Barn (including coop) fires are distressingly not-rare; the chances of one happening to you are small if you build, maintain and operate your coop sensibly, but it is good to be aware of the commonest causes of electrical fires in barns/coops.
They include, in no particular order:
--heaters or heatlamps falling onto combustible material such as bedding, or being mounted too close to combustible material e.g. too close to floor or wall;
--combustible material such as bedding, dust, feathers piling up or falling onto heaters or heatlamps;
--sparks from shorts due to rodents chewing wires or poorly wired connections, or from plugs that come half-unplugged and arc across the gap, igniting dust/shavings/feathers/whatever
--stuff being used that exceeds the electrical capacity of the circuit, with the result that wires heat up and set things on fire;
and (I dunno if this counts as electrical exactly) dust building up on anything mechanical and then starting a fire when the fan or whatever is run.
If you know the capacity of the circuit and are careful not to exceed it, make real sure the wiring is appropriate type and the connections are very correctly done, mount your electrical equipment safely and make real sure that combustible stuff is not allowed to pile up on or near it, you should be pretty safe. Frankly a considerable number of barn fires are things where 'you could see it coming', except -- this is important -- the person usually sort of knew it was a risk but decided that it would be ok in this case because of <whatever> or because it'd been that way for so long and nothing had happened.
Good luck, have fun,
Pat