Measuring the electrical safety in low-voltage distribution systems

The determination of both magnitude and duration of touch voltages is of paramount importance to protect persons against electric shock hazards, such as those caused by basic insulation failure in equipment/appliances, or direct contact with live parts. The magnitude of touch voltages depends on the impedance of the equipment grounding conductor (also referred to as Protective Conductor), which should be therefore properly sized. In the U.S., the National Electric Code (NEC) provides minimum sizes for the equipment grounding conductor; however, the NEC further clarifies that the equipment grounding conductor selected in the specific NEC table might need be sized larger to provide adequate electrical safety. In this paper, the authors discuss the major specifications that a “new generation” tester applied to NEC based low-voltage receptacles, or live terminals of feeder/branch circuits, should possess to indicate to users if a ground-fault would cause unsafe touch potentials. Such tester should also unequivocally identify improper wiring of the outlet (e.g. phase-neutral swap, equipment grounding conductor-to-neutral bond).