Abstract: A diagnostic meter base, used for testing the connections of a self-contained meter in a polyphase installation is described. The diagnostic meter base comprises a box-like casing, one end of which defines a socket with contacts to engage respective contacts of the meter, and the other end of which defines a plug with contacts to engage respective contacts of a permanent support socket of the installation. The casing houses a network of circuit elements each including a lamp which responds to current through the element, whereby the pattern of lamp responses is indicative of the pattern connections in the installation. The lamp bulbs are mounted in a well at one end of the casing, the well having a hinged cover.