An apparatus for online partial discharge testing includes a split-core radio-frequency current transformer sensor, an actuation subassembly, and a biasing member. The current transformer sensor has a first portion, a second portion, and a sensor hinge operably engaged between the first portion and the second portion. The actuation subassembly includes an elongate first member secured to the first portion of the current transformer sensor, a second member secured to the second portion of the current transformer sensor and configured to be movable relative to the first member, and a third member engaged with the second member and configured to translate along a length of the first member. The biasing member is configured to pivotally bias the first portion and the second portion of the current transformer sensor toward each other relative to the sensor hinge, with the biasing member secured relative to the first member.