Supply Rule 407.1 requires that for every electrical installation the consumer must provide his own earth electrode system by which the exposed conductive parts of his installation are connected to earth. That is, all exposed conductive parts of the installation shall be connected by protective conductors to the main earthing terminal of the installation and that terminal shall be connected to earth electrode(s) via an earthing conductor. This means that every installation is connected to be part of a TT system. Rule 407.3.f. (iii) further states that where the supply is taken direct from the Company's transformer or via underground cable, the Company may allow the consumer to provide a bonding conductor between his main earthing terminal and the Company's transformer earth or metallic sheaths of the service cable. Therefore, the installation will be operated as part of a TN-S system. However, this rule also states that in the measurement of the earth fault loop impedance or testing the operation of protective devices, the said bonding conductor must be disconnected, i.e. the design and commissioning of the installation must assume a TT system.