(20-03-2009, 02:01 PM)sugavanam nagarajan Wrote: I have some basic doubts.
Please Clarify the following:
At what conditions TPWS requires TSS alone? or wherever TPWS , there should be TSS and provision of OSS is optional - Does it mean like that?
How TPWS+ is working?
What is the Safety integrity level for AWS and TPWS?
-Sugavanam
If intervention by a TSS positioned just beyond the signal results in the train stopping within the SOD (Safe Overrun Distance) i.e. before it can reach a junction or head-on collision then it is effective; therefore no need for an OSS. Depends upon length of SOD (generally that of the overlap but depending on circumstances can be greater), its gradient etc. and the permissible speed of the line. Broadly however OSS are not normally required if speed is<40mph.
OSS are not optional- the standard lays down the minimum criteria when they are required and in these circumstances are effectively a legal requirement in the UK under the
Railway Safety Regs 1999. However they can also be provided in other circumstances if judged to provide useful risk mitigation; given that there is a requirement to reduce risk to be ALARP, the word "optional" does give the wrong impression but these additional ones wouldn't be needed if there were an alternative means to adequately address that risk, nor if the costs of providing were disproportional to the benefit gained.
TPWS+ is just another OSS placed even further from the signal, typically nearly 750m. Energised whenever signal at red, just as per the usual OSS. Obviously the pair of loops at the OSS+ site have a wider separation than the OSS; it just adds as an outer speed trap set at a correspondingly higher speed. OSS+ is provided when an intervention at the OSS would not stop the train within the SOD. Since the OSS intervention would generally do so for lines of under 70mph, OSS+ tend to be used for higher line speeds and in particular where a collision of 100mph or more could result (i.e. provided in the highest risk sites only).
I am unaware of any SIL claimed for TPWS. The fundamental concept of the way it operates is "badly flawed" from that perspective; certain failure modes are obviously present. However I am sure that very careful analysis was made of failure modes with the intention of making the whole system as reliable (and therefore as safe) as practicable BUT you need to recognise that it was not designed to be the ultimate in safety. It was designed as a system to provide a very useful amount of risk mitigation that could be reasonably easily and quickly installed; in the event it has actually delivered rather more risk reduction than specified. I suggest you read
Sir David Davies' Report which fundamentally demonstrated that better overall safety would be achieved by installing what was available than waiting to develop a theoretically more robust solution.
For discussion of SIL I very strongly recommend you read Roger Short's paper (IRSE News 142, Feb 2009). AWS was developed and in widespread use at least 25 years before the term SIL was invented (as indeed so is at least 90% of the signalling in the UK); the absence of a "SIL" doesn't mean that it is anyway unsafe.
