Insulated interrupters have been the norm for new work for many years, even when wired into the TC itself. A problem with non-insulated ones is the possibility that the 'knock off' portion could fall against the rail and remake the circuit.
Or you can wire a completely separate relay or SSI input circuit via an insulated type for axle counters, or where another adjacent piece of track is not part of the same train detection section but might also be fouled by a vehicle derailed through the traps.
Agreed, although they aren't normally required for traps where axle counters are employed, since a train dropping in the dirt won't have passed a head so there'll be no count out. There's also a suggestion to use axle counter heads (in treadle mode), instead of interrupters, for detecting potential or actual buffer stop collisions.
I forgot that. Even if the the vehicles completely leave the rails through the traps the axle counter can't clear of course. I like the idea of the non destructive sensor. Could be latched in the controls so manual reset required before normal service resumes.
Interrupters have been used with axle counters at trap points and at buffer stops on the areas I've worked on, although I've never stopped and thought about that point before either. Must do some reading into the rationale around it when I wake up later in the day.
Maybe it's something to do with registering that there has been a derailment, so while the vehicles completely leaving the track wouldn't risk the section showing clear, having a alarm circuit connected to an insulated interrupter explains to the signaller and technicians explicitly why the track is showing occupied and that it's not just an S&T fault.
No separate alarms in my experience, although in an SSI area it may be relatively straightforward to determine if it's the interrupter that's causing the section to show occupied by interrogating the SSI.
I might be dealing with a couple of buffer stop examples in the not too distant future. Still shown as interrupters on the scheme plan at present, but the options and method of operation have been considered 'behind the scenes' in anticipation.
There's a plastic insulation piece between the main body of the interrupter and the rail. I've marked this up on one of your photographs in yellow. There's also (not visible) an insulating ferrule surrounding the bolt that goes through the web of the rail and an insulated washer behind the nut that holds the interrupter onto the rail. I've marked these in green.
Generally, a non-insulated interrupter will have one cable connected to it, whereas an insulated one will have two.
Appearances aren't everything - I have a photo of an interrupter complete with insulation piece fitted, and two cables, yet it was uninsulated. Why, because one of the cables was terminated on the same rail the interrupter was fixed to.
