A method and apparatus for controlling the application of an alternating source voltage to a process, particularly an electrofusion welding operation, is provided to ensure correct voltage level and duration application. The method of controlling the application of an alternating source voltage to a process involves the source voltage being applied to a process for a portion of the voltage cycle between a starting point and a stop point, the, stop point being defined by a point on the voltage cycle where the voltage crosses a predetermined level, the method including:- analysing the source voltage to determine the frequency of the source voltage and/or the timing of points corresponding to the predetermined level in the source voltage; setting a time window based on the frequency of the source voltage and/or based on the timing of the points corresponding to the predetermined level for the source voltage; one or more points equating to the predetermined level falling within the window being treated as stop points and one or more points equating to the predetermined level falling outside the window not being treated as stop points. IMAGE
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