The Pros and Cons. I n theory, a wired in mains powered smoke detector is preferable to a battery operated unit, the argument being that the owner can omit to change the battery, or after a barrage of false alarms, remove the battery altogether. That theory is fine, however there is another side to the story. Attic fires are very common because of wiring faults, the high heat conditions in attics and the the deterioration of 120 volt wiring. When a fire occurs in an attic, the wiring melts and the owner to the detectors is destroyed, again killing the detectors.
Lightning is a common way for the appliances in the home to be destroyed.
However the sensitive components in alarm systems, burglar and fire units will definatly be fried and inoperative, but you need more than ever, when lightning strikes, to know about any fires the lightning started. Even a strike many miles away can attack alarm systems, since the most common way into a house for lightning is through the phone lines which connect the alarm to a monitoring station. Fires due to mains powered detectors are not unusual, often caused by arcing in the connecting wires, these often start fires in the attic above the detectors, thus destroying the detectors power source. Sometimes the detector will drop into the room it is protecting and start a fire there too. See the linked article.