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?
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.)
Breakers aren't normally meant to be mixed and matched...
Circuit breakers and panelboards, being as important for safety as they are, are tested extensively (UL 67 for panelboards, UL 489 for breakers) to ensure that they will function properly. (Unless you were FPE, who decided to try to hoodwink UL instead...) This testing includes making sure that the panelboard and breaker mate properly -- as thus, panelboards are listed to work with specific types of circuit breakers, as those are the types that have been tested under UL's watchful eyes and found to work satisfactorily.
While the various 1" breaker types are dimensionally similar to each other, the breaker-to-busbar attachment systems aren't quite the same. This means that while an alien breaker might fit and appear to work at first, the breaker-to-busbar contact may be poor, leading to overheating, arcing, damage, and even fire.
But there are exceptions, and UL has a way to denote them
It is possible for a manufacturer to design, build, and test a breaker for use in alien panelboards. These breakers are UL classified to reflect that UL actually saw the test results and said "these breakers are OK to fit into this other type of panelboard over there". The documentation for UL classified breakers (Eaton and Square-D both make UL classified breakers for 1" slot panelboards, and I believe Eaton also has a classified replacement for the Square-D QO) lists which types of panelboards a classified breaker has been tested and found to satisfactorily fit into.
If the circuit breaker goes in without having to pound it in, I would say that it is probably fine. There are circuit breakers that are designed to go into more than one type of panel. I know that Square D makes breakers that work in multiple breaker panels, including QP panels.
As long as there isn't a significant voltage drop across the breaker, don't worry about it.
I would go along and test the voltage drop across all of the breakers while you are at it. That might give you an early warning of a faulty breaker.
There is no need to worry about who manufactured your breaker. Siemens and SquareD are two of the larger manufacturers. In fact there is likely to be no difference as long as both are labeled as type QP. Your panel has a label on it which will tell you what type of breaker is required. For instance mine tells me to use type C or BR. The physical connection to the busbar is what the type ensures. Poor contact with the busbar could result in fire, which is what a breaker is intended to prevent.
If you are not clear on how to do this you should hire a licensed electrician. You do not want to burn down your house.
Incidentally, you may see other brand names at your local hardware store, Westinghouse, Eaton, and GE (General Electric) to name a few. These are all manufacturers of breakers. Again, the important thing is that the breaker matches the type listed on the panel label and that the amperage or current draw (expressed in Amps, A, or a number on the lever of the breaker) matches the ampacity of the wire. Typical Romex (wire) in residential construction is capable of handling 15A of current and must be paired with a 15A breaker (or smaller - but 15 is the typical size - I don't even think there is a 10 or a 5 at most hardware stores). Again, doing this wrong may equal fire/loss of property/serious injury/death. If any of this hasn't frightened you into hiring an electrician, then read the part about death again. It's not a trivial thing and finding one that "fits" is a recipe for disaster.
wow, some of these "electricians" spend way to much time on their soap box. the fact is, no your not supposed to mix and match breakers. BUT yes it is fine, its all BS by the manufactures to sell more of their own breakers. If the breaker fits, its fine. Ive been doing this a long time, forget about it, your fine.
By the way, who in the world gets this concerned about a random breaker in their panel, relax guy, I am sure your contractor knew what he was doing.