Abstract: An improved detector and alarm circuit for use with electric apparatus which is connected to power lines and where the body of the apparatus is insulated from ground is disclosed. This circuit varies the response time of an alarm in accordance with the magnitude of a sensed voltage thereby reducing the response time as the potential danger increases.

Abstract: The circuit is adaptable for electric apparatus, particularly trolley buses and the like, where the apparatus derives its power from a pair of electric power lines, and the body of the apparatus is insulated from ground. One of the pair of electric power lines is at, or very nearly at, ground potential. The circuit provides a known test resistance interposed between the body of the apparatus and the most negative of the power lines so that the test resistance is effectively in series with the leakage resistance between the electric power lines; thereby permitting a measurement between the body of the apparatus and the most negative of the power lines to determine current flow through the known resistance greater than a predetermined minimum. When the current flow through the known resistance is higher than the predetermined minimum, the body of the apparatus is at a voltage above ground sufficient that a hazardous condition might exist, and an alarm status is initiated.