Longest electrified stretch fed from one end?

[zwk500]

Isn't the phrase 'volts jolts, mills [milliamps] kills', because it's the current that does the damage, and even 25KV isn't always fatal.

 

[AM9]

Isn't the phrase 'volts jolts, mills [milliamps] kills', because it's the current that does the damage, and even 25KV isn't always fatal.

It's current flowing through the body, and particularly certain organs. So to get a certain current (>10mA sustained can be lethal, >100mA almost certain to be lethal) requires a voltage sufficient to overcome contact and body resistance. So in simple terms, at 750V a total resistance would need to be lower than 75kOhms for a severe shock and 7.5kOhms for a lethal shock. For 25kv, those resistances would be 2.5MOhms and 250kOhms respectively. Despite the 25kv needing a much higher resistance for survival, by design, coming into direct contact inadvertently with exposed conductors on those systems is much less likely, which is why it is inherently safer.

 

[Bald Rick]

Not necessarily. Ian Allan, in his autobiography, wrote of more than one Southern employee in WW2 who in the blackout stumbled over the live rail, but was saved by the voltage being generally reduced due to fuel shortage. Not a sure thing, but it can help.

I know people who have had a belt off the 750 and lived. My point, badly put, is that the voltage doesn’t really make a difference; it’s the current that’s the issue.

 

[LondonExile]

My point, badly put, is that the voltage doesn’t really make a difference; it’s the current that’s the issue.

The voltage does make a difference.

It's the current that kills, but Ohm's law (V=IR) also comes into play. At a basic level, rearrange the equation to get I=V/R, i.e. current is directly proportional to voltage. Now - if you're working with a current limited system where you simply cannot draw more than say 1mA, it's true that the voltage is going to be irrelevant, but if there is capacity to deliver a potentially lethal current, the voltage is still critical to the relevant safety of the system.

No electric system capable of powering trains is going to be current limited to the point that it is safe for humans to touch!

A car battery can potentially deliver 500A or so of current, but the fact it's only 12V means it has no chance of doing so through the human body.

 

[Taunton]

My point wasn't anything really to do with safety, but that it was consistently stated from HQ that the voltage at Lymington, far end of the section, was 750v, when in fact at the distance it is from the substation (original point of thread) you invariably only got a proportion of this, being a well-known issue to the electrical engineers.

The same problem used to afflict North Woolwich when it had the North London Line dc electrification. It seems to be a particular issue with dc single track branches, where the designers feel assured there will never be more than one train.

 

[Dai Corner]

My point wasn't anything really to do with safety, but that it was consistently stated from HQ that the voltage at Lymington, far end of the section, was 750v, when in fact at the distance it is from the substation (original point of thread) you invariably only got a proportion of this, being a well-known issue to the electrical engineers.

The same problem used to afflict North Woolwich when it had the North London Line dc electrification. It seems to be a particular issue with dc single track branches, where the designers feel assured there will never be more than one train.

As has been stated before, only under load. No current flowing means no voltage drop (ohm's law). Place a voltmeter across the relevant rails at Lymington or North Woolwich when there's no train taking power and it would read as near as dammit the same as at the feeder station.

If 450v was the minimum acceptable voltage that's what the designers would ensure was provided when the maximum required current was demanded.

 

[AM9]

My point wasn't anything really to do with safety, but that it was consistently stated from HQ that the voltage at Lymington, far end of the section, was 750v, when in fact at the distance it is from the substation (original point of thread) you invariably only got a proportion of this, being a well-known issue to the electrical engineers.

The same problem used to afflict North Woolwich when it had the North London Line dc electrification. It seems to be a particular issue with dc single track branches, where the designers feel assured there will never be more than one train.

It's a calcuated balance of operational reliability vs cost. f iIt is truly a single track with no parallel cables to reduce resistive losses, then the resistance of the steel, aluminium or composite conductor rail plus the steel running rail return will be in series with the supply. If there is a train at the remote end of the track drawing a high current (say 1000A for short periods), as suggested, the voltage at the end of the line will drop to c.450V. As soon as the driver backs off the traction power, the line voltage will rise to near 750V, - or even more if regen is allowed there (which I doubt). So in human terms, the risk is 750V for most of the time with occasional dips to as low as 450V.