Interesting One: Relay slugging

[SWer]

Does anyone have any theories on why the ZLR relay is slugged?
See attached scheme.

Thanks

[PJW]

Does anyone have any theories on why the ZLR relay is slugged?
See attached scheme.

Thanks

This sheet is only an extract of a bgger set of diagrams to which I don't have access right now, so based only on what is shown....

I guess that the preferred route from 1 to 7 goes via 101 Reverse as longer (and thus higher speed) points than 102. We therefore want 1B-2 to set if it can and 1B-1 only to try to set if 1B-2 can't. Obviously if one of the points is unavailable for a lie then there is no race condition, so we need to consider the situation when all of the NZLPRs and RZLPRs are up. Initially button 1 is pressed and as a result 1(S)R will pick and stick, subsequently when 7 button is pressed as an exit then 7(D)R picks so there will be a negative feed onto both 1B-1 NLR/RLR and also 1B-2 NLR/RLR.
1B ZLR is shown to be both slow to pick and slow to drop and it is the slow to pick that does the trick for us here, since it means that 1B-1 NLR.RLR will not initially have a positive feed available to it; this gives the time needed for 1B-2NLR to throw down, then 1B-2 RLR to pick up and hold itself up by its stick contact even after the push button ring normalises (until eventually 1 button is pulled by the signaller and 1(FM)R drops as a result). As soon as the 1B-2 NLR responds (which is why there is note that it must be the relay itself rather than a repeater) the feed to the 1B-1 NLR/RLR is prevented; i.e the preferred route sets and the non-preferred doesn't attempt to do so.
In circumstances when the preferred route is not available the 1B-1 relays do get their feed after the short delay imposed by the ZLR so therefore the non-preferred route is given its chance to set.

I guess that if the ZLR were not slugged then the circuit would not operate reliably and therefore a race condition would exist between the alternative routes. Remember that the NLRs are held up by magnets and so there needs to be a build up of current in the coil (large inductance) and then build up of flux in the magnetic circuit sufficient to overcome the magnet; when the relay is thrown it throws fast but there is probably a measureable delay between voltage being first applied to coils and its motion- the ZLR slug must cover that. I believe that half a 963 doesn't have a huge amount of slugging and somewhat less than a full size 933; don't have a spec available right now but I'd guess it may only be of the order of 150msec.