Overvoltage protection

I asked a similar question recently about the 912iS (virtually identical electrical system to your 915iS).  Here is that thread.  My conclusion was that you have to implement your own OVP scheme.

I use the Vertical Power VPX. It has an Alternator Field circuit.

You can use this to engage a Relay to pass the alternator citcuit.

The VPX has over voltage sensing and will automatically disables this circuit on over voltage.

I use a 40 amp relay with crowbar activation on my 912iS.  This also allows depowering the airframe in the event of smoke in the cockpit. Turning off the master will isolate the battery but 30 amps from the alternator is still available according to the rotax wiring diagrams.