Electric Shock Hazards Beyond 50/60 HZ and DC

Abstract

The effects of shock from alternating current (ac) power frequencies (50 and 60 Hz) and from direct. current (dc) has been studied for a century and thresholds for safety are now clearly defined in worker safety standards (i.e., NFPA 70E). However, shock thresholds for other frequencies and waveforms are not clearly stated in U.S. worker electrical safety standards. This includes frequencies of sub-radio frequency (RF) (1 Hz to 3 kHz) other than 50/60 Hz, RF (3 kHz to 100 MHz) and mixed waveforms, such as modulated dc. Of particular interest are 400 Hz, dc with significant ac ripple, and modulated RF. These waveforms are found in many commercial, industrial, and research applications, such as dc outputs of battery chargers, welders, etc., that may have a substantial 60 Hz ripple; modulated outputs of variable frequency drives for motors; inverters; and more. This paper presents what is known about such waveforms, from shock studies, international standards, and from accidents. Material reviewed includes existing standards (such as the International Electrotechnical Commission (IEC) shock standards), studies on shock effects, and a review of several accidents. Fourier analysis of mixed frequency waveforms will be presented which shows the primary frequency components which will determine response by the nerves, muscles and heart. A focus will be placed on the thresholds for injury for dc created by rectification, with remnant 60-Hz ripple.