ChuckHackett-844 wrote:For turnouts I use the following placement of insulated joints:
This results in no 'dead spots' where a train is not detected.
Back when we first undertook to signal our railroad, (occasionally-heated) discussion of what to do about turnouts came up. I had worked out a scheme based upon full-sized practice (which is similar to that in Chuck's illustration), but the track committee chairman wanted no cuts or other alterations done to either the stock or closure rails due to concerns about integrity, despite a mechanically-sound insulated joint having been developed. Also, an insulated throw bar ("bridal" in Chuck's illustration) was out of the question.
Hence we do not have block occupancy detection within turnouts. The adjustable delay-on-clear function in the block occupancy detector attached to a block that includes a turnout is used to "mask" the detection gap. As noted by Chuck, if a short train goes very slowly through such a turnout, or stops entirely within said turnout, it will "disappear" from the ABS. In practice, that hasn't been as much an issue for us as was originally anticipated.
Speaking of track circuitry, below is a photo of one of our insulated joints installed on the mainline. At the time of the photo (2013), the joint had been in service for about 7 years.
- Insulated Rail Joint
Below is the exploded view of an insulated joint.
- Exploded Insulated Rail Joint
Not shown are the stainless steel socket head capscrews, washers and self-locking nuts used to assemble the joint.
Next is a photo of the bonding jumper we use, this photo taken before the (machine-crimped) ring tongue terminals were soldered to the wire. The wire itself is 12 AWG THHN stranded. Incidentally, while it was still available, 20/80 rosin-core solder was used for soldering jumpers and other track circuit wiring. Nowadays, the most lead-rich rosin-core solder available through general distribution is 40/60.
- Rail Joint Bonding Jumper
Below is a photo of a bonding jumper that had been installed for approximately 10 years at the time the photo was taken (2013). It is still in service as of this writing.
- Installed Bonding Jumper
Note how the jumper is installed in a "bow tie" fashion. This arrangement causes the jumper to flex in the middle rather than at the ends as the rails expand and contract with temperature changes. One of the ty-raps had broken and fallen off, a problem that was noted with some other relatively old jumpers. We have since switched brands of ty-raps to eliminate this problem.
A test procedure used to verify continued block electrical integrity is that of comparing the track circuit voltage at the output terminals of the block occupancy detector (BOD) with the return voltage seen by the BOD's track relay, this being when the block is not occupied and the track is dry. In a Robinson failsafe circuit, the track circuit is a loop that depends on continuity through both rails in order for the track relay to pick (actuate) and indicate "block clear." Hence one can determine if bonding jumpers and other trackside wiring are electrically sound by seeing how much voltage drop occurs over the block.
In our ABS, older BODs operate with a 5 volt (nominal) track circuit; newer BODs operate at 3 volts nominal. A track circuit is deemed to be satisfactory if the return voltage is 4.5 or higher in a 5 volt track circuit, or 2.7 or higher in a 3 volt circuit. These minimum voltages allow for the increased leakage seen when the track and ballast are wet. Reliable operation is assured as long as the return voltage is at least 80 percent of nominal. Below that, the block may fail to clear, producing a false-occupied status.