Abstract: A method and apparatus, including an opto-isolation coupler, for monitoring an electrical parameter of a high voltage circuit. The apparatus comprises monitoring circuitry, a pair of input conductors, and the opto-isolation coupler. The input conductors are coupled to the high voltage circuit. The coupler comprises a holder and a Neon or other glow tube located therein for producing light upon the passage very little electrical current, e.g., in the microamp range, therethrough. The coupler includes a very high resistivity resistor coupled to the input conductors for providing the very low current to the Neon tube, and a phototransistor located within the holder and coupled to the monitoring circuitry to provide an electrical signal in response to the receipt of light from the Neon tube. The holder is a tubular member having a glass barrier wall separating the Neon tube and phototransistor and is encapsulated in a light-tight structure to prevent ambient light from reaching the phototransistor.