A hybrid D.C. power controller of the relay/circuit breaker type that uses a hybrid arrangement of hard contacts (4) and power FET's (18, 20) in cooperative functional combination as the arc quenching means for 270 volt D.C. power systems in low atmospheric pressure environments such as at 80,000 feet altitude in aircraft applications. Also, the relay is provided with arc horns (44c, 48b, 48c, 46c), magnetic field amplifiers (60, 62) looped stationary contacts (44, 46), arc splitters (64, 66), and gas ablating insulating members (60b, 62b, 72, 74, 64b, 66) to enable the relay to interrupt the power circuit if necessary without the help of the power FET's. The power FET's are controlled in both opening and closing the power circuit to afford arcless contact operation in normal load make-break situations. Current and voltage sensing (28, 32) and sampling (CS, VS) circuits determine when to turn the power FET's on after contact arcing has provided the required values. The power contacts (4) can be closed and opened by manual switch (TS) control and opened automatically under overload or short circuit conditions by instantaneous trip detector (ITD) control. Isolation contacts (2) in the power circuit are controlled by delay means (MMV4, MMV8) to close last and open first with respect to the power contacts.