Like defacto said, the APU has protective shutdown. When APU speed is over 95% it will shutdown if the turbine inlet temp. exceeds 2200°F. Some real general guidelines for EGT are, no load 550°C, 100 amp electrical load 585°C, two pack on with no electrics 665°C. These are upper limits of the normal operating EGT at 100°F OAT.