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Lost 11 kV neutral?

Chris Pearson

A chum of mine wrote this in a forum after Storm Arwen.

Yesterday morning the power seemed to be a bit below par, so I checked it with my tester, to reveal only 139 volts. Not had that before, it is usually all or nothing. We have our own 11,000 v cable (cost a fortune 40 years ago) running alongside our driveway (¼ mile) with transformer on pole near house. Discovered a Christmas tree had been blown over from our neighbour's land and snapped one of the two cables between two of the poles. The long end (from house) was lying on the grass and the short end (from supply) was dangling with the end about 6ft AGL - all still live and there's a public footpath next to the line.

The private 11 kV line has two wires so I assume that it is single phase although I have no idea where the neutral is connected. I do not know how the LV is distributed.

A 2 kW kettle would have a resistance of 26 Ω so it would draw 5.3 A at 139 V. In turn, that would draw 110 mA at 11 kV so it is conceivable that it was getting back to the transformer via terra firma.

I get a bit worried when people notice that the lights are dim, the kettle is slow, etc. and then get out a volt meter. Then they assume that the line voltage has dropped. A lay person might also reasonably assume that the cable at the house end would be dead.

Parents

  • 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. Astonished

Reply

  • 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. Astonished

Children
  • in reply to Chris Pearson

    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.

  • in reply to AJJewsbury

    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)

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