A device to measure direct current, DC, voltage in an insulated conductor, the device comprising: a housing; a conductive sensor physically coupled to the housing, the conductive sensor selectively positionable proximate to the insulated conductor without galvanically contacting the insulated conductor, wherein the conductive sensor capacitively couples with the insulated conductor; a conductive internal ground guard which at least partially surrounds the conductive sensor and is galvanically isolated from the conductive sensor, the internal ground guard sized and dimensioned to shield the conductive sensor from stray currents; a conductive reference shield which surrounds at least a portion of the housing and is galvanically insulated from the internal ground guard, the conductive reference shield sized and dimensioned to reduce currents between the internal ground guard and an external ground; a mechanical oscillator operatively coupled to the conductive sensor, in operation, the mechanical oscillator causes the conductive sensor to mechanically oscillate according to a mechanical oscillation amplitude and a mechanical oscillation frequency such that a distance between the conductive sensor and the insulated conductor varies periodically according to the mechanical oscillation amplitude and the mechanical oscillation frequency; a common mode reference voltage source which, in operation, generates an alternating current, AC, reference voltage having a reference frequency, the common mode reference voltage source electrically coupled between the internal ground guard and the conductive reference shield; a sensor signal measurement subsystem electrically coupled to the conductive sensor, wherein the sensor signal measurement subsystem, in operation, generates a sensor current signal indicative of current conducted through the conductive sensor; and control circuitry communicatively coupled to the sensor signal measurement subsystem, wherein, in operation, the control circuitry is configured to: receive the sensor current signal from the sensor signal measurement subsystem; and determine the DC voltage in the insulated conductor based at least in part on the received sensor current signal. a housing; a conductive sensor physically coupled to the housing, the conductive sensor selectively positionable proximate to the insulated conductor without galvanically contacting the insulated conductor, wherein the conductive sensor capacitively couples with the insulated conductor; a conductive internal ground guard which at least partially surrounds the conductive sensor and is galvanically isolated from the conductive sensor, the internal ground guard sized and dimensioned to shield the conductive sensor from stray currents; a conductive reference shield which surrounds at least a portion of the housing and is galvanically insulated from the internal ground guard, the conductive reference shield sized and dimensioned to reduce currents between the internal ground guard and an external ground; a mechanical oscillator operatively coupled to the conductive sensor, in operation, the mechanical oscillator causes the conductive sensor to mechanically oscillate according to a mechanical oscillation amplitude and a mechanical oscillation frequency such that a distance between the conductive sensor and the insulated conductor varies periodically according to the mechanical oscillation amplitude and the mechanical oscillation frequency; a common mode reference voltage source which, in operation, generates an alternating current, AC, reference voltage having a reference frequency, the common mode reference voltage source electrically coupled between the internal ground guard and the conductive reference shield; a sensor signal measurement subsystem electrically coupled to the conductive sensor, wherein the sensor signal measurement subsystem, in operation, generates a sensor current signal indicative of current conducted through the conductive sensor; and control circuitry communicatively coupled to the sensor signal measurement subsystem, wherein, in operation, the control circuitry is configured to: receive the sensor current signal from the sensor signal measurement subsystem; and determine the DC voltage in the insulated conductor based at least in part on the received sensor current signal. receive the sensor current signal from the sensor signal measurement subsystem; and determine the DC voltage in the insulated conductor based at least in part on the received sensor current signal.