What is a Short Circuit?
A Short Circuit (or simply a short) gets its name from the electrical energy finding a shortcut, an easier path from one side of the power supply to the other. And it will always take the easy way when it finds it.
A short circuit is bad. Unchecked, it will melt your power supply, your wiring, your plastic railroad ties etc., and worst case cause a fire. For this reason, all commercial model railroad power supplies (analog or DCC) have short circuit protection built in.
It's not much good using an ordinary fuse for protecting track power supplies because shorts will occur often enough that you will be blowing fuses all the time. Instead, electronic protection methods are used which recover automatically when the short is removed.
There are a number of things that can create a short circuit:
- A metal object touching the track such as a tool or watch strap
- A derailment - either a locomotive or some other rolling stock shorting opposite polarity rails
- Driving into a live frog turnout set against you
- A reversing loop or any other arrangement of track that allows a train to end up going back the way it came
- A locomotive crossing between unmatched power districts
- Bad track wiring - for example, uninsulated feeder wires coming into contact
- Bad decoder installation - resulting in the track pickups shorted together
- Non-DCC Friendly Turnout combined with out-of-spec rolling stock causing a short in the turnout
Why Should I Care?
Why worry about short circuits when DCC boosters have built-in protection? If you are operating a larger layout there are two big reasons why you should not just rely on your booster:
- A booster tripping will shut down the entire booster power district and all the trains running on it will immediately stop. This can be very embarrassing on a club layout because whatever mistake the operator made will be immediately obvious and annoying to all the other operators.
- Booster protection is really only intended to protect the booster itself. A large booster may be able to continuously supply up to about 5 amps without tripping. With poor quality wiring this current might be entirely due to a short and so 5A will be flowing continuously through whatever is causing the short. This might include fine guage locomotive pickup wiring which could overheat and melt the plastic insulation.
Short Circuit Protection
As mentioned earlier, all DCC boosters include electronic short circuit protection. Depending on the design these will trip either when a certain level of current is exceeded or a sudden increase in current draw occurs.
This is one good reason why you want to use reasonably heavy gauge wiring for the track power bus on a medium to large layout. If you only use standard hook-up wire (say 22 AWG guage) for a 40' track bus, the resistance (there and back) will be about 1.3 Ohms. Not in itself enough to prevent the booster tripping, but getting into the right ballpark, and it certainly won't help.
Test Your Track Wiring
You should test your track wiring by doing the Quarter Test. If the test fails, your wiring is causing the problem and has to be corrected.
Auto Reversing Booster
If you use two or more boosters on your layout then all except one should be auto reversing. This saves having to worry about matching the polarity of booster power district. Otherwise a locomotive crossing between districts will cause two boosters to fight each other - effectively the same thing as a short circuit.
Read the instructions as you may need a return path between the boosters to ensure proper operation of the auto reverse function
If the layout has any way for a train to go out and return so that it ends up facing in the opposite direction on the same piece of track that it started, then you have a short circuit. This might be a reversing loop or balloon loop, a reversing triangle (Wye), a turntable or a crossover on a "dog-bone" layout.
Obviously, to prevent a hard short circuit, the track must be double gapped at appropriate places. But a short circuit will still occur when a locomotive crosses the gaps going into a reversing section. This can be solved with a DCC auto reversing module.
Circuit breakers can be used to cut the power to sub-districts of a booster's power district when a short circuit is detected. This mitigates the effect of a short circuit by reducing how much of the layout gets shut down by an operator making a mistake.
This is a much cheaper solution than adding extra boosters.
The Digitrax PM42 Power Management System is an example of a multiple output circuit breaker.
There are disadvantages to using circuit breakers:
- They don't prevent short circuits, they just reduce the effect
- They are complicated to retrofit because the track power buses must be divided up into separate sub-districts and additional heavy gauge wire has to be run to each sub-district power bus.
Electronic Frog Juicers not only solve the problem of getting the frog polarity correct for live frog turnouts but also partially solve the problem of the operator making the common mistake of running the wrong way into a turnout (i.e. the points set against the train) thereby causing a short. If this happens, the frog juicer will automatically correct the polarity and no short will occur but of course it does nothing to guarantee that the train won't derail and then cause a short by a locomotive wheel touching the opposite rail.
Frog Current Limiters
Without a electronic frog juicer (or half an auto reverser), a live frog with the correct polarity will always be the wrong polarity if an operator runs into the turnout (or a crossing) the wrong way. This will trip the booster (or a circuit breaker) and shut down the power district until the short is cleared.
A solution to this is to use a frog current limiter. While it doesn't eliminate the short, it does two things:
- Prevents the booster (or circuit breaker) from tripping and therefore the power district stays up.
- Only the frog is shorted and the locomotive may be able to continue by taking power from its other pickups.
You can view both of these as good or bad. Good because no trains stop but bad because the train can continue on to the points set against it and almost certainly derail. And even if it doesn't derail, the reason the turnout was set the other way is probably because there is a train coming the other way and now there can be a head-on collision.
Let's look at a table showing a locomotive passing over a live frog with a current limiter. The frog is either correct or the wrong polarity. As the locomotive's wheel pass over the pickups may getting power from the frog, the other rails or a combination of both.
|Stock Rails Only|| Full Power
|Frog Only||Reduced Power
Low Level Short
|Frog+Stock Rails||Full Power
Low Level Short
As you can see, the table shows that there are no really bad cases (dead loco or bad short) except when the frog polarity is wrong and the locomotive has only one working pickup on one side. In that case the locomotive will stop. The orange case is where there is no short, but the locomotive isn't able to receive full power, typically this will be enough to get over the frog. The yellow case is where nothing noticeable happens to the locomotive but there is power wasted for a short time.
A couple of simple devices can be used as current limiters:
- An 12V automotive bulb (such as a taillight bulb)
- a 10W 50 Ohm wirewound resistor
In any case the device must be able to withstand a continuous short without overheating.
The wirewound resistor is low enough resistance to provide some power to a locomotive pickup if no other pickups are working (that's why we made the frog live), yet high enough not to trip the booster or any circuit breaker. 10W is beefy enough not to get excessively hot to the touch.
Doing the math, the 10W resistor when shorted will be taking the full voltage of the booster (let's say 18V), which means 360mA flowing continuously. Doesn't sound much, but the power dissipation is volts x amps = 18 x 0.36 = 6.5W. Easily enough to burn out a small resistor. (We assume DC here, the DCC calculation is a lot more complicated, but the figures would be slightly lower).
The light bulb trick will also work and can dissipate more heat and therefore we can get away with a lower resistance and the locomotive gets more power when its only source is a pickup from the (correct polarity) frog. A neat effect is that the bulb will light up to tell you when you've cause a short circuit! The disadvantage to this solution is that you need to consider what happens if you leave a locomotive in a shorted position for more than a few seconds (the yellow case in the table). The short circuit current will be flowing up one wheel pickup and down another wheel pickup, therefore flowing through the locomotive's internal pickup wiring. An 25W automotive bulb might draw as much as 3A from a DCC supply which will exceed the current carrying capacity of any wire smaller than 24AWG gauge (for comparison, DCC decoder installation wiring is typically 30AWG). For this reason, a higher resistance wirewound resistor might be safer.