On Tue, 2 Dec 2008 07:49:35 +0000, "Clive D. W. Feather"
wrote:

In article , Steve Fitzgerald
] writes
Traction current is fed at +420v (positive) on the outside rails and -
210v on the centre rail (negative) the sum of these giving a traction
feed of 630v dc.

They are 'loosely' tied to earth through resistances in the sub
stations that feed the supply. Despite being an electrician in a
previous life and now a driver, I struggle to get my head round
'loosely tied to earth' and what it means in real life.

There are resistors (I've seen various figures between 1 ohm and a few
hundred ohms) connecting the power rails to earth. Because they are in a
2:1 ratio, in the absence of anything odd going on you get a small
current flowing through those resistors and thus a potential divider.
For example:

+----- +ve{630V supply}-ve -----|
| |
Outer Centre
rail rail
| |
|----[210ohm]---+---[105ohm]----|
| | |
|
Ground

In the idle state, there will be 630V across the pair of resistors which
add up to 315 ohms, meaning 2 amps will flow through them. Since the
central tap is grounded, the outer rail will be at +420V relative to
ground and the centre rail at +210V.

Suppose that something shorts the centre rail to ground, bridging out
the 105 ohm resistor. We now have:

+----- +ve{630V supply}-ve -----|
| |
Outer Centre
rail rail
| |
|----[210ohm]---+---------------|
| | (short) |
|
Ground

The centre rail is at ground potential and the outer rail is at 630V
because of the power supply. 3A now flows through the resistor. If,
instead, we short the outer rail to ground, a similar change happens.

Finally, what happens if some strange event causes a 210V potential
between the outer rail and ground? We get:

+----- +ve{630V supply}-ve -----|
| |
Outer Centre
rail rail
| |
|----[210ohm]---+---[105ohm]----|
| | |
|----{210V}-----|
| |
|
Ground

The difference between the 210V and the 630V supply means that the
centre rail will spring to -420V relative to ground. There will be a 1A
flow through the 210 ohm resistor and a 4A flow through the 105V
resistor.

In other words, the resistors keep the rails at the +420V/-210V position
*unless* something else happens to alter this. Hence "loosely tied". But
at all times (absent a major short) the two rails are 630V apart.

They can be both at the same potential if there is a break between one
conductor rail feed and the substation (or only one of the pair of
switches feeding from the next section is closed) and you have a train
in section. There will be no voltage between the rails but they will
definitely be alive, leaving a trap for anyone who ignores the
standard "do not touch any rail" warnings,