A Technique for Measuring Net Charge Through Transient Arcs which Occur Across a Contact Breaking an Inductive Load

When a contact interrupts the circuit to a relay winding or another inductive load, severe cathode erosion can occur due to transient arcs associated with high voltage breakdown. Since the volume of this erosion is a function of the net charge conducted through the transient arcs and since the approximate erosion rate per coulomb is known for most contact materials, the contact erosion life can be estimated from a knowledge of the net charge transfer per operation. This paper presents a circuit technique to readily measure the net charge passage per operation without analytical treatment. The circuit uses current probes to monitor the arc current transient, integrates all positive and negative arc current surges separately, and subtracts the two results to yield an output proportional to the net charge flow through all the arcs resulting from the single inductive break. The detection circuitry has a response time of 2.5 ns; however, the holding time. characteristics are such that the output can be readily mete-ed using a digital voltmeter. A plot of measured Palladium cathode erosion under different load conditions versus that predicted using the net charge measuring circuit is presented to illustrate that the correlation is good.