New install. Earth fault triggers before the Multis (that are in 3P) when Neutral are connected on the output - but not if they are not connected. Nothing is connected to AC out, except a RCD breaker (that doesn't trip). I believed the transfer relay connected neutral in with neutral out?
Changing the breaker before to one without ground fault then everything seems OK. And if neutral on the output are not connected together in a ac out breaker, then also everything works fine.
Could it be that one of the new Multis introduce a ground fault themself? Nothing else is connected, other than AC IN and a breaker on AC out1.