Tuesday, January 12, 2021

Electrical considerations for a lift bridge

When adding a removable section of tracks to span a gap in the layout such as the entry door to the layout room some thought has to be given to a few problems. 

How will any needed wiring pass through the bridge ?

My train room was built in the back half of a garage that is about 70 years old.  The concrete floor was in poor enough condition that I elected to build a raised floor which ended up being about 9 inches above the concrete garage floor.   Anticipating that layout wiring may need to span across the doorway I included a 1 inch diameter electrical conduit that connects each side of the doorway.  This conduit travels down the wall on each side and under the ramp that leads from the garage floor at the door up to the raised floor if the layout room

A pull line had been left in this conduit when it was installed so it was easy to pull in an 8 conductor, 18 gauge cable at this point in the layouts development.  Each end of the cable was terminated on a terminal strip.  A new pull line was also pulled in at the same time so future cables can be installed the same way.


How are the tracks on the bridge itself connected to track power ?

In this case there are 2 tracks that go across this bridge with each being connected to a different DCC Specialties PSX-1 DCC circuit breaker.  While there are special door hinges used for electrical door locks that could be used, these are quite expensive so I am just using 4 flexible wires.  These wires are 18 gauge but are made up of many smaller strands and that is what makes them more flexible.  The ends are connected to terminal strips so can easily be replaced if movement eventually causes any of them to break.  

What prevents a train from approaching the bridge when the bridge is open ?

At the end of the bridge that does not have the hinge,  there is a magnetic switch within the frame that the bridge fits into and it's activating magnet that is within the end of the bridge itself.  The bridge has to be fully closed for the switch to activate.   

The magnetic switch controls this group of 5 single pole relays.  4 of the relays control track power, 1 for each rail of 2 tracks.  When the track is installed, it will have an additional isolated gap a few feet from either side of the bridge.   This will stop a train from approaching the bridge when the gate is up.  The 5th relay will be to control a warning light outside the door indicating that the gate is lowered.


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