usually have many times higher resistance than a typical supply cable, so earth-fault currents are relatively low. In the TN-S or TN-C-S systems none of the "earth fault" current necessarily passes through the earth, as there is a metallic circuit for the entire earth-fault loop: adequate ADS times may often be achieved by normal OCPDs. However, the connection to the earth itself is always relevant, since the earth forms a mildly conductive surface that we cannot easily avoid (e.g. a person standing on the ground and touching a metal appliance or tap or building-framework that is connected with the electrical installation's protective earthing system). In TT systems the installation's earth electrode needs to have low enough impedance to operate protection if a safe voltage (usually taken as 50 V) between the installation and remote earth is exceeded; in TN systems the system's neutral point needs a low resistance connection to earth to prevent a fault between a line conductor and some unintended earth electrode from displacing the neutral-point potential to a dangerous level compared to the earth. Bonding is the connection of conductive parts together, to reduce the voltage between them. This is an important measure for electric shock protection. When this protective function is the purpose of bonding, BS 7671 describes the bonding by the term "protective equipotential bonding"; this does not mean that the bonding guarantees perfect equipotentiality, but just that it reduces the differences of potential. In the following, this formal term is abbreviated to "bonding". Without adequate bonding, dangerous voltages could arise between conductive parts that can be touched simultaneously, either due to problems outside the installation, or to faults in the installation. Main bonding connects "extraneous conductive parts" such as mains water / gas supply pipes, structural parts of buildings, communication cable sheaths, lightning protection systems etc., to the main earthing terminal. These parts could otherwise introduce potentials that are different from the potential of the installation's earthing system. The main bonding avoids dangerous differences in potential being introduced into the installation, between e.g. two different pipework systems, or between pipes and electrical system ECPs. Supplementary bonding connects simultaneously touchable conductive parts in local parts of an installation: the parts may be pairs of ECPs on different circuits, or an ECP and extraneous conductive parts. This reduces the voltage between them, even in fault conditions. Supplementary bonding is particularly used in situations such as bathrooms,