prevent overheating of the load as would be the case if the trip time remained constant in all circumstances in such a case trips occurring within a short time of one another would not permit sufficient cooling of the load such that damage could result the invention also provides a mean for tripping when one or more of the phases is lost that is to say if power on one of the conductors 10 12 or 14 ceases that condition is sensed and will cause a trip independently of any corresponding increase in the two remaining operative phases in particular a series of three comparators 80 82 and 84 are respectively connected to receive signals indicative of the sum of the current through the associated phase and the average current through all phases as illustrated in fig1 as can be appreciated from fig2 the input to the comparator 84 is on a conductor 86 which in turn is connected to a junction 88 of two resistors 90 and 92 the resistor 92 may be connected to the output of the operational amplifier 48 to receive the average current signal therefrom while the resistor 92 may also be connected to the junction 30 to receive the signal representing current in the associated phase therefrom like conductors and resistors similarly connected provide inputs to the comparators 80 and 82 the outputs from the comparators 80 82 and 84.