Another Darwin award candidate?

[ARW]

Do you think that I should quote that in court if the CPS decide to press charges [1] on my assault case with the apprentice?

[1] Not heard about it anything for a while.

[Adrian]

I'm not sure it'll help, however true it might be.

[newshound]

More like 30 kV/mm. Think spark plug gaps.

[ARW]

At 16 years being I was young and stupid and I took my mum's car for a day out in Blackpool with a couple of friends.

But you sometimes do live and learn from your mistakes.

Never again will I visit Blackpool - what a shithole. We took the car to Bridlington the next month and had a really good time.

[mick]

Depends on the voltage and the shape of the conductor. In dry air, with rounded conductors at standard temperature & pressure, the breakdown voltage is roughly 33kV/cm. The distance would fall in even slightly damp air or to non-spherical conductors (such as human bodies). The overhead line where he was would have been at 25kV AC, so he would have probably been within mm of the wire before he was killed.

[Tim Lamb]

Yebbut... hardly atmospheric pressure and dry.

[Chris J Dixon]

No, in old money the rule of thumb was 25 kV per inch. On nominally 25 kV railway systems, the protective spark gaps were set at 4.5 inches (1), expected to flashover somewhere in excess of 112 kV.

(1) Now metricated to 114 mm IIRC. I've probably got the drawings in the loft, but I'm not climbing up there right now.

Chris

[F]

No, just someone clearing up what turns out to have been a misunderstanding.

And I don't normally respond to Trolls. Now go away and play on the railway.

[F]

Thanks, all, for the figures. I thought I had seen previously that simply standing on top of a train would cause a flash-over.

[Tough Guy no. 1265]

Most people don't care about a minor thing like that.

Then fix your killfile.

[Johnny B Good]

If it's the one where the 'wind powered generators' were based on resonant wires 'singing in the wind' driving transducers to generate electric current, then I guess I remember it. In this case, the wires had been sandblasted by, well, desert sands!

Once the archaeologists had figured out the function of these mysterious 'Harps', the solution was to restring enough of them with wires protected by a sandblast proof plastic coating to get a small section of the sophisticated infrastructure functioning again.

Even back then I had the wit to consider that this was a gaping plot hole in the story simply on account if there were such a plastic coating, it was going to considerably dampen (ie absorb energy from) the resonance, considerably reducing generator output (damping the resonance should have been solely the transducers' function in extracting power from the wind).

The implied notion that the deceased civilisation had been considerably more advanced in that it had replaced the antiquated notion of 'windmill' styled turbines with hi-tech "Harps", rather begs the question as to why they hadn't also come up with a superior version of protection against sandstorm damage.

Of course, on revisiting the story (from memory), there's a much bigger plot hole than that one now that we more fully appreciate the folly of 'wind power' as a significant source of alternative energy to power the needs of even a really energy thrifty civilisation.

The story was really a fable to demonstrate just one way in which a global civilisation could fall into catastrophic decline by neglect of the longer term consequences of adopting a technological strategy that leads to an absolute dependency. History and pre-history is littered with many examples of such folly whereby the civilisation in each case has simply 'Slept walked' into a dead end canyon.

[Vir Campestris]

Obviously this is escapist literature, designed to take the reader away from the problems of the real world.

Published 1964. 4 years after Silent Spring (which I also have - although a later edition). And long before most people woke up.

Andy

[Huge]

Quote away, but I suspect it won't do you much good.

[Johnny B Good]

Yes, but at 10 to 15 Bar!

The dielectric strength of dry air under NTP conditions is 3MV/m which works out at 3 KV/mm. Humidity raises the dielectric strength of air very slightly, contrary to the behaviour of pure gases such as nitrogen and sulphur hexafluoride where it decreases with humidity.

Typical spark plug gap values of 0.63mm (25 thou) and 0.7mm (28 thou) only need about 2KV or so to create a spark in open air. When under compression some 10 to 15 times greater voltage is required to jump the gap (some 20 to 30KV).

I had good cause to appreciate the dependency of breakdown voltage on air pressure one cool night, about 40 years ago, when riding home on my FrankenAJS/Matchless 250CSR when it suddenly lost power and took to severely misfiring.

