When setting up a fire alarm is the voltage low enough so that there is no need to turn power off ?
Well if you are a trained professional, you "might" take that chance. Not worth it though, best to cut power, do the job and turn back on. Yes, you may need to reset clocks, sprinkler system timers, etc., but better that then a trip to the ER.
My husband--who is an electrician--just hard wired 4 fire alarms. He turned off the power.
all I needed to hear
The power needs to be turned off whenever you are dealing with electrical connections.
Turn the power off always. The fire alarms have step down transformers to lower the 120V voltage to 24V, but the voltage going into them is always 120V. I have replace many electrical components on the fly without turning power off, but I know what I'm doing.