Abstract: Structure for and a method of determining which of a plurality of switches in a series circuit first opens after the circuit has initially been completed, for determining line voltage failure to the series circuit, and for monitoring open switches in the series circuit on start-up, comprising means for and the steps of sensing line voltage failure and the condition of each of the switches and providing sensed signals in accordance therewith, converting the sensed signals into coded output signals representative of the desired information relative to switch and line voltage conditions, and subsequently decoding the coded signals and driving a digital display device therewith to provide a visual readout of line voltage and switch condition. Provision is made for manually decoding the coded signals and providing a display in accordance therewith whereby the first fault in the series circuit prior to the circuit having been completed may be determined.