Abstract: Power distribution networks need to monitor the electrical power at points around the network. A unit which senses the electrical characteristics of the network can also sense other environmental characteristics of the electrical network at that location. An electrical sensor unit attachable to an insulated electrical distribution cable comprises at least one electrical sensor for at least one of current, voltage, and phase angle of electrical power being transmitted in the cable, a source of electrical power for driving the electrical sensors, and a socket for receiving an environmental sensor and comprising connections for the source of electrical power. The unit comprises an annular body defining an axial passage through which an electrical conductor can pass, the annular body housing an electrical sensor in the form of a current sensor including a loop extending around the axial passage.