or other electrical components connected to the bus even if there should be a short circuit or other such fault in the bus when the power supply section 10 and the test system as a whole are shut down all of the switches 1 2 3 and 4 are in their open positions as shown in fig1 in accordance with this invention before the test system is powered up and more particularly before the main power supply 12 is turned on a self testing procedure is carried out by the power supply section 10 in accordance with the routine shown in fig2 this routine is initiated by closing switch 1 usually manually the closing of that switch supplies power to the central processor 16 voltmeter 18 and to the switch 3 in the test power supply 28 at this point switch 2 is in its open or cpu position so that the processor can operate its booting routine but is unable to control the overall test system if the boot procedure is not successful processor 16 logs a power up failure and may be arranged to shut down the system by opening switch 1 and or to signal the failure by activating an indicator 38 in any event the failure marks the completion of the.