09-03-2009, 10:53 PM
Someone (who may not want to be identified so I have not done so) has asked me the following questions in an email. I think it best to reply in public since I suspect many others may be interested. Few are actually easy answers!
1. Very pertinent question.
TPWS would perhaps be more accurately called- "SPAD Risk Reduction System. Although it does intervene and give a level of protection it falls well short of a true Protection system. It certainly gives no warning before it acts. Therefore I agree that "Protection and Warning" is a double misnomer; however it is the name it has been given. I have no idea why (aprat from Public Relations / politics)- we just have to accept it.
2. The TPWS TSS loop is just beyond the signal (to act AFTER the SPAD) whereas the AWS is on the approach (generally 180m) to give a warning at a time that the driver can take a further look at the signal having been alerted that it is not at Green. Therefore the answer is AWS.
However where a TPWS TSS is insufficient, then one or more TPWS OSS loops are provided to act as a speed trap prior to a signal at Red. An OSS is almost certainly further from the signal than the AWS and therefore for such signals the answer is TPWS:OSS! [Of course unless the driver is driving inappropriately, passing over TPWS loops has no effect so are "invisible" whereas an AWS always gives eithr a bell or a horn.]
3. There are actually practically no POSAs yet on NR; the only ones I know of are on CTRL line (see current IRSE News article) which was not implemented to tradiitional NR standards anyway. I would expect a Control Table (albeit very simple content) for POSA aspects- these have yet to feature in IRSE Mod 3 layouts for the exam (though have come up in written essay type questions).
4. oooh- very political..........
Double Red arguably improves safety at the expense of performance. It is needed on relatively high speed lines where TPWS cannot otherwise stop a train within the Safe Overrun Distance beyond the junction signal and the resultant collision would be high consequence; hence the signal on the approach to the junction signal is held to red as well. Dead Double Red is now certainly felt to be too draconian; Conditional Double Red (whereby the outer signal is allowed to clear after a train has already passed over its OSS) is how we'd generally implement. Unlikely that the IRSE Control Table layout would feature such a scenario; again more likely to be an essay type question I think.
5. TPWS is indeed the alternative to fail-safe design. Brakes are applied only if the train picks up the relevant frequencies- there can be on-board failures or trackside failures that would prevent, so certainly NOT failsafe. However such failures are quite rare, the trackside is reasonably well self-proved and this checked in the aspect of the signal in rear, the on-board performs a self-test each time a cab is opened so that faults cannot develop and remain undetected for a long time.
It therefore can be assumed to be present and effective for the vast majority of the time and hence provide extremely useful risk mitigation; it cannot be absolutely relied upon. AWS is generally regarded as pretty much fail-safe; but there are failure modes that are not........to be honest the whole concept of the "black and whiteness" that fail-safe implies is really not how we should view the world in 21st century.
6. By a plate (black bar on white background) that is still generally called "Auto plate".
7. The road user does not actually have to; not relevant to them.
The educated railwayman when driving their car can however tell, primarily by looking whether there is a floodlight for the road surface (so rail driver can see whether the crossing is clear) and a Driver's Crossing Indicator light for the rail driver (that confirms the road lights are adequately displayed etc)- if these are present then it is clearly an ABCL. Another hint may be the quality and alignment of the rail track / speed of passing rail traffic (as ABCLs can only be utilised at low speed whereas AHBC may be used at up to 100mph).
Quote:Dear Sir,
I have some basic doubts.
Please Clarify the following:
1. As TPWS is not giving any warning to the driver - Why we are calling as Train Protection And Warning System?
2. If the signal has both TPWS And AWS,normally which one will be faced by train first?
3. Why there is no control table for POSA?
4. When Two Red Rule will be preferred?
5. What is the alternate solution instead of fail safe design?
6. How the driver identify If controlled signal is passable signal?
7. How the road user identify AHBC from ABCL?
1. Very pertinent question.
TPWS would perhaps be more accurately called- "SPAD Risk Reduction System. Although it does intervene and give a level of protection it falls well short of a true Protection system. It certainly gives no warning before it acts. Therefore I agree that "Protection and Warning" is a double misnomer; however it is the name it has been given. I have no idea why (aprat from Public Relations / politics)- we just have to accept it.
2. The TPWS TSS loop is just beyond the signal (to act AFTER the SPAD) whereas the AWS is on the approach (generally 180m) to give a warning at a time that the driver can take a further look at the signal having been alerted that it is not at Green. Therefore the answer is AWS.
However where a TPWS TSS is insufficient, then one or more TPWS OSS loops are provided to act as a speed trap prior to a signal at Red. An OSS is almost certainly further from the signal than the AWS and therefore for such signals the answer is TPWS:OSS! [Of course unless the driver is driving inappropriately, passing over TPWS loops has no effect so are "invisible" whereas an AWS always gives eithr a bell or a horn.]
3. There are actually practically no POSAs yet on NR; the only ones I know of are on CTRL line (see current IRSE News article) which was not implemented to tradiitional NR standards anyway. I would expect a Control Table (albeit very simple content) for POSA aspects- these have yet to feature in IRSE Mod 3 layouts for the exam (though have come up in written essay type questions).
4. oooh- very political..........
Double Red arguably improves safety at the expense of performance. It is needed on relatively high speed lines where TPWS cannot otherwise stop a train within the Safe Overrun Distance beyond the junction signal and the resultant collision would be high consequence; hence the signal on the approach to the junction signal is held to red as well. Dead Double Red is now certainly felt to be too draconian; Conditional Double Red (whereby the outer signal is allowed to clear after a train has already passed over its OSS) is how we'd generally implement. Unlikely that the IRSE Control Table layout would feature such a scenario; again more likely to be an essay type question I think.
5. TPWS is indeed the alternative to fail-safe design. Brakes are applied only if the train picks up the relevant frequencies- there can be on-board failures or trackside failures that would prevent, so certainly NOT failsafe. However such failures are quite rare, the trackside is reasonably well self-proved and this checked in the aspect of the signal in rear, the on-board performs a self-test each time a cab is opened so that faults cannot develop and remain undetected for a long time.
It therefore can be assumed to be present and effective for the vast majority of the time and hence provide extremely useful risk mitigation; it cannot be absolutely relied upon. AWS is generally regarded as pretty much fail-safe; but there are failure modes that are not........to be honest the whole concept of the "black and whiteness" that fail-safe implies is really not how we should view the world in 21st century.
6. By a plate (black bar on white background) that is still generally called "Auto plate".
7. The road user does not actually have to; not relevant to them.
The educated railwayman when driving their car can however tell, primarily by looking whether there is a floodlight for the road surface (so rail driver can see whether the crossing is clear) and a Driver's Crossing Indicator light for the rail driver (that confirms the road lights are adequately displayed etc)- if these are present then it is clearly an ABCL. Another hint may be the quality and alignment of the rail track / speed of passing rail traffic (as ABCLs can only be utilised at low speed whereas AHBC may be used at up to 100mph).
PJW
