This SAE Aerospace Recommended Practice (ARP) covers the requirements for ground power equipment that supplies 115/200 V, 3-phase, 400 Hz output capable of powering aircraft type loads requiring MIL-STD-704E/DFS-400 quality power. All forms of ground power including mobile and fixed systems are addressed by this document. High voltage (270 V DC) systems are not included in this specification.

The intent of this document is to provide specifications for engine generators, motor generators, and solid-state converters tailored for performance with modern aircraft, applicable to all unless otherwise noted. Modern aircraft are requiring more power at a higher quality without interruption which places new constraints such as No-Break-Power-Transfer (NBPT) operation on the ground power units (GPUs). The protective trip limits are designed to allow a two tier protection utilizing both the aircraft's electrical power generating systems (EPGS) and the GPUs protection. Some of these specifications will place some new design constraints on the manufacturers of GPUs but will help promote compatibility with old and new aircraft with all types of ground power equipment.

While GPUs are designed to provide the same quality of power as the aircraft's EPGS, there are differences in rated capacity. Engines utilized in GPUs must be sized for the peak kW specified which is why a 125% overload is specified for GPUs. Increasing the size of the engine to match the aircraft's EPGS typical 150 to 200% overload rating would not be practical. The 150 to 200% overload rating is designed for abnormal conditions (such as fault clearing) and the use of a GPU to clear aircraft on-board faults is not recommended. GPUs that utilized engine generators are also self-excited which implies that they are not capable of providing output under excessive fault current since their output voltage is utilized to excite the generator. Most aircraft EPGS have a permanent magnet pilot machine that provides the excitation supply to allow for fault clearing current. GPUs are typically rated for a 0.8 power factor which differs from aircraft EPGS which are rated at 0.75 PF. An engine generator will have to have slightly larger engine to accommodate a 0.8 PF when compared to a 0.75 PF which infers that 0.8 is actually a conservative rating for GPUs.