Abstract: Apparatus (1) for use in disturbance evaluation systems based on disturbance extraction, which apparatus (1) comprises measuring means for obtaining signal frequency both before (4) and after a disturbance (3) in an electricity network but excluding use of the disturbance transient itself, and interpolation means (5) for interpolating frequency across the disturbance interval to reflect the underlying frequency trend (9) uninfluenced by the disturbance transient.

Abstract: Apparatus (16) for use in estimating a fault level in a network (2) providing an electric power supply to consumers, which apparatus (16) comprises: (i) upstream calculator means (18) for calculating upstream impedance by utilising changes which occur after and consequent upon changes in the electrical power supply due to consumer removal or supply of electricity to the network (2); and (ii) downstream calculator means (20) for: (a) calculating a static downstream impedance which is based on measurements before and/or after the time when the changes in the electrical power supply occur; (b) measuring the actual downstream impedance at the time when the changes in the electrical power supply occur; and (c) obtaining the difference between the calculated static downstream impedance and the actual downstream impedance.

Abstract: AC calibration apparatus (2) comprising an analogue to digital converter (4), and in which the absolute gain of the analogue to digital converter (4) is dynamically assessed by applying a reference AC signal which is in addition to a sampling signal required for measurement purposes and which is a square wave of a known amplitude at a precise sub-harmonic of the sampling frequency of the analogue to digital converter (4).