HomeMatic HM-LC-Sw1-PB-FM Installation And Operating Manual page 46

If the test is completed without errors, the
actuator transmits a wireless telegram containing
its status information. To prevent all actuators
from transmitting at the same time when power
is recovered (after a mains power failure or a
disconnection, for example), there is a random delay
before the actuator transmits. During this time, the
device LED flashes (as in teach-in mode). If the
delay is very short, this flashing may be almost
imperceptible.
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