The Quattro has 2 AC inputs that automatically switch, so it's curious that you're using a manual switch. I don't see it as cause for concern though. From the manual:
"Two AC inputs; integrated switch-over system between shore voltage and generating set. The Quattro features two AC inputs (AC-in-1 and AC-in-2) for connecting two independent voltage sources. For example, two
generator sets, or a mains supply and a
generator set. The Quattro automatically selects the input where voltage is present. If voltage is present on both inputs, the Quattro selects the AC-in-1 input, to which normally the generating set is connected."
AC-out-2 is designed to shut off in the event of
battery operation:
"The load on AC-out-2 is disconnected immediately when the Quattro switches to
battery operation. After AC
power becomes available on AC-in-1 or AC-in-2, the load on AC-out-2 will be reconnected with a delay of approximately 2 minutes. This is to allow a genset to stabilize."
If I understand correctly, you're expecting a seamless transfer when you manually select the genset as
power source on AC-in-1. What does the Quattro do when you switch the inputs from mains to genset and back? What is your
current limit set to on AC-1? Is the Quattro set to UPS mode? This might force a disconnect if the Quattro doesn't like the look of the incoming waveform.
What is the startup amperage on the
watermaker? AC-out-2 is limited to 25A. If you're close to the limit, undersized
wiring or poor connections could increase the draw.
Lastly, a
wiring fault between the Quattro, the selector switch and the genset might be convincing the Quattro to operate as if it were on battery power, disconnect AC-out-2, and reconnect after 2 minutes to allow the genset to stabilize.