EMB ERJ 145 ER/LR
Flight Crew Air Transport Pilot (ATP)
Situational Awareness
Aircraft Equipment Problem Less Severe
Flight Crew Diverted; Flight Crew Landed As Precaution
Aircraft; Human Factors; Procedure
The aircraft had been operated with generator 4  MEL'd prior to the departure.  Per the MEL procedures; the APU had been running during the entire flight to provide a 4th generator.  In cruise at FL360; without warning or any erroneous indications; the APU automatically shutdown; with the associated Master Caution and EICAS message APU FAIL.  We also saw a lightning flash nearby; and feared that the failure was associated with a possible lightning strike. The Captain ran the appropriate QRH; which directed us to NOT attempt any restart of the APU since it failed in flight.  He then contacted dispatch and maintenance through ACARS to discuss our options to either continue or consider a diversion.  After several confusing messages; he contacted dispatch/maintenance via voice. It was eventually decided that given our present position; a diversion was the best course of action for a precautionary landing.  The subsequent arrival and landing were uneventful.The cause for our concern was that due to the generator MEL; and with the APU failure; we were now down to 3 operating generators.  Any further generator failures would present a further degraded electrical system and the loss of electrical components.  The aircraft was technically un-airworthy; operating outside of the guidelines per the generator MEL. After several ACARS messages with dispatch and maintenance; the Captain decided to contact the company to have a verbal discussion.  Further compounding the issue was confusion on the part of maintenance in helping us deal with this malfunction.  The QRH clearly states that if the APU fails in flight; to NOT attempt any restarts.  At least 2 ACARS messages and then several times the Captain was verbally instructed by maintenance to restart the APU and that 2 attempts were permitted.  Unfortunately; maintenance was reading the 2nd half of the QRH procedure which address APU failure during a start attempt; and presumably while on the ground.  They had skipped over the first indention of the procedure which pertains to in flight failures.  We had discussed the situation; and agreed that we were going to comply with the QRH; and NOT attempt any restarts. It is imperative that everyone involved understands where to find appropriate procedures; and how to read and apply them.  Several minutes were wasted trying to convince maintenance that he was reading the wrong part of the QRH procedure; and that what he was instructing us to do could have had detrimental effects. Eventually; the Captain simply told him that we were not starting the APU; and that we were going to divert.
EMB-145 First Officer reports being dispatched with the number four generator inoperative and the APU generator taking its place. In cruise at FL360 the APU fails and the crew elects to divert to a suitable airport.