is 240v really that dangerous

[mrstitt]

Back to the original topic. If you ever speak with DNO people they will often tell you that its LV that they fear the most. 11kV and above tend to be arc risks - you get blown back as you approach the equipment. Also its worth noting that MV and HV equipment normally has several layers of safety - you would have to be deliberately reckless to come into contact with a live MV or HV contact that hasn't been fully isolated (or your working under an appropriate live working regime).

LV is lethal because for three reasons. A) its less likely to be fully isolated B) live working is more likely to be permitted for reasonably complex tasks (its only really used in HV for very basic maintenance) C) when you make contact with an LV circuit your unlikely to be blown away from it, you may simply cling onto it.

I believe that when the British Railways Board was experimenting with different voltages for overhead electrification in the 50's, they set up a line out of Glasgow Central and used various transformers to test different voltages. One thing they found was that trespassers on the railway who came into contact with 25kV equipment needed burns specialists, whereas those who came in contact with <3kV needed a coroner.

I would much rather be seeing a burns specialist than a coroner any day.

 

[bernardgreen]

A bare 25kV conductor is far enough away from the ground that the average trespasser will have difficulty making a prolong two point contact with both ground and 25kV Hence the shock will be fleeting. Separation at lower voltages, such as third rail ( 600 to 700 volts and often DC ) can be as small as 12 inches so prolonged contact is easy to achieve.

A short duration shock will stop the heart but not damage it and the heart auto re-starts after the shock. ( that is how a defbrillator works ) A prolonged shock stops and hold the heart stopped during which time with no blood flow damage to various organs including the heart will occur

 

[mrstitt]

750V and 1.5kV overhead is more common than you think. In the UK we use it on most of our light rail systems.

< 3kV is used for the OHLE on many non-express networks in mainline Europe. If you ever see the roof of a Class 373 Eurostar power car you will see the two pantographs - an MV one for 25kV in the UK and on french LGV lines, and an LV pantograph for use on other lines in Europe at 3kV and 1.5kV.

European rail networks only started to adopt MV voltages for high powered express systems like the TGV and the ICE. Although they all have different flavours - Germany uses 16.27Hz, which must make life difficult for German Paramedics wishing to use an ECG machine on a railway platform.

 

[SimonH2]

I don't know if it was an urban myth or not, but someone I used to work with told me about a chap who worked in a gantree crane in a steelworks. The chap got out of his cab and needed to go to the gents - only the gents is a long way to go from a gantree - so he relived himself from the cab door. When your doing your business its helpful to have something to aim for, so he chose a bus bar that powered the crane motors. Urine is rich in salt so very conductive. Now imagine the entry burn....

Which, wincingly, brings to mind a story an old farmer told me years ago. Again, dunno if it's a myth or real, but the suggestion was that people had peed on third rail power tracks. I can well believe that people would try it, and unless their shoes and the ground were reasonably well insulating,I imagine the results would be more than unpleasant

 

[mrstitt]

Well the inside of your fire hose is soft membrane tissue with a large surface area, not very much impedance. I wouldn't try it if you gave me boots that looked like 132kV bushings.

 

[PBC_1966]

No, I understand they die by suffocation as the chest is unable to breathe.

Obviously the current will paralyze heart and lung operation, but with a current of several amps passing through the head, I find it hard to believe that the person isn't rendered unconscious within just a few seconds, even if not absolutely instantaneously, although I know there has been some controversy about it. It will probably be entirely academic in a few more years anyway, given that there are now only a few states retaining working electric chairs and those are likely to be retired soon.

750V and 1.5kV overhead is more common than you think. In the UK we use it on most of our light rail systems.

Many of the old British tram and trolley bus systems were 500 - 600V.

London Underground adopted a system unique in the world, so far as I'm aware, with its 3rd & 4th rail electrification system. Nominal voltage was set at around 630V, but with resistance earthing at the supply station putting potentials on a one-third/two-thirds split: +420V on the outer rail, -210V on the central rail during normal operation. The system permits continued running in the presence of a single earth fault, the potential on the non-earthed side then rising to the full +/-630V approx. for the duration of the fault. Overground stretches of tracks shared with British Railways used an earthed central rail with full positive potential (or higher) on the outer rail for compatibility.

 

[JohnD]

with a current of several amps passing through the head, I find it hard to believe that the person isn't rendered unconscious within just a few seconds,

yes, at high voltages, the brain comes to the boil as well, but this takes a little time

 

[SimonH2]

Warning, wandering off topic ...

