A Bit of an Odd One

[west lakes]

An incident occured in this part of the country, last month, where a number of electricity customers lost their supply: -

At 20:34 on 23/5/15 alarms were received from ............ along with fault
passage indication from .............. Substation

Report then received at 21:04 of a Hot Ait Balloon strike on an 11,000V overhead line near Fellside Sth Transformer pole. The damage was within 150ft of a 33,000V line

The circuit was made dead via remote control at 21:07 affecting 344
customers

There was no one on site when staff arrived and the balloon had been
removed

The line was later repaired and supplies restored

The repairs took well into the next day and involved maintaining supply by generators.

But the sheer risk to the balloon crew of packing up without any assurance the situation was safe is absolutely amazing.

In this part of the world we get usually one or two aviation related incidents a year where contact is made. Usually balloons or hang gliders, fortunately there have been no deaths as yet!

A case of not keeping a look-out on what is going on at ground level?

[B Fraser]

Unfortunately, wire strikes are quite common. Balloon crews carry OS maps overprinted with "half mil" information for navigation so pylons are marked. Crews are also trained to look left and right to spot lines of poles and so avoid gift wrapping the National Grid. Sadly, even the best pilots have been known to decorate power lines with half an acre of bright nylon.

I will add a good story when I get the time to write it properly.

Brian.

[onetrack]

I fail to understand how it took 3 minute to de-activate the circuit once a fault was identified. I have been informed that high voltage lines have circuit breakers that kick out in milliseconds and that the current draw required to make them kick out is extremely low - around 5 amps. I would have thought that balloon contact with a HT line would initiate immediate circuit de-activation. I wonder what the "alarm" is, that is fitted to the line, that merely indicates "a fault", but which alarm still keeps the circuit live?

[cockney steve]

I would have thought that balloon contact with a HT line would initiate immediate circuit de-activation

In that case, every time a bird alighted on a HT distribution line, the power to hundreds of consumers would be constantly tripped-out.
touching a high-voltage conductor is, not of itself, dangerous. The nasty bit occurs if you , at the same time, get too near a neutral/return point.
bridging HT to a low potential(IIRC, Neutral is maintained ~5V above" earth") is not to be recommended if you seek to reach mature years.

In my spotty youth, Iheard several anecdotes from a college lecturer in Evening Class...his daytime job was with CEGB (Central Electricity Generating Board ) He told us that the maintenance crews would traverse the live lines crossing the Thames in London.....A sort of gondola would be hooked (from a well-insulated platform) onto the wire....the men would mount and pull themselves hand-over hand to the next pylon on the opposite side of the river.
Perhaps balloon- fabric is sufficiently conductive to carry a trip-current if wrapped across several wires at varying potentials.....Hang-gliders, with their metal frames, would cause a somewhat spectacular bang in the same situation.

Westlakes is the professional, perhaps he can expand on the subject.

[west lakes]

OK!

The line that was contacted was a 3 wire 11,000 volt circuit.
That 3 wires physically separated by a metre and 6 m or greater above ground level.
It is supported on wood poles spaced about 100 m apart.

Now if the contact was made mid span it is likely the added tension would bring the conductors close enough for there to be a direct short between them. (big flash, big bang)
Closer to the poles it would require the current to be carried by the fabric of the balloon, which as a man-made fibre is quite unlikely.
In this case there was a short which actually snapped at least one of the conductors and it would have been seen and switchgear would have operated

THIS DOES NOT MAKE IT SAFE NOR IS IT CLASSED AS DEAD UNTIL THE COMPANY HAS PHYSICALLY INTERVENED ON SITE AND CONNECTED EVERY CONDUCTOR TO EARTH.

Yet the balloon operator chose to quickly pack up, with no idea if the line was safe or not and leave site, no doubt to avoid being identified and getting a big bill for the repairs.

It is also common on overhead line circuits that some form of auto reclose is in use, in this case the circuit would trip and then a few seconds later reclose. A common addition to overhead circuits to maintain supplies when the likes of, say, a bird contacts two conductors and then falls to the ground.
This could have occurred in this case and the circuit become live after the first trip.

The only time a neutral comes into play is on 230/400V lines, and as it must be bonded to earth it is always at 0 volts on the network