Comment by krisoft

It is a very practical amount of fait.

There are two fire detection loops for each engine.[1] Even if both fails (because they get shredded as you say it) the system will report an engine fire if the two loops fail within 5s of each other. (Or FIRE DET (1,2,3,or APU) FAIL, if they got shredded with more than 5s in between without any fire indications in between.)

The detection logic is implemented directly below the cockpit. So that unlikely to have shredded at the same time. But even if the detection logic would have died that would also result in a fire alarm. (as we learned from the March 31, 2002 Charlotte incident.)[2]

In other words it is a very reliable system.

1: page 393 https://randomflightdatabase.fr/Documents/Manuel%20Aviation/...

2: https://www.fss.aero/accident-reports/dvdfiles/US/2002-03-31...