mains and low voltage sounders in same building

[thepprentice]

I've changed a few faulty sounders on a fire alarm, however, there are at least two sounder circuits, with both mains powered and low voltage ones. Is this very common?
I changed 5 sounders and put low voltage ones in, however I didnt notice one was mains powered as when we tested the panels, it went BANG!!! There are two main panels for the building, which cover different floors.
Is this just bad designing?

 

[bernardgreen]

Always read the label..........

 

[plugwash]

I would guess it was different parts installed by different companies and/or at different times.

 

[breezer]

most unusula to have mains sounders on a fre alarm since they wont work in a power cut