There are two main scenarios where the ProxySG appliance enters fail-over mode; each enter fail-over at different rates:
System Power Loss: When a ProxySG appliance is configured to fail-open and all power is cut from the appliance (e.g. power outage or power cable removal), the built-in hardware relays should go immediately into fail-over mode.
System Hang: If the ProxySG appliance hangs instead of losing power, there might be a short delay (a maximum of 5 seconds) before the hardware bridge goes into fail-over mode.