Using LEDs to highlight possible fault combinations the CP301 Industrial Check Plug from Martindale is an invaluable piece of equipment for checking sockets in factory and on site installations.
To be used on 110V centre tapped circuits +55, 0, -55, it has a yellow 110V 16A CT plug fitted to BS4343/IEC309.
The Martindale CP301 110V Socket Tester shows line 2 fault, centre tap fault, line 1 or 2 to centre tap reverse and line 1 fault.