Make sure you have the correct transfer switch for the type of generator being installed.
If the generator neutral is bonded
to the frame by the manufacturer, a transfer switch that breaks the neutral (three pole)
Here is a basic diagram for a typical generator with a bonded neutral.
However, if the generator neutral is not bonded
to the frame, a transfer switch with a solid neutral buss (two pole)
is should be used.
Here is the basic diagram for a typical generator with a floating (not bonded) neutral.
Hope this helps,