I quickly realised that by using less throttle, I was able to suppress the misfiring. This meant I merely had to use lower gears and carefully nurse the throttle whilst remembering to keep the revs up when having to come to a stop at junctions.

When I finally got back home (luckily, only a matter of the last 5 or 6 miles on familiar roads), I removed the number one suspect, the spark plug and was quite bemused to discover that it was no longer adorned with an earth electrode.

No wonder I'd had to baby the throttle! Even so, with nearly a tenfold increase in gap length, I was amazed that the 12v car ignition coil I'd fitted, after upgrading to 12v electrics using a 10 cell NiFe battery, had managed to provide enough voltage to work at all.

Throttling back reduced the charge of air/fuel mixture enough to reduce the peak 10 to 15 bar close to top dead centre of the compression stroke down to something in the region of just 2 to 3 bar.

It was a salutary lesson regarding how spark voltage demand varies with throttle settings, useful to know when, for whatever reason, you start suffering from a misfiring petrol engine (especially when it's a single cylinder engine!) since you can often keep it running on reduced power demand using lower gear ratios than usual until you either complete your journey or find a suitable place to stop where you can attempt a quick diagnosis and repair in relative comfort and safety.

Of course, modern cars (and motorbikes) all use 'sophisticated' ECUs which may or may not know how best to manage such problems. This knowledge may only prove useful with older vintage vehicles and limited to helping in the diagnosis of misfiring/power loss problems with 'modern' vehicles. Ah well, such is the cost of progress. :-(

[Bill]

I thought you ought to be reminded of Britain's darkest hour. For 3 minutes.

[Johnny B Good]

More likely circa 10mm with 25KV ac with its peak of 35KV. Assuming a conservative body resistance of 1K ohm after the contact areas become carbonised, you'd be looking at a power level of just over half a megawatt (half a mega-joule per second). Bones are surprisingly conductive so body resistance might even reduce to less than a hundred ohms under these conditions meaning a power level of 5MW or greater.

Even if he'd had to stretch out on tippy toe to get with 10mm of touching the line, thus leaving the possibility that muscular spasm could have flung him clear within a tenth of a second, the damage, even if limited to the effects of the lower estimate of 50Kjoules would have likely had fatal consequences even if he hadn't been killed outright like a bug in a fly zapper.

I reckon there'll be H&S consequences which may make the poor lad's mum one very rich lady if she plays her cards right. The scene looked just like "An Accident Waiting To Happen"(tm) imo.

I'm not advocating that the stupid be rewarded, far from it. In this case, I think the stupidity lies with the railway company rather than the accident victim.

[Dave Plowman (News)]

;-)

[Dave Liquorice]

Wouldn't like to bet on the ECU throwing in the towel and refusing to play though. It might not know that a plug has lost the elctrode, but it will know about the missfire and tweak the injector, timing, and possibly spark power for that cylinder to remove the miss fire and get the best burn.

I've told the tale of friend who came to visit, complained that car wasn't "lacked power", and struggled on the motorway hills. Took out the plugs, each one had an umbrella of deposit on the electrode and the gap was more like 2 mm. I very much doubt a traditional points 'n coil, mechanical advance, engine would have run at all. Couldn't get new plug so just cleaned and gapped as best I could and put 'em back. Report from chap after he got home was he had trouble keeping it below 80...

[Dave Liquorice]

I don't think he would have to, 16, 5'6" ish, the pentographs on top of engines operate at 2 to 3' so even if the wagon top was 2' lower than top of engine the line is around head height.

As you say massive amount of energy available, explosivly boil any water, fly in a fly zapper...

Hole(s) in boundary fence?

[tony sayer]

Around an inch in dry "ish" air, can be a lot less when its damp.

But there will be some leakage current before the grand flashover.

But do they teach children in schools these days or was he just showing off and thought that the lines weren't live as no trains were running on that line?.

A misconception that many people have...

I bet your average Joe hasn't a clue what the lines are or do even, and if they do probably thinks there're much the same and his house mains at home.

There yer go!, ask a few mates what sort of volts they think they are carrying..

Preferably non DIY ones too;!...