Conrad Electronic RM100B868P1 Bedienungsanleitung Seite 48

For the normal (periodical) function test by pressing the test key
up to the pressure point for about 2 seconds, the smoke alarm
just being operated/tested is activated.
Only in case of a smoke alarm (e.g. trigger with cigarette smoke)
are all smoke alarms connected by the cable (and the smoke
alarms connected with one another by radio) triggered.
The red LED only flashes for the smoke alarm which triggers he
alarm.
If for a "genuine" smoke alarm one or several smoke alarms do not go off,
check the wiring (observe the correct polarity!).
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