I want to ask those that might know if the current ratings on slide switches matter when you use them to power a frog on a DCC layout. My situation: Atlas Code 55 turnouts with live frog Lenz DCC (12V @ 5A) Miniature Slide Switches used to control turnout throw AND route power to the frog I am using SPDT slide switches (which I don't know the current rating of) right now with the layout powered by DC (Tech II powerpack) and everything works fine. However, I am getting close to connecting my DCC system and I started to worry about blowing the slide switches when I power up the layout at a full 12V and 5A of power. I see a lot slide switches available with many different current ratings (0.3A @ 120V, 0.5A @ 12V, 2A @ 60V, etc.) Will this be a concern when I power up DCC? Will I need to replace all of my miniature slide switches already installed? Any help would be appreciated and I can add details if I left anything necessary out.
The short answer is: if the switch handled the current with DC it will handle the current with DCC. The long answer is a little more complicated: While everything is working normally the switch would only need to supply power to whatever is sitting on the track the switch supplies power to. If the switch only feeds the frog it might need to be able to power two locomotives for a brief period. Depending on the length of the locomotives in a consist you might be able to get the last axle of one unit and the first axle of the next unit on the frog at one time. This still isn't much current. However, you can run into a problem if something shorts the frog, like a derailment on the frog. If the frog is shorted the switch may end up having the full current capacity of the booster pass through it, in this case 5 amps, until the boosters overload circuit cuts the current. This is not something specific to the DCC setup but would also be a problem with a DC setup. So if you weren't having a problem before you still shouldn't. Actually a DCC setup may be a little safer in this situation then a DC setup because the boosters seem to respond to shorts faster then DC power packs.
Thanks Mark. This information is very helpful. I guess then, when buying new slide switches for my new extension I should try to get the highest power rating possible?
Ideally you should use a switch that can handle the full current available in that section of track. Depending on the size of your layout you may want to look into segmenting the layout into power districts. If you install electronic circuit breakers like the DCC Specialties PSX-1 in between the booster and each power district you can reduce the power available to each district. What this allows you to do is set a current level at which the circuit breaker will electronically trip. This does several things, first it limits the current to a lower level then the full five amps. Secondly, the circuit breaker trips so fast the components involved in the short circuit don't have time to melt. A third benefit is it makes troubleshooting easier; if something shorts one power district you know the problem is in that district and you don't have to search the entire layout.
Don't panic! The rating given for most switches is the breaking capacity - ie. it's ability to interrupt the circuit. The carrying capacity is usually quite a bit more (you may find this value in the maker's datasheet for the switch). As a general rule the higher the voltage the lower the current because ... well the volts help jump the gap as the switch opens. For a turnout frog, and indeed other MRR blocks, the switch won't normally break the circuit while much if any current is passing, so you don't need a high rating. Even if overloaded on the odd occasion, at the voltage we use there will not be an instant disintegration of the switch - it'll just knock a few more cycles off it's life. Buying new I'd aim for something that is rated at say 2A @ 50V, but if I had 1A/50V or 0.5A/120V switches lying around I'd use them without worrying too much; maybe test one on DCC using the penny short to make sure there's not a big volt drop across the switch.
