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[email protected] · September 24th 05, 12:03 AM posted to uk.transport.london

In article ,
() wrote:

In article ,
(Clive) wrote:

In message ,
writes
No, it is main line. However only a small amount is lost (and keeps
on escaping until the tripcock is reset) and so has no effect on the
overall main line air pressure on the train
Please explain why the Main Line should be affected by operation of
the Tripcock?
--
Clive

I haven't got easy access to the Alstom diagrams and notes at present
(they're buried up in the loft), however, from what I remember, the
basic principle is similar to a control governor, main line governor
etc.

Main line air on one side of the piston face (it may well be a
diaphragm) in the pressure switch pushes the piston against a spring.
If there is sufficient main line air, contacts will be made. If the
main line air is reduced or lost, the spring pushes the piston back and
the contacts will be broken.

When the tripcock valve is operated, the main line air under the piston
is vented to atmosphere, and the spring pushes the piston back. There
is a restricted flow of air into the tripcock valve, so that when the
valve is open, only a minute amount of air is coming from the mainline
pipe and going to atmosphere. This leak is easily overcome by the
compressors. When the tripcock is reset, the valve is closed, the air
builds up under the piston and the circuit is made. The air leak also
acts as an indication that the train has been tripped, although whether
it is designed to do that or not I don't know.

Note
The above is only a very rough guide, the exact working is probably a
bit different, but it shows the principle.

If the train is front tripped, the safety circuit (A & B) is broken. If
the train is rear tripped, the round train circuit is broken. Either
way, an emergency brake application will automatically be made. If the
tripcock cannot be reset, the contacts can be by-passed, but the
tripcock cannot be pneumatically isolated in the same way as is done
with those on the train line pipe. There is no need.

If I manage to find the diagram in the next couple of days, I'll stick
it on a web site.

Roger

I've looked up the information:

A feed comes from the main line pipe via a choke (to limit the amount of
air lost when the tripcock is operated) and goes to three
(diaphragm operated) tripcock pressure switches (Safety A circuit, Safety
B circuit and the Round Train circuit). The feed also goes to the tripcock
valve.

When the train is tripped, a valve is moved, uncovering an exhaust port
and the main line goes to atmosphere. Because of this, the pressure in the
pressure valves drop and the contacts on the Safety and Round Train
circuits open. This causes an emergency brake application. The trip valve
will remain latched open until the tripcock is reset, and a small amount
of air will be escaping to atmosphere. When the tripcock is reset, a
spring will move the valve back, covering the exhaust port. Main line can
now build up in the pressure switches and the contacts will close.

There test switches, used as part of the driver's train preparation,
which will also vent air away from the tripcock pressure switches in order
to simulate the train being tripped and thus test the operation of the
pressure switches.

Roger