When using an external transfer switch ahead of a Multiplus to accommodate both grid and generator inputs, the transition between both active inputs can cause a large current spike. A transfer switch that provides a large gap between the two active sources would prevent this spike.
A transfer switch is typically designed to switch between sources in the shortest period of time so that the loads downstream of the switch continue to operate. However, this fast switch prevents an issue for the Multiplus AC input.
The Multi synchronizes its inverter to the AC input then closes the AC input relay so that the inverter can assist the AC input connection and also so that it can provide interrupted power to the loads connected to it's output. When the AC input is lost the input relay opens to prevent back-feed. When the AC input reappears, the Multi syncs to it then closes the AC input relay.
When the AC input is fed from a transfer switch, the AC input conditions, especially phase, can change almost instantly. This connects the new AC source to the Multi's inverter which is no longer phase-synchronous with it's input. This out of phase condition causes AC source and Multi's inverter to fight each other. A large current will flow initially until the Multi detects the loss of synchronization and opens the AC input relay. Currents could exceed the transfer switch's contact rating, the source could sag and/or the Multi could overload. One user reported the Victron ATS "chattering" in this situation.
The Quattro has separate inputs for grid and generator and can sense synchronization (or lack of it) when a transition between it's AC inputs is needed. The Quattro will open both of it's AC input relays, synchronize to the new source then close that input relay. This behavior should be emulated in the external transfer switch.
The Multi will open it's AC input relay very rapidly when the incoming AC source disappears. So it seems the best way to avoid high current during the transition is to introduce a gap in the AC signal to the Multi so that it's input relay will open. Some transfer switches that use a motor to open one set of contacts and close another should be slow enough to cause the necessary gap. Likewise, a manual switch with an off position will generally provide sufficient time for the Multi to open it's relay. For relay-based transfer switches, two interlocked relays along with control logic to insure one relay opens a fraction of a second before the other closes may be needed.
Another way to address this is to have the Multi open it's input relay before initiating the source transfer. Again, this requires control logic and would require using a digital input on the Multi to momentarily activate "ignore AC input".