Date: 201910
Local time of day: 1801-2400
Location: ZZZ.Airport, US
Flight conditions: VMC
Light: Night
Aircraft: B757 Undifferentiated or Other Model
Operator: Air Carrier
Operating under: Part 121
Flight plan: IFR
Mission: Passenger
Flight phase: Cruise
Airspace: Class B ZZZ
ATC / advisory: CTAF ZZZ
Aircraft component: Electrical Distribution
Component problem: Malfunctioning
Reporter organization: Air Carrier
Reporter function: Pilot Flying; First Officer
Reporter qualification: Flight Crew Air Transport Pilot (ATP); Flight Crew Instrument; Flight Crew Multiengine
Second reporter function: Pilot Not Flying; Captain
Anomaly: Aircraft Equipment Problem Critical; Deviation / Discrepancy - Procedural Published Material / Policy
Detector: Person Flight Crew
When detected: In-flight
Result: Flight Crew Diverted; Flight Crew Landed in Emergency Condition; General Maintenance Action; General Flight Cancelled / Delayed
Contributing factors: Aircraft
Primary problem: Aircraft

Narrative: We were at FL390 overflying the ZZZ area from ZZZ1 to ZZZ2 when we noticed a flicker of the lights followed by the EICAS messages indicating the left generator had gone off line. This was confirmed by the OFF light on the left gen control switch. The plane was on autopilot and held it; as we went into the QRH to rectify the issue. The first step of the QRH gave us one attempt to reset the generator; which we did. Upon that attempted reset; we instead lost the entire L AC bus and; in turn; all components associated with it. As Pilot Flying; and to make sure our first priority was safely flying the aircraft; I began to hand fly as the autopilot was kicked off and to make sure we were in full control. We completed the L GEN OFF QRH which led us to starting the APU to acquire the extra generator and then went to get the L AC BUS OFF QRH as that was our new issue. The first step in that procedure is again to try and reset the generator. That is only allowed once; we had already done it once; not to mention it had made the situation worse; so we couldn't perform that step again. It was at that time that the cockpit was getting very warm and I noticed my ADI lost all color including the artificial horizon. We noticed the equipment cooling overheat light had illuminated so we switched to alternate and regained full imaging on the ADI. The Captain also switched his flight instruments to the FO's (First Officer) side as his had failed with the L AC bus. As we returned to the QRH to continue the procedure; we noticed our screens were getting dim and slightly flickering and since we were basically right over O'Hare; we made the decision to get the plane down to the ground before things continued to get worse. The Captain [Requested Priority Handling] and we were vectored for a descent back to ZZZ. The Captain informed the FA (Flight Attendants) on what exactly was going on. We had no ACARS function so we could not contact anyone else but had ATC relay what information they could. The FA's informed us that we had an FO in the back who was willing to come up and help be an extra brain on the flight deck which was extremely helpful. She looked up our landing speeds and informed us of some of the components we would no longer have from the QRH; while the Captain and I were getting the things we could finished with the QRH procedure and preparing for landing in ZZZ. Once established on the ILS to Runway XXR; at about 4000 ft.; we transferred controls to the Captain for the landing and the emergency trucks followed us to the gate.


Second narrative: While flying from ZZZ1 to ZZZ2 at cruise altitude of FL 390 we experienced a left generator bus off light. Immediately after attempting a reset of the generator per the checklist we experienced the loss of the left AC bus. We then ran the checklist for loss of the AC bus but discontinued the checklist after the first step which call for resetting the generator bus which we could not do because only one attempt was authorized and was already attempted during the first abnormal checklist. In the meantime we experienced an equipment overheat and the Flight Attendants complained of possible fumes in the cabin. We switched to the alternate mode for the equipment overheat and under the circumstances decided to [Request Priority Handling] and divert to ZZZ. The approach and landing at ZZZ was uneventful.


Synopsis: B757 flight crew reported a diversion after losing the left AC Bus.