ARAFOAL, Treadle types, Importance of HR contacts in TPWS circuit

[shekar413]

Hi,

Could anyone explain and share documents regarding the following,

1. What is the importance of ARAFOAL and How it is applied at Level crossings?

2. What are Treadle types and Explanation about QRR and QNR relays?

3. HR front and back contacts are used in TPWS circuit for Relay operated signalling approach. What is the significance of those contacts?


Thank you,

Regards,
shekar

[PJW]

Hi,

Could anyone explain and share documents regarding the following,

1. What is the importance of ARAFOAL and How it is applied at Level crossings?

2. What are Treadle types and Explanation about QRR and QNR relays?

3. HR front and back contacts are used in TPWS circuit for Relay operated signalling approach. What is the significance of those contacts?


Thank you,

Regards,
shekar

ARAFOAL stands for At Red And Free of Approach Locking
If a signal is replaced to danger and there is no Approach Locking provided at the site, then it is possible that the level crossing can be immediately opened to road traffic by mistake.  For example read the RAIB report into the accident at Moreton on Lugg.

Treadles are of a variety of types; nowadays tend to use electronic devices for the treadle function. Traditionally though treadles are mechanical devices with a lever being depressed by a passing wheel flange. 

• Those which only have one arm are operated by any train that passes.  When the arm is slightly depressed the "top" contacts are broken so the repeat relay (the QNR) drops.  When the treadle arm is fully depressed then the "bottom" set of contacts  are made so that repeat relay picks (the QRR).
  
  Those which have two arms are directional treadles.  Obviously both arms are depressed by every passing train, but depending on which is hit first either the contacts are operated in the normal way as above.
  

Not quite sure which sets of contacts you are referring to.  The TPWS has to be energised when the HR is down (signal at Red).  For the aspect of the signal in rear to clear requires either the TPWS proved active, or the signal showing a proceed aspect; hence the VCR relay is fed via the HR contact for times when the TPWS proving will be lost because that signal has cleared.

[shekar413]

Thank you PJW