The metal sheath was bonded to each metal wiring device to ensure earthing continuity. Grounding transformers are part of an earthing system of the network. All of the switch mechanisms have no exposed metal parts requiring grounding (earthing). A grounding (earthing) system must be designed. The TT earthing system is used throughout Japan, with RCD units in most industrial settings. The assigned IEC appliance class is governed by the requirement for earthing or equivalent protection. Bonding and earthing are the usual ways charge buildup can be prevented. Various testing instruments are available for earthing resistance tests. Bonding and earthing are the usual ways by which charge buildup can be prevented. The choice of earthing system can affect the safety and electromagnetic compatibility of the installation. Charging is done via a single-phase or three-phase network and installation of an earthing cable. This test shows the resistance offered by the earthing rods with the connection leads. Regulations for earthing systems vary considerably between different countries. They are also used for neutral earthing. Several different earthing systems are in use.