How does a TCA work?

[MichaelAMW]

This has probably been asked but I can't find a suitable thread and and Google hasn't been my friend. Question, more or less as stated: how does a track circuit actuator prevent a train being "lost" when it might fail to operate a track circuit correctly, e.g. owing to dirty rails etc. You can assume I did ok in my GCSE double science when it comes to how technical you might want to be. Alternatively, please direct me to the thread that explains it.

I'm curious because of comments raised elsewhere about leaves on the line this time of year and how the TCA can help with train detection in those circumstances.

 

[swt_passenger]

The preferred terminology is track circuit assister. That should increase Google's success rate I think.

Instead of just relying on an electrical short circuit between the rails, the unit's equipment superimposes an AC voltage between the rails. The frequency used (180 kHz or something similar) helps to break down any increased resistance in the contact patch.

 

[edwin_m]

The TCA is effectively a single-turn transformer. There is an incomplete loop of copper tube within the bogie and as stated a signal generator circulates a high-frequency current through this. The other "coil" is the circuit formed by the wheels, axles and rails, which therefore carries a current at the same frequency through all four wheel-rail contact patches on that bogie (or whole vehicle in the case of a Pacer).

For reasons not fully understood, this high frequency current reduces the impedance of the contact patch at the lower frequencies (DC up to a few hundred hertz) used by track circuits, and therefore makes it more likely the train will be detected. The TCA is not effective if the rail is fully insulated, for example by heavy leaf contamination, but there is a track-mounted device called at TCAID that can be fitted in the worst areas, and detects the much lower induced current caused by a TCA-fitted train.

 

[najaB]

For reasons not fully understood...

Not fully understood by yourself, or by science? Just curious if this is a true unknown - might make a decent research project if I ever get back into academia.

 

[Sunset route]

The preferred terminology is track circuit assister. That should increase Google's success rate I think.

Instead of just relying on an electrical short circuit between the rails, the unit's equipment superimposes an AC voltage between the rails. The frequency used (180 kHz or something similar) helps to break down any increased resistance in the contact patch.

Funny the rule book TW5/21 still refers to them as Track circuit actuators (TCA)

 

[HarleyDavidson]

Not sure exactly how they work, all I know is that I can't take a train out with it not working.

 

[edwin_m]

Not fully understood by yourself, or by science? Just curious if this is a true unknown - might make a decent research project if I ever get back into academia.

When I was with BR Research (who invented it) nobody seemed completely sure. I wasn't involved in the original design but was part of a couple of projects to fit TCAs and TCAIDs. I think the Research archives went to RSSB so they may have something on the basic physics.

 

[HarleyDavidson]

Ger meen to say "Ye canna change the laws of physics Jim" sorry edwin_m?

 

[tsr]

The preferred terminology is track circuit assister. That should increase Google's success rate I think.

Instead of just relying on an electrical short circuit between the rails, the unit's equipment superimposes an AC voltage between the rails. The frequency used (180 kHz or something similar) helps to break down any increased resistance in the contact patch.

My employer, a v. large commuter TOC, seems to know them as actuators, at least from everything that I've read about what I need to know about them (regrettably not much). Certainly in the fault finding docs.

 

[swt_passenger]

My employer, a v. large commuter TOC, seems to know them as actuators, at least from everything that I've read about what I need to know about them (regrettably not much). Certainly in the fault finding docs.

It is Ellis's encyclopaedia that says assister is the preferred term, I wasn't really trying to prove a point, it's just that you do get more hits with assister than actuator.

I also found a couple of RSSB research papers that call them TC Assisters, but they also refer in the pre-amble (quoted just below) to them being 'sometimes referred to as TC Actuators'. This one for example, uses Assister in the title of a 'steering group':

Track Circuit Assisters (TCA) Steering Group Remit

1 Introduction
1.1 This document defines the working arrangements for the Track Circuit Assisters Steering Group (TCA SG) in accordance with SIC Protocol, Issue 4.
2 Purpose / Scope of the Sub Group
2.1 The TCA Steering Group will report to the Vehicle / Train Control & Communications System Interface Committee’s (V/TC&C SIC), Train Control Technical Sub Group (TCTSG) and comprise of representation from across the industry.
2.2 TCA are sometimes referred to as Track Circuit Actuators and are used to improve train detection dependability by assisting shunt performance.

http://www.rssb.co.uk/Library/groups-and-committees/2011-remit-tca.pdf

 

[edwin_m]

BR Research was very insistent on calling them Assisters, but as usual with the railway it has several names for the same thing.

 

[30909]

“What’s in a name?” Romeo and Juliet (II, ii, 1-2) if they do the same job?

 

[tsr]

“What’s in a name?” Romeo and Juliet (II, ii, 1-2) if they do the same job?

Indeed. I was not trying to prove a point, just emphasise that "actuator" is not now an unusual name. Obviously the technical purpose still stands!

 

[Phil.]

Funny the rule book TW5/21 still refers to them as Track circuit actuators (TCA)

That's because it's the correct word. It actuates or makes it work. It's incorrect to refer to it as a track circuit assister because it's not assisting the track circuit, it's making it work. The entire rail system - indeed the country - is full of incorrect terms that have been accepted into common use.

Just another example of the degradation of the language I'm afraid.

 

[Juniper Driver]

That's because it's the correct word. It actuates or makes it work. It's incorrect to refer to it as a track circuit assister because it's not assisting the track circuit, it's making it work.

Just another example of the degradation of the language I'm afraid.

I learn't the 159's in 2001 and that what I have always know them called as (even in training).Track Circuit Activators.I will check my 159 manual to see how it's worded,though.

 

[edwin_m]

I think the point of "assister" is that (as I posted further back) there has to be some amount of conductivity between the wheel and the rail - if there is very little or none the TCA is ineffective. The use of "actuator" could lead to the false impression that its very presence will ensure the track circuit operates.

 

[Barmay]

Many moons ago I was involved in the design and development of the TCA system whilst working for the now defunct BR Research. Most of what has been stated above partially goes to explaining how it works just think of it as a transformer with the windings separated by a big air gap and you will begin to understand how it all works.