Electrical systems involving a Boeing 737-476, VH-TJG, 160 km W Melbourne Airport, VIC on 14 May 1996

The aircraft departed Melbourne with an unserviceable auxiliary power unit (APU). During climb the left engine driven generator also failed. As the aircraft was left with only one serviceable generator,(the right engine driven generator) the flight returned to Melbourne. These problems had been intermittently occurring for a couple of days. After extensive investigation maintenance found that the APU start switch was unserviceable. After the switch was replaced the APU operated satisfactorily. The left engine driven generator and electrical contractor were subsequently confirmed to be unserviceable and were replaced. Subsequent operations have been satisfactory.