A device (10) is disclosed that is suitable for testing an earth connection (30) that is isolated from a mains electricity supply. The device (10) comprises means (14, 40) for electrically connecting the device (10) to the earth connection (30), and means (16, 32) for electrically connecting the device (10) to an electrically conductive item (20) having a capacitance relative to an adjacent surface of the earth that is within a pre-determined range of capacitances. Furthermore, the device (10) includes means for generating an AC signal and delivering the AC signal to the electrically conductive item, and means for determining whether the resistance between the earth connection (30) and earth reference potential is less than a maximum earth resistance value. This determination is achieved by comparing the frequency of the generated AC signal with a pre-determined range of frequencies.