A ground circuit voltage detector for monitoring stray ground voltages occurring in dairy equipment to provide both an instantaneous warning indication and a sustained alarm indication. The voltage detector is connected between the equipment to be monitored at a sensing input and a separate ground. The sensed voltage is filtered and applied to a first comparator through an averaging circuit. If the average ground voltage exceeds the threshold level of the first comparator, a warning indication is provided which is proportional in strength to the sensed ground voltage. The first comparator is connected in series with a second comparator through an integrator. If the threshold level of the second comparator is exceeded, a sustained alarm indication is provided. A test circuit applies a voltage to the detector input to ensure the detector is operating properly.