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Placing an Auto Reverser on a Layout With Circuit Breakers

redking56@...
 

The common wisdom seems to be to isolate not only the reversing section but to also keep the auto-reverser separate and apart from any circuit breakers controlling other power districts. In other words, an auto-reverser should not be placed inside a power district controlled by a circuit breaker. That raises a question in my mind.

I am building a new layout, and for all practical purposes, it is a double mainline dogbone with crossovers in the middle of the dogbone. I am installing reversing sections at each end of the layout where the mainlines loop around so that I can keep all four mainline tracks in phase in the middle of the layout.

Following accepted protocol, I have installed separate buses for each of the four reversing sections. However, since the reversing sections are merely part of the mainline tracks, it occurs to me that the auto-reversers could input from the double mainline bus which is a separate power district controlled by its own circuit breaker. 

My reasoning is that if there is a short on the mainline which shuts down the power district, it will also shut down the reversing sections which are nested inside the mainline power district. I don't see a problem with this because until the short is cleared trains in the reversing section probably ought to stop anyhow since they are operating on mainline tracks. On the other hand, if there is a short in the reversing section, it would seem that you would want trains on the non-reversing portion of the mainline to stop as well to avoid problems with trains nearing the reversing sections.

Am I thinking clearly on this issue?

Rich

Don Vollrath
 

Rich, your thinking is logical except for consideration of how the auto-reversers actually work. When a loco enters an A-R section of opposite polarity there is a momentary short circuit created. The pulse of unwanted current to the track is sensed by the A-R controller causing it to reverse the polarity to relieve the short. This allows trains to keep running. If that same pulse of current also passes through a non-reversing type circuit breaker there is a time/race condition where the circuit breaker may interrupt the current first... before the A-R unit has time to react... causing a shut-down rather than a polarity flip correction by the A-R section. The conventional way to eliminate that possibility is to power the A-R controller directly from booster mains rather than after or through any power district circuit breaker. The brand/model choice of A-R unit and CBs can certainly affect the result. Many of the A-R control units have their own internal CB that cuts off all current to the A-R track section if there is indeed a short circuit within the A-R section and flipping the polarity does not resolve the conflict. [See the PSX-AR and/or OG-AR series for examples] There is no need for any additional current limiting or circuit breaker action. If the pulse of current through the A-R unit is constricted it may not flip the polarity as desired.

DonV

redking56@...
 

Don, thanks for that response. You're right.  I had not considered how the auto-reversers actually work. Fortunately, I have run separate buses for each of the four reversing sections. Incidentally, the circuit breakers are PSX units and the auto-reversers are PSX-AR units.

Rich

Don Vollrath
 

Rich... A followup on your original line of thinking is that a short circuit anywhere on the layout should stop all trains. This makes perfect sense on a single operator layout if you plan on letting a train run continuously on the main line loops while you busy yourself operating another to perform switching moves. This can be accomplished by dispensing with separate protected power districts (except for A-R sections) and let the single booster trip when there is a shorting problem, regardless of where it exists. Just be careful to be sure that the booster trip level is not likely to cause damage if it keeps recycling and pay attention to quickly find the issue and remove power while you fix the issue.

DonV 

redking56@...
 

Don, thanks for that follow-up analysis.

In the process of building my new layout, I have created 8 power districts. The four reversing sections are separate power districts, each controlled by a PSX-AR which also serves as a circuit breaker. The other four power districts are each controlled by a separate PSX circuit breaker. I have run separate buses from the output side of each PSX (PSX-AR) to the respective power district. So, in the event of a short in any one of the power districts, only that power district will shut down until the short is resolved.

Rich

Tom in Texas
 

The PSX-AR will not work reliably if at all if installed downstream from a PSX circuit breaker. The PSX-AR has its own circuit breaker in addition to be an auto reverser so it should be powered directly from the booster

Tom in Texas