Confusion about ground comes from the fact that it is used to mean several different, often conflicting things. In this case, “ground” just means a return path for current to travel. Something at a high voltage has to be connected to something at a lower voltage for charge to flow and current to pass. Kind of like a ball on a surface has to be connected to a lower surface if you expect it to roll without pushing it.
“Ground” also refers to a safety measure where an alternate path for current to travel is provided so that if there is a short circuit, the high current will follow that path instead of a path through the nearest human.
“Ground” can also mean the common point of reference relative to which all other voltages in a circuit are measured. When I say I am 5’9", I don’t mean I’m five feet and nine inches higher than sea level. I mean I am five feet nine inches above the ground.
A circuit needs a return path. In some circuits, the return path is designed to be at the same electric potential as the surface of the Earth, which can be a good idea as it makes that part of the circuit less dangerous, but it’s not physically required: You could have a perfectly good circuit with the two legs at a million volts and a million ten volts. In some circuits, the Earth itself is used as the return path, with just one actual wire, but I’m not sure of any specific examples where that’s used (it’d usually be nearly as easy and more efficient just to have two wires).
So how far is it from the third rail to the next closest rail? If a rat manages to put one foot on the third rail and one foot ( or a tail ) on the other, then it gets a shock, possibly enough to toast it. Otherwise it’s as safe as a bird on a wire.
But the NYC third rail system looks very similar, including the insulating “cover” over the third rail so that it is a little harder to make causal contact.
Hmm… The rat could easily touch the ground underneath and the third rail at the same time. I know Chronos said that concrete (and by extension, the stone ballast) would be a good insulator, but 600v might be enough to get a deadly current through the rat. I’ve certainly given myself some unpleasant shocks by standing with bare feet on concrete and touching a hot piece of metal (in a very poorly wired shop). Although, even packed stone is probably a lot better at insulating than a concrete slab, with a smaller number of contact points from rock to rock. Is the ballast further separated insulated from the rest of the earth, for this purpose?
Is the third rail inside the housing? Could I walk on top of it without doing harm, but if I stuck my hand in the middle gap, I’d contact the electrified part?
The “housing” (shiny white part) is just a flimsy sheet metal cover to keep leaves & junk from falling on the rail. More like an umbrella than a cover. It wouldn’t support a person.
The 3rd rail itself is the rusty thing in the shade held up by the two conical supports
I am surprised to see wooden cross ties. It’s been a long time since I saw any in Spain where it probably been 30 years or more since they started using concrete ties and I doubt there are any wooden ones left.
Many years ago (Aug '84, apparently), Electronics Today International magazine published a circuit diagram for an electronic mouse killer - basically a capacitive discharge device. But the author only wanted to kill rodents, so the two PCB pads (big outer ring and inner pad with bait) ran with a fairly high potential, but no current to speak of. When the two pads were bridged by some critter, the circuit evaluated the resistance across the pads, and if it was in the rodent zone, triggered the capacitative discharge, killing the rodent and throwing the poor creature across the room, and then resetting. Of course, the successful operation of the device required knowing one vital piece of information: the internal resistance of a mouse. The designer must have gone through several mice to determine that information.
So, to figure out if a rat will be killed by rail to concrete contact you need the internal resistance of the rat - and if you find the ETI article, it may just have it.
I’ve heard of this too, but I’ve been unable to find any examples.
I worked as an electrician for many (30) years, first in construction then maintenance. Then I came to a much easier way to make a living as a tech. So I’ve been around electricity a lot, both high and low voltage, and would surely have encountered the practice of using earth as a conductor if such a practice existed. Plus I’ve done some research. Nada. Except for the unlikely comparison to radio transmission, all circuits use a return wire.
A big fat virtual hug (manhug) to anyone who can show me.
Peace,
mangeorge
When I checked out the third rail in the New York subway, there was no cover at all over it. Funny thing is, I remember the rail inside the tunnel, but not in the station. This was back in the 60s.
The third rail for BART, the SF Bay area subway, is covered for it’s entire length.
NYC subway third rails are covered, but the cover is wood in most places. The contact shoes on the NYC subway ride along the top of the third rail, under the cover.
Here is a closeup showing the third rail and contact shoe.
Here you can see the 750-volt third rail used on parts of the UK overground network. As you can see, it’s uncovered but there is quite a distance between it and the running rail. Even with this arrangement I don’t think many larger animals such as foxes and badgers end up being fried.
Probably not quite the example you’re looking for but they used to run single-wire phone lines in some rural areas. They were subject to a lot of interference though from electric trains, mining, storms, etc. and also certain areas where the ground has fairly high resistance. I doubt they’re used any more. I thought the military and miners used single-wire detonators that used the earth as a return, not sure about that though.
As mentioned below firetowers also (in NY at least) use to run a single line telephone line from the tower down to where ever it hooked into the telephone network, using earth ground as the return.
Also FWIW just as a experiment I used a pole driven into the ground as the return to run a electric drill with just the hot going to it. The speed of the drill increases as it was driven further into the ground (safety information omitted, don’t try this at home)
I don’t think “return” is technically correct (I know, a nitpick) in these situations, given that it’s unlikely that the same electrons are going back to the source. More likely they’re being dissipated into the earth trying to equalize the charge in the area. The charge in the area is pretty much a diminishing circle, (or upside-down hemi-sphere?) depending upon soil conditions.
We did decide on the “flow” theory vs the “bump” theory, didn’t we?
I remember clearly looking out the back window of a subway car and seeing the bare third rail alongside the other two rails. I think it was about 1/2 the distance between the main rails away from the pair. I spent some months at the Navy Yard, and rode the subway a lot.
This was over forty years ago, and, though unlikely, could have been on Chicago’s El.
No, I wasn’t stoned.
The same electrons never return to the source - as a matter of fact, in an AC circuit, they never even leave the generator. Even in a DC circuit, it might take day for an electron to complete the circuit - the drift velocity is only a few cm/sec.
But that’s for a low resistance DC circuit, where you have a good conductor for the entire circuit. What happens to the slow-poke electrons if you lift the negative (grounded) conductor at the source?
I had forgotten about the AC back-and-forth electrons.
Thanks for the synapse stimulation.
