What Does No Break Power Transfer (NBPT) Mean?

Several specifications and brochures concerning 400 HZ Ground Power Units (GPU) mention protection for No Break Power Transfer (NBPT). Few explain why it is included or what this requirement actually means.

Originally when power was switched from internal aircraft sources to an external source, the internal source connection was broken, before the external connection was made. This was a “break before make” transfer. It meant that there was a time without power during the transfer. This is shown in Figures 1, 2 & 3.

In Figures 4, 5 & 6 the internal and external sources are tied together before the aircraft internal side breaks. This is a “make before break” transfer. Power to the aircraft electrical loads is never interrupted. This is referred to as a No Break Power Transfer (NBPT).