Multi-phase, gas insulated electrical switchgear with optical voltage and/or current sensors are described. Compensation for contributions to sensor readings from non-associated conductors is provided. A processor is connected to voltage and/or current sensors which are placed near conductor whose voltages and/or currents are to be measured. The processor can receive outputs from those sensors and compensate the outputs for contributions which are attributable to sources other than the conductors which are intended to be measured.