(29-03-2010, 10:47 PM)Peter Wrote:(29-03-2010, 08:22 PM)Archie Wrote: Could you be so kind to just go over the timer relay (WJR) within the circuit, 7.5 seconds I believe and it's main function, is it to do with in case of an obstruction, and and detection cannot be achieved in this time.
Without going into detail of how it works, its purpose is to cut off the motor after a time if the detection is not made up, mainly to stop the motor burning out as would happen if it continued to run until the techs arrive. There is normally some sort of clutch that slips or hydraulic bypass to deal with the time in between the points stopping moving on an obstruction and the motor cutting out.
Why 7.5 secs? Who knows!?!?!?
My guess is that some people wanted 5 and others 10 so it was a British compromise! To be honest with longer heavier switches 7.5seconds is a bit too short. I remember that at Cogload we originally had to put two in series to get 15 seconds as the WJRs are non adjustable- I think that the SSI TFM time value is slightly more. It might even be selectable on a "per point" basis with certain interlockings.
Re detection- I would argue that detection must theoretically be both the open and closed switch, although I agree with Peter that the open switch position is often not explictly measured but only inferred by the integity of the multiple stretchers (and whereas this seemed adequate for very many decades, Potters Bar and particularly Lambrigg provide contrary evidence).
BR Western Region has always distinguished between neat detection (that only states where the points actually are) and correspondence (which proves that it accords with the requested position). Other Regions generally incorporated the "called position" into the very first detection relay and therefore made no such distinction. The latest NR circuits has gone against that policy. Hence the words are used inconsistently depending on context which is confusing to the newcomer but something one learns to live with; the industry is not completely homogeneous. Signallers almost always say "detection" when they actually mean "correspondence" and so do many signal engineers including myself- I find myself only really using the two terms when the difference is important in the context and I tend then to say "pure detection" to emnphasise that it isn't "correspondence". Probably more than Achie wanted to know!
PJW