most inverters of this type are full of electrolytic caps ...so that means their should be a way to precharge them... most of the time it is a resistor limiting the current to charge the caps until a voltage level is reached before the electronic drive that controls the scr's starts. This maybe a precharge switch.. or part of a automatic start which needs only the closure of a large switch or breaker... if the precharge is seperate then another switch or breaker would need to be closed after which a short delay it will start the inverter.
joe