What caused this burn out?

[Refugee]

Although repairs have been carried out the burn pattern can still clearly be seen on the wood however very little if any to vegetation close by.
The heat clearly came from the drop wire running to ground level as the wood is only burned on one side and only to the top of the drop wire.
The drop wire is on the south side of the pole.

[ppppenguin]

Looks like an 11kV 3 phase circuit. The 3 red cables each have a single core and the pole is a transition from underground to overhead. All very commonplace. The red cables would have replaced older ones, probably paper insulated lead sheathed. My guess would be that one of the old cables developed a fault and overheated. This wouldn't necessarily have triggered a protection relay in time to avoid the damage seen.

[Refugee]

There is a clue a little way out of shot and also I spotted a power sparkie supervising a temporary generator while some work was being done on another line a mile or so away whom recalled having to rush out and fix it.
It is actually quite unlikely that the cable went faulty. The repair included up-rating of the cable.

[Ed_Dinning]

Hi Gents, could even be a 33KV circuit, count the insulator sheds. If so, even by present day standards quite a bit of power available.

Ed

[ppppenguin]

Though I'd hope the protection arrangements on a 33kV circuit would be rather better. Though if was a high resistance joint that just got worse it's hard for any protection system to pick that up.

A few years back we lost power at home. Evenutally they brought in mobile generators and hooked them up to a couple of the local substations. That went on for a few days. The problem was a blown 11kV underground circuit. ISTR it made quite a nice hole in the pavement. There were 2 blow ups as the 11kV circuit was a ring and they re-energised it until another section failed under the increased load.

At low voltage (230V/400V) underground joint box failures are common enough, especially in central London. Some of them go bang quite nicely. It's wonder that you don't get more deaths and serious injury.
http://www.bbc.co.uk/news/uk-england-london-22934155

[Murphyv310]

Hi.
Well I'm no expert in this field at all but to me it looks like a serious flash over has taken place.
Poor insulation, damp surroundings, vegetation, perhaps heavy rain, who knows. These things happen and why speculate..... maybe the answer will never be known.

[Refugee]

The speculation stopped when I looked at the gear owned by a shiny new 200 amp per phase customer 100 yards from the pole.
The original cable was quite a bit smaller and south facing so in the daytime when demand is greatest the sun was adding heat to an already overloaded cable hence the upgrade the engineer told me about. The line is indeed 11Kv.
The trouble is that bean counters failed to keep up with new technology and failed to get the cable uprated when the new customer was connected.
There was a carbon footprint

[ppppenguin]

Maintenance on the LV network is usually on the basis: "if it blows, fix it". There's very little preventive maintenance. I suspect it's cheaper that way. I don't know about the 11kV network but I suspect there's relatively little preventive work and again it may well be cheaper to fix it when it blows rather than look for trouble in advance.

[Terry]

At low voltage (230V/400V) underground joint box failures are common enough, especially in central London. Some of them go bang quite nicely. It's wonder that you don't get more deaths and serious injury.

When I turned up in the City for a job interview in 1969 I found Threadneedle Street in a state of chaos with smoke pouring out of the footway chambers and clumps of people hanging around where they had been evacuated from their offices - and no power, of course.

The force of the bang was sufficient, apparently, to hurl the footway chamber lids ten feet in the air. Fortunately it occurred after most people were safely at work in their offices and nobody was hurt. If it had happened at the height of the rush hour it might have been a very different story ...

[ppppenguin]

I think I've related the story of the exploding cast iron joint box. This was at the top of a pole outside my aunt's house. It had been dripping pitch for a while and she'd reported it. One evening it went off with a very loud bang, red hot cast iron fragments everywhere. Blew the fuses at the sub. The leccy people worked overnight to restore power but my aunt was definitely not happy.