earth fault with the supply side hanging in the breeze.

But the 'return' cable was in contact with the ground - so the supply should be able to see the loss of current from the normal L-L circuit - in much the same way as an RCD (although the actual current monitoring is I think done via a single coil in the connection between the HV star point and Earth, rather than looking for the an imbalance in currents in live conductors).

I think they earth relay should trip at a few amps - so in this case the current would be limited by by the LV load and the transformer windings ratio. Had the supply end hit the ground directly a trip should have been more likely - or indeed if the LV load been significantly higher.

    - Andy.

Being 11kV there's likely (not guaranteed) to be a neutral earth resistor that would limit current further.

Further thought: Some networks run with an Arc Suppression ("Peterson") Coil which by design limits earth faults to relatively low currents, and thereby delay tripping to allow customers to stay on the network for some hours while said faults are found. This does not make them touch-safe!

(For example, Western Power Distribution note)

I've had both extremes

This is in a rural area overhead line 3 phase ABC type distribution from local substation fed with 11 KV underground incoming from somewhere.

So I've had the 130V scenario regularly in the past - less so now, I never even bothered ringing up - just a weary sigh when I came into the house and found what was happening

Also had the opposite where everything rises to 278 volts (being the highest I recorded) which is even more exciting.

I've had the 130V scenario regularly in the past

The other, possibly more likely, cause of low voltage is one of the HV lines being lost (e.g. just one HV fuse blowing) - which leaves two of the HV (normally delta) windings effectively connected in series across the remaining pair of lines. In residential areas that often leaves two-thirds of of customers down a street with around half normal voltage and the remaining third puzzling what all the fuss is about.

   - Andy.

The joys of Dyn substations