Most regulated power supplies have power protection built into them. LION uses regulate wall-warts for some circuits about the layout of him. Still, fuses are necessary. The main track power of LION is 10.2 volts at 12 Amps. With resistors built into my track circuits, even a dead short will not reach 12 amps, and so the power supply will keep on providing ampeers until the resistor burns up. That CAN cause a fire.
LION likes the trick with an automotive tail light, obviously not an LED light, put this in series with the layout, there will not be enough current drawn to light the bulb, it will simply act like a wire. But if there is a short, all that will happen is that the lamp will light up.
Layout of LION has as many as eight trains running all at once, and this would be enough of a draw to cause the lamp to glow. So LION just added more lamps until the circuit worked as required. A bank of lamps in parallel in series with the layout.
ROAR
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Those wall-wart supplies usually have a one-time fuse buried within them, after which you have an ugly paperweight with wires. (It could, however, be of use when you need a weight to glue down roadbed or track.)
Ideally, you should use a fuse rated a bit below the supply, so, a 4-amp fuse on a 5-amp supply. In my experience, a supply will operate a bit above its rated output. But, the purpose of a fuse is two-fold. First, it should protect the supply in case of an overload or short, and second, it should protect your house in case something should go wrong that might cause a fire.
Be safe, not sorry.
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Having 5 DC power supplies for lights (3 circuits), signals (1) and Tortoises (1), I realize I added fast blow fuses to 4 of 5 but not for the Circuitron 500ma Tortoise power supply (looks like a wall wart). I'm getting ready to add a fuse to that one also and it occurred to me that I'm not sure what fuse can be used.
I wonder if I need to use a fuse lower than the rated current output (0.5 amps). I assume the power supply has an internal fuse, fusible link, etc that will cause it to die if excessive current occurs? Is that true or likely? I would also guess that since it is rated at 0.5 amps output that it has a bit of tolerance before self destructing; i.e., I can use a 0.5 amp fuse that will blow and protect the supply at pretty close to the fuse rating. Of course, I have some 0.5 amp fuses and am not keen to mail order one slightly lower rated unless necessary.
I could not find any clarity with a google search but suspect some of you with electronics background or learnings will have the answer.
As always, thanks!
Paul
Modeling HO with a transition era UP bent