Short circuits and reversing loop gaps

Recently on the NCE DCC forum, Mark Gurries, an well known USA electrical guru,

Electrical Engineer DCC Website & NMRA DCC Clinics: http://www.markgurries.com

posted this regarding Reversing Section gap placements and has given me permission to copy his info to others who may not be part of the NCE DCC forum
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In the following discussion, I am assuming we are talking about AutoReversing devices (ARs) that use short circuit detection for operation.

There are many individual who have stories about why they made the gaps they way they did (Offset or wide), I submit that anything more than a simpler than a common insulated rail joiner is really a sign of a independent electrical problem that is interfering with the normal operation of a reversing section. In other words, the modification beyond the basic gap requirement is a band aid.

I do want to say I understand the big picture here and if a given solution works for a given individual, then I am all for that because in the end all that matters is your trains run reliability for you. But you will never hear me say to do any special with the gaps because electrically nothing special needs to be done.

Background

Definition: What is a short circuit? It is not ZERO ohms. A short circuit is operating condition on the track where the “load" is asking for more current than power source (Booster) can provide. You can calculate why the resistance of short circuit is from the booster specification of your DCC system

R(short) = V(booster) / I(booster). So a 14V, 5A booster will be: R(short) = 14V / 5A = 2.8 Ohms.

So as long as the resistance value of the short circuit is less than 2.8 ohms, then you have a short circuit. What makes up the resistance is found in all of the conductors involved in carrying the current such as the track bus, track feeders, rail size, wheel resistance, wheel brushes, and if there a lighted car or engine is involved, the wire resistance inside. So there are a lot of variables involved. This is part of the reason why one should use larger gauge track wiring such that all wiring starting at the rail feeders and back to the booster is very low resistance. It is the only resistance variable we can control. Anything starting at the wheel on up is beyond our control. More reliable short circuit detection will happen with lower below the rail resistance because we give more playroom for the unknown resistance above the rail. Stated another way, the system is more forgiving of the unknown resistance and still result in a very low “total resistance" value for a reliable short circuit.

Definition: Reversing section is a section of track where the polarity of the power on the two tracks feeding into the reversing section is exactly the opposite polarity of the other. To prevent the short, the reversing section must have insulated rail joiners or “gaps” on both rails of both track paths the feed into the reversing section. AKA double Gaps. This only prevents a permanent short circuit but in of itself does not address the reversing sections polarity. The polarity of the reversing section must match to the track path in which the train is entering into the reversing section.

Discussion of AR operation

Most autoreversers work by looking for the short circuit that is created across ANY gaps on any rail on any track path feeding the reversing section and as a result change the track polarity of the reversing section ASAP. If there is no short, the AR will not change the polarity of the reversing section as it should.

Although short circuits are consider a bad thing, in the DCC specific case it is a good thing because the action of the AR will address the short circuit faster than your booster can react. The short circuit is so brief, you will never notice it. It should be noted that independent of a reversing section, there are many brief unnoticed shorts circuit that occurs when a train runs abound a given layout depending on qualify of the track and the wheels involved. Clearly by eliminating all you can find will result in smoother operation of the trains. You may never get rid of all of them. You can never escape the derailment which results in a series of random shorts that end with a solid short. The point here is that DCC boosters are designed with a short circuit being a NORMAL electrical event in the operation of a model railroad. The fact that a AR is using short circuits as a detection mechanism to perform its function is not going to harm anything and as such should not be feared in anyway as being a bad thing.

Gaps recommendations.

All that is required to establish a short is at minimum a metal wheel ELECTRICALLY bridging between the two railheads of the same rail separated by the insulated rail joiner or gap.

1) YES: Narrow Gaps. The desire gap size is the smallest gap you can create as long as the gap integrity is maintained and will not fail with age or rail expansion and contraction stress. Using double insulated real joiners meets this requirement. Cutting gaps in a given rail WITHOUT installing a insulator does not meet this requirement for it is asking for trouble some day. The rails will eventually move and short to each other. Not if, but when.

2) NO: Wide Gaps. Intentionally making the gap larger then a wheel contact area only delays the short circuit, it does not eliminate it. It comes at the expense of creating a dead spot in the rail which can be problematic with low axle count engines. The short will eventually occur when two axles picking up power on same rail on each side of the gap occurs. So instead of the short circuit current passing through a simple metal wheel tread, it now has to travel through the truck wiring which is a resistance variable beyond our control.

3) NO: Offset Gaps. They serve no electrical purpose as far as the AR is concerned. Again all the AR is looking for is a quality short circuit and it does not matter which rail it occurs upon. Offset rail gaps only force the short to occur on the gap closest to the train. One can argue that this is potentially less reliable in creating a solid short since your only having one wheel cross the gap as opposed to having two wheels cross their respective gap at the same time creating a better double short. The more reliable the quality of the short becomes, the less problems you will have with the AR operation. Stated another way, weak shorts can create operational problems with ARs if the short circuit current is not high enough and long enough to trigger the AR.

If the booster shuts down before the AR trips requiring you to attempt to use #2 and #3 above, then this is a sign of a independent electrical problem somewhere between the booster and the reversing section. I know first hand in debugging reversing sections a lot of the AR problems are traceable to the layout wiring and/or the location of the autoreverser relative to the booster. It is solvable. I also recognize we do not all have the skills to figure that out and in the end, making it work is all that counts. Its your layout and your fun in using it that counts.

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His website is full of info re DCC
https://sites.google.com/site/markgurries/