Shocking (!) demo of the danger of arc flash

[beam me up schottky]

Sorry about the pun in the title ! However, it is true, this is a sobering demonstration of the potential danger of getting on the wrong side of electricity, lets hope nothing this bad ever happens in out sheds !

[Pamphonica]

In BT the main 50V bus-bars in the exchange are rated at 50,000A or so. Incautious dropping of spanners across this 50V supply resulted in fireworks and disappearance of the spanner! Spark not arc of course.
The poor test chap in the film was very patient wasn't he? Especially when they let him burn; no rushing in with extinguishers!

[glowinganode]

That's interesting Jeremy, I guess there was a bank of lead acid batteries hanging off the end of the busbars.
This is one of my favourite arc flash demo's.
I've seen the aftermath of such an incident, I'll try and dig out some photo's.
Rob.

[Pamphonica]

Each exchange had a battery room. The batteries were made up of lots of large concrete cells, cast in-situ, lead acid type. Each was about the size of an under-counter fridge, from memory. The idea was to keep the exchange runing for up to an hour, in case the standby generator had starting problems! No smoking in the battery room or bang!

[AlanBeckett]

In days of old you could calculate the required capacity. A 'good' exchange in the '60s needed about 1A per Erlang of traffic.
http://en.wikipedia.org/wiki/Erlang_%28unit%29
Alan

[Skywave]

Hmm - thought-provoking, if nothing else; certainly frightening.

However, there is one detail that puzzles me: perhaps I need enlightening: if you can do so, please do.
If a technician prepares to do maintenance work on such plant and wears the requisite protective clothing, does said clothing include a full-face totally black face mask? If so, how can he then see what he is doing?
Moreover, judging by the sheer intensity of the arc & flash in that video, without a totally black face mask, surely the technician will suffer eye damage; possibly blindness.

The reason I say all this is that when I last did some arc welding using such a face mask, (albeit many years ago), I could not see anything at all until the arc struck. It was really a question of aiming the rod at the point where the arc was to be struck, but not making contact, then flipping down the face mask and quickly striking the arc. Only then could I see what I was doing.

Al.

[BG405]

Modern welding masks have a sensor and a liquid-crystal panel in the visor - much more user-friendly!

Not sure about the ones in the video though.


Brian

[AlanBeckett]

Modern welding masks have a sensor and a liquid-crystal panel in the visor ...

And splendid they are
I was using mine yesterday.
Alan

[Skywave]

Thank you; your replies read & understood.

Al.

[beam me up schottky]

The face shield in the video is a normal, passive, clear type, apart from being resistant to the arc flash. I think the flash is so brief that any eye problems would be temporary. The blackening is caused by the flash, metal and plastic vapour.

I don't think there is an automatic type welding visor that big, I know mine is a fraction of the size of the shield shown in the video and the largest I've seen are still substantially smaller.

The thing that shocked me the most is how common this arc flash danger is when working on industrial type installations. There are quite a few video's on youtube, some very disturbing to watch. Even highly trained technicians are seen to be making bad mistakes. I suppose it's easy to become complacent when someone has been doing the job for years without incident, so corners are cut and the odd's conspire against them.