Short Circuit

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A Short Circuit gets its name from the electrical energy finding a shortcut, an easier path from one side of the power supply to the other.


DAMAGED-DIESEL.jpg

Protecting your layout from the excessive currents which flow during a short circuit event can prevent damage to your models and Digital Command Control System. It doesn't take much time or current to cause the damage shown above.


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.

Causes

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:

  1. 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.
  2. 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

Booster Trips

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

Auto Reversers

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

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.

Current Limiting Devices

You will often hear solutions involving inserting a resistor (regular or PTC fuse) or automotive lamp in series with the track to limit the current if a short occurs.

Not a good idea.

By limiting the current you are not solving the problem. Fuses protect your equipment from damage should something short out. Replacing the fuse repeatedly isn't solving the problem. Eliminate the cause of the short first. Use an NMRA gauge to check turnout clearances and that your wheels are in gauge.

Since many DCC systems can supply upwards of five amps, limiting the current via an additional impedance only means that not all the current can flow through the short. You may still have three or four amps flowing. This solution worked fine in the days of analog control with low current. DCC has much higher current capability.

Additional impedance in the DCC track circuit can negatively affect operation of over current devices present in the booster, as well as power management devices.

Automotive Lamps

This idea appeared in the 1960s, using taillight bulbs to limit current. The idea is that at low current draw, the filament would be a low resistance, with a minimal voltage drop. In the days of analog control, many small, low current power supplies were used to provide power to the layout's blocks. So the idea had some merit. The typical 1156 lamp would draw ~2.5A at full brightness, so when a short occurred it limited the current and glowed brightly.

NMRA Data Sheet D7q mentions using 12V lamps to limit current, yet advises against using lamps in a high current environment. It recommends them only in low current (less than 1A) situations. As DCC systems are typically supply 3 or more amps, this method is not appropriate.

Why is this not a good idea with DCC?

  1. DCC boosters tend to supply more current, typically 5A, and will trip before the lamp does anything.
  2. If you are protecting multiple power districts with multiple lamps, the booster may shut down when two or more shorts occur.
    1. When the lamps cool, the booster will re-energize the track, immediately shut down, re-energize...
  3. This technique limits the current. So the maximum current flowing will be the amount limited by the lamp. Meanwhile, about 2.5A of current is still flowing, the longer it flows the more damage is possible. If not cleared promptly, the short will do damage.
  4. The bulbs themselves get hot. More than enough to melt or scorch something nearby. Given enough time or flammable material nearby, this may cause a fire.

Again, this does not solve the problem, it just masks it. Eliminating the cause of the shorts is the only solution.

Frog Juicers

Electronic Frog Juicers not only solve the problem of getting the frog phasing correct for live frog turnouts, partially solving the problem of the operator making the common mistake of running the wrong way into a turnout (i.e. the switch set against the train) thereby causing a short. If this happens, the frog juicer will automatically correct the phase and no short will occur. It does nothing to guarantee that the train won't derail and then cause a short via a locomotive wheel touching the opposite rail.

Additional Reading

DCC Short Circuit Lamps by Dick Bronson