I just noticed that all of the branches from my upstairs service box are run off of 15 or 20-amp type QP circuit breakers, except for one 15-amp line that is for the hard-wired smoke alarms- it uses a Square D type breaker. What is the difference between these two, and can they safely be mixed in a breaker box?
Electrical – the difference between Square D and QP single pole circuit breakers, and can they be mixed
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Related Solutions
That could be a split bus panel, meaning that there are two sets of power distribution buses. It used to be allowed by something called the "6 handle rule" (which allowed up to six of them in a service panel) back when circuit breakers cost a small fortune.
They were wired like this:
And might look like this: 
Note the gap in the breaker buses at center where there are no breakers, but you can see heavy black wires connecting to the right end of the secondary buses.
The cables from the meter connect at right (which would conventionally be the top of the panel) and feeds the six double breakers. Five of them feed directly to heavy loads: range, water heater, dryer, baseboard heaters, etc. The top-left double ganged breaker feeds the second bus in the left half of the photo.
In this configuration, any circuit fed from the lower bus would be shut off if either its own breaker were switched off, or the feed breaker—which also shuts off almost everything in the building.
Something is wrong with the wiring to the ceiling fan or with the connections to the switch.
Possibly in between the last succesful test and the first unsuccessful use of that switch something has damaged the wiring (e.g. a nail into the wall? strain on the wires?)
I would isolate the circuit at the breaker box, test for voltage at the switch and then test the wires attached to the fan switch for unexpected continuity between live & neutral, live & earth. I'd then detach the switch from all wiring and test for continuity across it's contacts with the switch in each state.
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Best Answer
Some research says that a QP circuit breaker is the 1" or 2" wide breakers that protect a 110V or 220V circuit respectively, and is made by Siemens. Square D is another manufacturer of circuit breakers and service panels.
Ideally, you would want to have all your circuit breakers be manufactured by the same company that makes your service panel; the manufacturer will -- one hopes -- certify that their breakers will work correctly in their panels. Some manufacturers produce breakers with the same mechanical fit as others, in which case it would be up to that manufacturer to say if their breakers will work in a panel made by the other.
I'm assuming that the Square D breaker actually fits in the service panel, so to determine if it's OK, you'd have to do some research to see if Square D certifies their products to work with the panel, or if the panel's manufacturer does the same.
(Side note: per NEC2008, smoke detector circuits have to be protected with an AFCI if there's a detector in a bedroom. This might explain why that circuit is different.)