redundancy for track circuit

[onestrangeday]

Hi Signalling Professionals:

Recently, I have encountered a track circuit design philosophy which I have doubted the actual benefit that it brings to the railway system.

is it better to implement track circuit with redundant function ? (that's if one track receiver fails, the system will automatic change to other receiver unit for the same track circuit, so the track circuit will work normally as usual). Anyone got ideas ? or has anyone seen this kind of design philosophy implemented for track circuit ?

[Peter]

Hi Signalling Professionals:

Recently, I have encountered a track circuit design philosophy which I have doubted the actual benefit that it brings to the railway system.

is it better to implement track circuit with redundant function ? (that's if one track receiver fails, the system will automatic change to other receiver unit for the same track circuit, so the track circuit will work normally as usual). Anyone got ideas ? or has anyone seen this kind of design philosophy implemented for track circuit ?

We saw that the Swiss had done something similar for the Lötschberg Base Tunnel where, for reliability, they completely duplicated the axle counter train detection. Based on the logic that they were SIL4 systems, if the two outputs disagreed on the state of the section, selecting one over the other would be unlikely to result in a genuine wrong side failure since there are many more reasons that the section that was showing occupied would be showing that when it should be clear.

There is the question as to whether you have doubled the wrong side failure probability since you are always selecting the least restrictive and therefore always select the one (if there is one) that wrong side fails. That said, if the chance of a WSF on a SIL4 system is almost negligible, double negligible is still small.

Something to consider is the method by which you select one output or another - how reliable is your mediation system.

Peter

[onestrangeday]

Hi Peter:

ok, I see, so there is actual implementation on using 'duplicating' system for train detection purpose. I agree that it is very important for the logic of system to decided which one is normal (if other system is down) in order to determine the presence of train, otherwise WSF could occur. So have you seen any redundant design for track circuit ?

Hi Signalling Professionals:

Recently, I have encountered a track circuit design philosophy which I have doubted the actual benefit that it brings to the railway system.

is it better to implement track circuit with redundant function ? (that's if one track receiver fails, the system will automatic change to other receiver unit for the same track circuit, so the track circuit will work normally as usual). Anyone got ideas ? or has anyone seen this kind of design philosophy implemented for track circuit ?

We saw that the Swiss had done something similar for the Lötschberg Base Tunnel where, for reliability, they completely duplicated the axle counter train detection. Based on the logic that they were SIL4 systems, if the two outputs disagreed on the state of the section, selecting one over the other would be unlikely to result in a genuine wrong side failure since there are many more reasons that the section that was showing occupied would be showing that when it should be clear.

There is the question as to whether you have doubled the wrong side failure probability since you are always selecting the least restrictive and therefore always select the one (if there is one) that wrong side fails. That said, if the chance of a WSF on a SIL4 system is almost negligible, double negligible is still small.

Something to consider is the method by which you select one output or another - how reliable is your mediation system.

Peter

[PJW]

Not redundancy in that form- of course the most common causes of track circuit failure are insulation deterioration / low ballast resistance due to weather conditions, damaged jumpers and rail connections rather than what is generally regarded as track circuit equipment; therefore not useful to duplicate track feed or relay end as wouldn't make impact on reliability.
In addition, providing a second track relay would alter the characteristics of the whole track circuit, so it is not as simple as might first appear; the electrical disconnection of one relay would affect the current flowing through the other.

Certainly NR practice is to duplicate all track leads, jumper connections etc., so some of the more unreliable components are duplicated.

Conversely London Underground does duplicate TI2I receivers but this is done for reasons of increased SAFETY rather than increased RELIABILITY; both relays must be energised for the track to be considered clear, this to avoid a "single point of wrongside failure". Whereas NR basically put full confidence in the operation of each 930 series relay in interlocking circuitry, LU always insist on wiring two relay coils in parallel and then use contacts of each relay in series with each other: "to be sure, to be sure".

Indeed NR do themselves sometimes use two track relays for the same dc track circuit and require both to be up for the track to be considered clear. This however is for a slightly different reason; it is protection against a combination of "residual voltage" (ballast battery effect) and a broken rail / disconnected jumper- there is the usual track relay and a track feed relay at the other end of the track circuit.

There are also a few sites where the usual train detection is by track circuits but in addition there is axle counter detection. This can be similar to what Peter has described for the Lötschberg, but actually it can be a form of degraded mode. Instead of a the same number of track sections as provided by TCs, there may only be a single axle counter section for the entire area. Hence there isn't the same degree of flexibility / capacity but still a method of keeping some train service in the event of a TC failure.

Hi Peter:

ok, I see, so there is actual implementation on using 'duplicating' system for train detection purpose. I agree that it is very important for the logic of system to decided which one is normal (if other system is down) in order to determine the presence of train, otherwise WSF could occur. So have you seen any redundant design for track circuit ?

[Peter]

Hi Peter:

ok, I see, so there is actual implementation on using 'duplicating' system for train detection purpose. I agree that it is very important for the logic of system to decided which one is normal (if other system is down) in order to determine the presence of train, otherwise WSF could occur. So have you seen any redundant design for track circuit ?

No. I am not sure that anyone has seen the need for the marginal reliability benefit vs the cost.

In aswering a question like this, you would need to think what failure modes would be avoided by the two receivers, and what failure modes will make both receivers show the track occupied. Would it really be worth it.

Peter

[onestrangeday]

thanks for the information provided. The reason I ask for this question is that one of the track circuit system I've seen does have two receivers and two transmitters for same track circuit. (transmitter works in hot standby--only one transmitter is working at the time, whereas receivers works in parallel, both receiving and comaring the track circuit level), in simple term everything has backup, it certainly costs much higher but reliability is enhanced.

Personally, I have not seen this kind of implementation in the past, it's also the reason I would like to ask has anyone seen this kind of design. But I can see it's not usual practice in UK.

Hi Peter:

ok, I see, so there is actual implementation on using 'duplicating' system for train detection purpose. I agree that it is very important for the logic of system to decided which one is normal (if other system is down) in order to determine the presence of train, otherwise WSF could occur. So have you seen any redundant design for track circuit ?

No. I am not sure that anyone has seen the need for the marginal reliability benefit vs the cost.

In aswering a question like this, you would need to think what failure modes would be avoided by the two receivers, and what failure modes will make both receivers show the track occupied. Would it really be worth it.

Peter

[kumarangovindan]

Hi,Signalling Professional,
In INDIA Parrallel redundency is provided for Track circuits with axlecounters, track circuits fails ,if axlecounter clear in the failed track circuit area concerned signal can be taken off.

[PJW]

As I understand things, in India a driver cannot simply be "called past" a controlled signal at danger purely verbally by the signaller, but needs to be handed a physical written authority by a handsignalman. Getting the person to the remote site can cause long delays and perhaps it is this which justifies the diversity of axle counter and track circuits

Hi,Signalling Professional,
In INDIA Parrallel redundency is provided for Track circuits with axlecounters, track circuits fails ,if axlecounter clear in the failed track circuit area concerned signal can be taken off.