Electrocution on railways

[drivin_me_nuts]

On the box last night there was a story about the number of people killed on the railways. The reporter went on to say that with DC lines (3rd rail) there was a tendancy for you to hold on to the rail) and so, the power had to be cut before you could be rescued and pronounced DAS.

Is this holding on a 'feature' of a DC current or does AC do the same?

[alfa pint]

I thought it was the other way round! Although thinking about it, a mate of my dad's died by shorting a spanner out over the battery and suffocating on the exhaust fumes while he was unconscious.

[Steve-O56]

From what I understand (I work on the tracks not electrification) The overhead lines run 25000v AC which as the current alternates throws you off, where as the 650v DC conductor rails keep you drawn towards them as it is just a constant current. Personaly i've only ever seen one lad be electrocuted by overheads and he was thrown away pretty quickly.

[kambites]

As I understand it, an electric current passed through your muscles in one direction causes them to contract and the other way to expand, so DC will cause your muscles to lock in one direction, whereas AC will cause them to spasm (if that's the right word).

So if you grab hold of a DC wire, your hand can lock closed around the wire. With AC that can never happen.

Edited by kambites on Friday 16th July 09:30

[ZesPak]

alfa pint said:

I thought it was the other way round! Although thinking about it, a mate of my dad's died by shorting a spanner out over the battery and suffocating on the exhaust fumes while he was unconscious.

did he by any chance got off flight 180 before it took off?

[john_p]

kambites said:

So if you grab hold of a DC wire, your hand can lock closed around the wire. With AC that can never happen.

Hence why you use the back of your hand if you're not sure something is live*


* and if you are mental, and lacking a multimeter, and working on wiring you're not sure is safe

[tank slapper]

Steve-O56 said:

From what I understand (I work on the tracks not electrification) The overhead lines run 25000v AC which as the current alternates throws you off, where as the 650v DC conductor rails keep you drawn towards them as it is just a constant current. Personaly i've only ever seen one lad be electrocuted by overheads and he was thrown away pretty quickly.

There is a clip on one of the video websites that shows a guy getting electrocuted by overhead wires in India. He was walking along the roof of the carriage, and put his arm up. He wasn't really thrown clear, but just fell down immediately.

[davepoth]

With AC the electricity will go from +240v to -240v 50 times a second. So that's 51 (?) times a second that the voltage will be 0 and you can pull your hand away.

[Mattt]

tank slapper said:

Steve-O56 said:

From what I understand (I work on the tracks not electrification) The overhead lines run 25000v AC which as the current alternates throws you off, where as the 650v DC conductor rails keep you drawn towards them as it is just a constant current. Personaly i've only ever seen one lad be electrocuted by overheads and he was thrown away pretty quickly.

There is a clip on one of the video websites that shows a guy getting electrocuted by overhead wires in India. He was walking along the roof of the carriage, and put his arm up. He wasn't really thrown clear, but just fell down immediately.

Then caught fire!

[Zad]

davepoth said:

With AC the electricity will go from +240v to -240v 50 times a second. So that's 51 (?) times a second that the voltage will be 0 and you can pull your hand away.

Unfortunately, it is at 0 volts for an infinitely short period of time though.

[spikeyhead]

Zad said:

davepoth said:

With AC the electricity will go from +240v to -240v 50 times a second. So that's 51 (?) times a second that the voltage will be 0 and you can pull your hand away.

Unfortunately, it is at 0 volts for an infinitely short period of time though.

but it is there 100 times a second.

[Dogwatch]

It really surprises me the number of people (not all dimwits either) who think the third rail current is switched off between trains.

Fortunately most of them have the sense not to try it out.

[Ross1988]

tank slapper said:

Steve-O56 said:

From what I understand (I work on the tracks not electrification) The overhead lines run 25000v AC which as the current alternates throws you off, where as the 650v DC conductor rails keep you drawn towards them as it is just a constant current. Personaly i've only ever seen one lad be electrocuted by overheads and he was thrown away pretty quickly.

There is a clip on one of the video websites that shows a guy getting electrocuted by overhead wires in India. He was walking along the roof of the carriage, and put his arm up. He wasn't really thrown clear, but just fell down immediately.

