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.
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.
(30-12-2011, 02:55 AM)onestrangeday Wrote: 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 ?
PJW