Quote:Thank you for giving me the quick answers,
I still have doubt in following:
-- NZLPR/RZLPR CIRCUIT:
*Providing only WZR contact is enough (to prove point is free to operate) for picking the relay
*what is the need of providing NLR/RLR contact?
-- Why WZR made slow to release?
11600: To prevent preselection
please explain
Indeed WZR proves points free to operate; however the functionality required is "points already correct or free to operate into that position". Imagine a crossover on a double track line, with a route set over tha A end normal. The points are therefore locked normal, but you'd still want to be able to set a route over the B end normal for the opposite direction, wouldn't you?
WZR is energised by point key in its centre position. Turning point key to Reverse needs to throw down the NLR and pick up the RLR- it needs to do this whilst the WZR is still making its front contacts; hence the WZR needs to remain up for a short period after the centre contact of the point switch has become open circuit, therefore made slow to release. Hence the call is effective only if the point switch has recently been in the centre position . Therefore if turned Reverrse at a time when the points weere locked, say track circuit over them occupied, then they would obviously not respond at that moment, but the WZR functionality also stops them responding 5 minutes later when the track circuit beomes clear. If they were permitted to move then they would seem to have operated "by themselves"- the call to Reverse would have been stored due to the switch position and we'd say that they had been subject to "pre-selection". The WZR stops this from occuring; hence "anti-pre-selection".
PJW