nigelcliffe wrote:If a short does not cause the command station to shut down, and thus remove all current from the track, then the layout is badly wired.
If people persist in supplying layouts with high current command stations, without any moderation of the maximum current in a given section of track, then the layout is not wired in a defensive manner.
The problem could be avoided with good wiring practises and appropriate current limits to sub-areas within the layout.
As I'll soon be wiring some track for DCC, what words of experience can you share with us about good and bad practice in supplying track?
I did think that DCC controllers came with cut-offs that shut down the power in the event of a short. There seem to be enough warnings made about the problems caused by a short circuit stopping all activity on an entire layout that I'm surprised something like this could happen.
You could try the backnumbers of Snooze where Mick Moignard wrote an extensive series of articles on DCC. I expect Mick will be at Warley show on the Sunningwell Command & Control stand (a DCC supplier from the Oxford area).
The biggest problem comes if the command station doesn't shut down in the event of a short. This is checked with the "coin" test. Put a coin across the track to create a short circuit (and try everywhere, not just one place). If the command station (or power-district cut-out) doesn't shut down, your wiring is poor and needs fixing.
The failure to shut-down comes because there is too much resistance in the wires, so rather than >5A flowing (in a 5A rated system), about 4.5A flows. The command station cannot tell the difference between running two dozen trains and a short drawing 4.5A, so supplies the current. 4.5A at 14v is 63Watts, a lot more than my soldering iron !
The failure of the coin test is usually because a piece of thin wire has been used, or a weak solder joint. For everything other than the shortest bits of connection, use decently thick wire for the main track power "bus" around the layout. Very short pieces of thin wire are generally fine (eg. to track dropper wires), their resistance won't be significant. Andrew Crosland (designer of the Sprog DCC programmer) did some calculations on the use of copper foil as layout wiring; the resistance of the foil was too high to be used as the main bus wiring, though it may be useful in shorter lengths for local power distribution.
Next biggest comes from the way some command stations shut down and reset. Quite a few will try to reconnect power automatically. They do this by pulsing power at the layout to see if it is cleared. Not much of a way around that.
Finally, lowering the current cut-out threshold.
I think it is wise to divide anything but the simplest layout into electrical sub-sections. These can be disconnected for fault finding. One or more sub-sections could be combined into a "power district" (to use the US term which will be found in many documents). You can have as many power districts as you like. The power district is protected by its own cut-out device, set to the lowest current you can, consistent with being able to run your trains. For almost everything in 4mm scale, that's well below 1.5A for one or two locos moving within a power district. A small layout might have a single power district.
Commercial electronic power district cut-out devices are available from firms such as DCC Specialities who make the PowerShield range which have a good reputation and can be set to various current levels. MERG have a design for one, but I don't think it has reached production as a kit yet. Some people report using commercial resettable fuse devices, I'm not sure whether these are truely effective.
A different approach is current-limiting devices, rather than cut-out. The cheapest of these is a 21W car indicator bulb in series with one of the track feed wires. If the layout is wired into power districts, the bulb can act as a current limiter for a single power district (so several bulbs for several districts). The bulb will illuminate at about 1.8A of current, and limit the current at this level, so a short beyond the bulb is limited to that power, and will never shut down a command station with a higher current rating. The things to remember with lightbulbs is that they are working as a current limiter - THE CURRENT IS STILL FLOWING !!, though the voltage after the illuminated bulb is fairly low. And, a 21W car bulb can get hot, so put it in a proper bulb holder in a ventillated place, not under the scenery where it's a fire risk !
Using current-limiting light bulbs is a contraversial topic on various DCC forums. On the whole, I wouldn't use them, and would go for cut-out devices. I did fit bulbs for someone who was having problems with their DCC system re-setting in a user-hostile manner if the command station detected a short.