[tonyarkles]

> A binary sensor reading the wire going to the speaker isn't some magic modification that invalidates the safety

That’s where you have to be careful about what you’re calling “safety”. There’s “does this, even a little bit, make it less likely that my fire alarm will go off at the appropriate time?” and there’s “will this modification invalidate the UL certification of the fire alarm?”

If reasonably designed (e.g. not low impedance) you could pretty readily conclude that the answer to the first question is No. However… the answer to the second question is almost certainly Yes, which can transition from irrelevant to “a very big deal” if there’s an incident that gets fire marshals and insurance companies doing an investigation.