Learn Russian: На всякий случай мы купили новый датчик, как бы не начался пожар. - Just in case, we bought a new sensor, lest a fire start.

This is the main rule for clauses of apprehension using как бы не (lest / for fear that). The particle бы signals the subjunctive mood, which in Russian always requires the past tense of the verb, even when you are talking about avoiding a potential future event.

Does the не in как бы не mean the fire won't start?

No, it's the opposite. The phrase как бы не expresses a fear of an unwanted event happening. The не doesn't negate the possibility; instead, it expresses the speaker's desire that the event doesn't occur. It translates best to 'lest' or 'so that [X] doesn't happen'.

Why is the subject пожар at the very end of the sentence instead of before начался?

In Russian, placing the subject at the end often emphasizes it as the new or most important information. Furthermore, with verbs of appearing, beginning, or existing (like начаться), it is extremely natural and common to put the verb before the subject.

Why use the reflexive начался instead of just начал?