Tore S Bekkedal <toresbe(a)ifi.uio.no> wrote: As do Russian electric trains. 4 kV. Ouch. My dad told me a story of something similar happening in Russia with an unlucky railroad maintenance worker. Apparently the vertical spacing between the roof of a train car and the high voltage wire is high enough for workers to be able to walk on the roof under the wire and not touch it, even though it doesn't seem that high when looking from the ground. But that unlucky worker was carrying a wrench in his hand and had it stuck up in the air... When I was an intern on the Moscow railroad I witnessed another incident, this one fortunately without fatalities or human injuries, but demonstrating the power of that DC supply. On Russian and probably many other railroads the rails carry both the power current (return) and signaling current. The power current is DC, the signaling current is AC (at very low voltage of course, 6V). The track is divided into block-sections, each block-section monitored as a unit and reported as free or occupied to the rail traffic control system. Block-sections are separated by insulated rail joints. Each end of a thus isolated block-section is connected to a transformer whose other (higher voltage) winding is connected to a wire pair going back to the rail traffic control centre. Simple enough on diesel-powered railroads. It was a little more complicated on electrically powered railroads. The thick copper wires from the ends of each insulated joint were connected to transformer windings like before, with a separate transformer for each side of the insulated joint. The high-voltage windings were connected to signaling lines like before. But the low-voltage windings, the ones connected to the rails, were centre-tapped, and the centre tap from the transformer on one side of the insulated joint was connected with a thick copper wire to the centre tap on the other side. This way the DC power current effectively passes right through while the insulated joint acts as a barrier to the signaling AC current. The town where I lived and the railroad station on which I had my internship was on the end of a low-traffic branch of Moscow railroad. The station itself had electric centralisation, the term used in the Russian railroad technical documentation for the rail traffic control system at a station. The track stage between that station and the next, however, was unmonitored. What this means is that at the station the left and right rails had the low signaling AC voltage between them, but out on the stage the two rails were simply connected. Thus the last insulated joint at the station boundary had only one transformer box next to it, the ends of the winding connected to one side and the centre tap connected to the other side. Well, one fine day a track maintenance vehicle knocked that transformer box down, and it fell tumbling down into the ditch. Apparently the track maintenance workers didn't notice it and went on with their business. But the monitoring circuit was broken of course. Rail circuits are normally closed. An open rail circuit corresponds to a track section that is either busy (the train axles short the circuit and all the current goes through them rather than through the relay on the other end, so it reports as open) or broken (a rail broke under load or was removed by terrorists, etc). So when that transformer box got knocked down and the circuit broke, it just lit a red light on the panel in the rail traffic control room indicating a busy track section. We (the traffic controller lady and me-the-intern) noticed it and went to check it out, but the trains still had to go on schedule while a crew was dispatched to mount the transformer box back where it belongs. So we let one or two trains through, using the manual override button to override the track busy status. (The manual override buttons have seals on them that you have to break in cases like this when you have to use them, and LOTS of paperwork must be generated.) So we let those trains through. Now think what happened when the train crossed that insulated joint and continued on our side, our station being the end of the line. From what I understood when I was there, our end was not the one supplying train power, so when a train was on our side, its return current had to go through that insulated joint, i.e., through the transformer centre tap. With the transformer gone, where did the current go? Well, it arced right through the insulated joint and melted the ends of the rails. Pretty impressive. MS