electro-Physics: 17.3409 volts

From: Michael Sokolov <msokolov_at_ivan.Harhan.ORG>
Date: Tue Dec 14 13:27:51 2004

Tore S Bekkedal <toresbe_at_ifi.uio.no> wrote:

> Norwegian trains run DC...

As do Russian electric trains. 4 kV.

> I've seen a tagger actually climb up on the
> roof of a train, and before I could run to the veranda of my appartment
> to tell him to get the fuck down, the dumb fuck stood up. He illuminated
> the entire train exchange for a millisecond, and then all that was left
> of him was one of his feet, that apparently hadn't made contact with the
> grounded train roof. His charred remains were still burning 20 minutes
> later, until the firemen came and cleaned up the mess.

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.

Received on Tue Dec 14 2004 - 13:27:51 GMT

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