Talking of third rail systems, am I the only one who spots "taking liberties with the science" in films ?
I recall one set in America, where towards the end the copper is chasing one of the baddies down the Subway tunnel. The baddie finds one foot trapped when the points he conveniently is stood in change - and to avoid giving the copper the satisfaction of an arrest then puts his other ankle against the power rail - cue dramatic acting and special effects smoke.
At the time I recall thinking two things :
1) It's leg to leg, so probably not much lecky is going to reach as far up the torso as the heart.
2) He's wearing shoes and socks, so no direct contact anyway

Only the other night SWMBO told me off for this sort of thing. She was watching Die Hard with a Vengeance, and I dared to comment on how the two goodies (played by Bruce Willis and Samuel L Jackson) had been through things that should have left them in full body casts - but were still able to run around and fight the baddies.

 

[PBC_1966]

Talking of third rail systems, am I the only one who spots "taking liberties with the science" in films ?

Not at all! Giant arcs flying all over the place when somebody touches 120 or 240V, computers which have smoke pouring out of them and then explode spectacularly when presented with a paradox.......

I recall one set in America, where towards the end the copper is chasing one of the baddies down the Subway tunnel.

The Taking of Pelham One Two Three , in which a gang hijacks a New York City subway train:

http://www.imdb.com/title/tt0072251

 

[Dave54]

Obviously no fuses or circuit breakers in Hollywood?

 

[stillp]

For some reason US executions sometimes have groups of spectators.

I believe they are there as witnesses.

 

[stillp]

I don't know if it was an urban myth or not, but someone I used to work with told me about a chap who worked in a gantree crane in a steelworks. The chap got out of his cab and needed to go to the gents - only the gents is a long way to go from a gantree - so he relived himself from the cab door. When your doing your business its helpful to have something to aim for, so he chose a bus bar that powered the crane motors. Urine is rich in salt so very conductive. Now imagine the entry burn....

Which, wincingly, brings to mind a story an old farmer told me years ago. Again, dunno if it's a myth or real, but the suggestion was that people had peed on third rail power tracks. I can well believe that people would try it, and unless their shoes and the ground were reasonably well insulating,I imagine the results would be more than unpleasant

I used to work with a guy who'd been with CEGB in the fifties. He told me they found a lot of corrosion on some Pyro in a basement area of his power station, and couldn't understand why, until someone working late followed the night security guard and caught him urinating on the wall with the Pyro on. After repeated warnings, he was still doing it, so they laid a sheet of weldmesh on the floor in his preferred area and connected a Megger between the weldmesh and the outer sheath of the Pyro. A few turns of the Megger handle later, security guard exited screaming. Afterwards he relieved himself somewhere more appropriate.

 

[Dave54]

I don't know if it was an urban myth or not, but someone I used to work with told me about a chap who worked in a gantree crane in a steelworks. The chap got out of his cab and needed to go to the gents - only the gents is a long way to go from a gantree - so he relived himself from the cab door. When your doing your business its helpful to have something to aim for, so he chose a bus bar that powered the crane motors. Urine is rich in salt so very conductive. Now imagine the entry burn....

Which, wincingly, brings to mind a story an old farmer told me years ago. Again, dunno if it's a myth or real, but the suggestion was that people had peed on third rail power tracks. I can well believe that people would try it, and unless their shoes and the ground were reasonably well insulating,I imagine the results would be more than unpleasant

I used to work with a guy who'd been with CEGB in the fifties. He told me they found a lot of corrosion on some Pyro in a basement area of his power station, and couldn't understand why, until someone working late followed the night security guard and caught him urinating on the wall with the Pyro on. After repeated warnings, he was still doing it, so they laid a sheet of weldmesh on the floor in his preferred area and connected a Megger between the weldmesh and the outer sheath of the Pyro. A few turns of the Megger handle later, security guard exited screaming. Afterwards he relieved himself somewhere more appropriate.

Not saying it's not true, but there was a play on the TV I remember from back in the 60s. Two younger blokes and an older bloke working in a switch room of some sort. The older guy used to go and pee around the back, and they did the same thing to him. Details are a bit vague to me now but it was definitely on.

 

[SimonH2]

... caught him urinating on the wall with the Pyro on. After repeated warnings, he was still doing it, so they laid a sheet of weldmesh on the floor in his preferred area and connected a Megger between the weldmesh and the outer sheath of the Pyro. A few turns of the Megger handle later, security guard exited screaming. Afterwards he relieved himself somewhere more appropriate.

There's a flip side to that.
At my last place, the factory manager had been a few places. One was a tinplate plant (IIRC). They went through a process of modernisation and automation, and eventually got rid of a few people.
After that, they found they couldn't quite get the pickling tank working properly and plating quality was suffering. Eventually they came to an arrangement with the guy they'd made redundant, who after making sure his consulting fee was secure, made an admission : "well it's a long way to the loo, so I'd been suing the tank". And that was part of the chemistry the new automated system didn't control

 

[stillp]

Yes Dave, I remember that! (Sad or what?)