There are times when there is a need to measure the voltage and/or current at the plug point supplying a piece of equipment. Voltage is simply achieved with the aid of a double adapter. Current, however, requires that the live cable be separated from the Neutral and exposed such that the current clamp can be placed over it. Apart from being a little untidy, there are some who will claim it a little dangerous too.
The adapter featured here achieves two purposes. The first is to present the voltages in a standard test format being 4mm shrouded sockets. The second is to present the current in two modes. The first being a loop where a current clamp can be clamped on, the second an internal current transformer (for use with oscilloscopes etc.) with its output presented via a BNC connector.
Unfortunately there appears to be no power quality company making such adapters so it's down to a bit of 'home-brew'. The one presented here is made from a cheap and nasty timer with the timer module removed.
The circuit is extremely simple. In this case the internal current transformer (not critical if not fitted) is set for 1A = 1V, and 20A = 1V. The first is used for very small loads such as lamps (especially when determining harmonic contents etc.). The second is for all normal loads.
The CT is a 2000:1 ratio PC mount miniature CT. A DPDT switch is deliberately used as this ensures the voltage reaching the socket is good. If only the burden resistor is changed then any defect in the switch can cause false readings. If only the external loop is required, then these parts may be omitted.