TPWS OSS+

[Archie]

Hi there, can anyone explain the function of the OSS+ and the circumstance in which it is used.

Many thanks

[PJW]

Hi there, can anyone explain the function of the OSS+ and the circumstance in which it is used.

Many thanks

The function of any TPWS fitment, TSS or OSS is to make an emergency brake application. For a TSS situated at a signal the SPAD has happened and therefore the intervention is irrespective of speed whenever the loop is energised (signal at danger) An OSS is positioned on the approach to the signal and acts as a speed trap- it too is energised when relevant signal at danger but only intervenes if the train speed is over a specific limit (set by the loop spacing) which is determined to be the lowest speed at which a SPAD is regarded as "extremely likely" to occur. By starting the emergency braking early in anticipation of a SPAD, this improves the chances of containing the overrun within the safe area beyond the signal.
The standard TPWS fitment (as per the retrofit standards for national rollout c 2000) provided a TSS and, except where speeds were so slow compared with SOD distance, also an OSS. This was positioned according to a laid down tabulation, but effectively a compromise between
a) false interventions and
b) failing to intervene when it should.

On this basis TPWS was broadly effective for speeds up to around 60mph (depends on gradients, SOD length, train characteristics etc). In order to cope with higher speeds (i.e. have enough distance to bring the train to a stand within the SOD) then need to provide an additional OSS positioned further away from the signal. This is identical to the OSS but with a high set speed (loop spacing) toreflect that even a train that is going to stop will still be going quite fast here. For technical reasons this additional OSS can be a maximum of 750m from the signal; this is called the OSS+ and gives safety benefit where the maximum permissible speed is such that the OSS by itself is not adequate. However there is a cost-benefit analysis to be performed and going back to upgrade a existing site is expensive so therefore the sites which warrant fitment are those where there could be a junction collision at high speed.

Life has moved on and for new works / alterations there is now a requirement to prove that the TPWS is sufficiently effective at all critical signals. The terms standard OSS, OSS-, OSS+ are not applicable in this scenario; each site is now calculated individually and the positions of the various OSS are optimised- start st the signal work back to place OSS1 such that it provides protection at the speed for which the TSS is no longer effective, then work back to place OSS2 so that it takes over once OSS1 no longer effective etc until get protection at the max permissible speed.

Hope this helps; to aid future reference I am repositioning this thread under the Train Protection sub-topic.