Method and apparatus for providing isolation from hazardous voltage

Abstract

An isolation system for allowing a single measurement circuit to measure both low and high voltage signals from separate low and high voltage buses, respectively, while maintaining substantial isolation protection for humans or animals potentially exposed to the low voltage bus. A central relay coupled to the measurement circuit selects between the low and high voltage buses and thus provide one level of isolation. Two mutually exclusive relays on either side of the central relay provide another level of isolation. Since the central and the two mutually exclusive relay are all rated at the basic insulation level, the low and high voltage buses are separated by at least two relay levels thereby meeting the double insulation or high voltage testing standard during normal operation. For additional protection, should one or more of the relays fail, dual spark gaps limit the maximum voltage to an intermediate voltage level. Clamp circuits further limit the voltage appearing on the low voltage bus to below the hazardous voltage level. The spark gaps protect the maximum voltage appearing across the clamp circuits and the clamp circuits further limit the maximum voltage across the low voltage bus. Furthermore, in-series fuses isolate the high voltage from the low voltage bus if the high voltage hazardous condition exists for a significant period of time. Thus, at least three levels of protection are provided.