How long must I wait after turning off the XMT 304 before working on internal components?

You must always wait 60 seconds after power is turned off before working on the unit, or alternatively check the input capacitor voltage and confirm it is near 0 before touching any parts. Significant DC voltage remains on input capacitors after removal of input power on inverters. Failure to discharge capacitors before touching internal parts is a serious electrical shock hazard. (Page 10)