I was shown this video during a track safety briefing, and as I understood it the overheads over there are infact DC.

We were taught to use our jackets turned inside out to drag people off DC lines.

I just hope I never get into a situation where that is an option.

[neilb62]

When I worked on DC trains (as a Driver) we were taught that DC current causes muscle contraction, therefore if you grab hold of something live you'll stay grabbed!
When fault finding we always tested components with the back of the hand so it would be thrown off if we made a mistake!

[Simpo Two]

tank slapper said:

There is a clip on one of the video websites that shows a guy getting electrocuted by overhead wires in India. He was walking along the roof of the carriage, and put his arm up. He wasn't really thrown clear, but just fell down immediately.

It took about two days for PH to remove my post in which I included the clip. The reason given was 'It showed somebody dying'. Well death comes to us all, and putting your head in the sand isn't going to change that. But it might serve as a reminder to others than electricity is a bit dangerous.

Anyway, it's easy enough to find by searching for 'indian man electrocuted on train video'.

[PeteG]

Simpo Two said:

tank slapper said:

There is a clip on one of the video websites that shows a guy getting electrocuted by overhead wires in India. He was walking along the roof of the carriage, and put his arm up. He wasn't really thrown clear, but just fell down immediately.

It took about two days for PH to remove my post in which I included the clip. The reason given was 'It showed somebody dying'. Well death comes to us all, and putting your head in the sand isn't going to change that. But it might serve as a reminder to others than electricity is a bit dangerous.

Anyway, it's easy enough to find by searching for 'indian man electrocuted on train video'.

fk a duck. That was just... "hey, he's walking along- BZZZZT" and then just... death. No scream or anything.

[Yertis]

Can anyone explain why your domestic 240v supply is sometimes lethal, sometimes not? I'm very careful when doing anything involving sparks but nevertheless I've inadvertently given myself a bit of a jolt a couple of times. Why aren't I dead?

The worst jolt I ever had was off the HT lead of an Audi one damp morning. Not nice, shook me up a bit.

[spikeyhead]

Yertis said:

Can anyone explain why your domestic 240v supply is sometimes lethal, sometimes not? I'm very careful when doing anything involving sparks but nevertheless I've inadvertently given myself a bit of a jolt a couple of times. Why aren't I dead?

The worst jolt I ever had was off the HT lead of an Audi one damp morning. Not nice, shook me up a bit.

What kills you is almost always the current that flowa through your heart. It only takes about 10mA, which can be supplied by a small battery. Ohms law tends to stop us suffering, as out skin and tissue resistance is fairly high, so it is unlikely that anything less than 50V will ever kill you, then you need to be holding on to the positive in one hand and the negative in the other.

In a domestic scenario, it's usually a case of touching the live with your hand and the current path also contains your capacitance to ground which is fairly small rather than any direct route, so little current flows. If your other hand is touching a good ground, such as a well earthed water pipe or the neutral line it'll do you less good.

Princiapply the current will make muscles contract, hance experienced sparkies using the backs of their hands to check to see if something, ie the bus bar running across a factory roof, is live, it's easier doing that than messing around with a meter when on the top of a high step ladder.

Fortunately most of my work rarely goes above a few volts, though occaisionally I've been hit with 100V or so of very high frequency, which doesn't twitch the muscles, just burns holes in your fingers.

[SeeFive]

In the days before electronic ignition, when we were testing the spark on engines, we used to just bounce it off our fingers - after making sure we were not standing in a puddle or anything silly of course. My dad could actually stall a mini by spreading his fingers across all 4 plug leads (in the days of open caps). It saved walking around to switch the key off when start stopping the engines.

That was DC, and you could feel the pull on your finger into the plug lead, so I would say DC pulls and AC throws, although given the amperage, it is not something I try to do. Having said that, if no meter is available, I always test wiring using the back of my hand so will I grab fresh air if I cop a belt.

Note - do not try this on today's cars with electronic ignition. Although the amperage is still low, the volts are very high and it will hurt you a lot, and if you have any health issues, low amperage will kill you.