Abstract: A monitor (FIG. 1) of disturbances of normal voltage/current parameters of a power line 12 consists of input A/D conversion means 18 by which the actual parameters are sampled and digitized and applied to a store 36 controlled by a main microprocessor 30. A real time clock also provides time markers which are stored as flags with the sample data. The store has a plurality of locations which are addressed in a loop whereby newly digitized sample data replaces that stored previously. A secondary microprocessor 54 also receives the digitized measured parameter data which it compares with stored nominal parameter data to detect a disturbance, the detection causing the store to store subsequent sample data sequentially in different locations. Thereafter the main processor reads the data from the store to provide samples stored prior to, and subsequent to, the disturbance and the real time relationship between them.