Abstract: A method and device for testing the quality of a ground path, particularly although not exclusively for a telecommunications system. The device has terminals for connection to respective lines, such as tip and ring. A terminal is provided for connection to the local ground. Voltage-sensitive switches of increasing threshold voltage are alternatively connected between the local ground and each of the tip and ring lines via a rotary or other selector switch and the terminals. Current will flow if the local ground potential exceeds the threshold voltage of the voltage sensitive switches. A device responsive to mains frequency, e.g. a butt-in head set, provides an audible sound (mains hum) indicating the ground potential rise exceeds the threshold of the selected voltage-sensitive switch.