Cable insulation testing method

A method for determining the state of a cable insulation parameter of a cable in an electrical system, the method comprising the steps of: providing, by an alternating current (AC) voltage source, a plurality of AC signals between a conductor of the cable and an electrical ground, wherein the plurality of AC signals comprise a plurality of frequencies, in which each AC signal comprises a different frequency greater than zero 501; measuring a frequency response for each of the plurality of AC signals; determining a plurality of phase shifts 502, each of the plurality of phase shifts begin between each injected AC signal and the corresponding measured frequency response, extrapolating the phase shifts to provide a parameter of the cable insulation 504; and an associated system. The parameter of the cable insulation may correspond to an insulation resistance, an insulation capacitance or an insulation resistance leakage current value for a direct current response for the cable. Providing the AC signals may be carried out whilst the conductor is either operation or not operational. The cable may be a signalling cable for transmitting signals between components of a railway signalling system.