High impedance earth point

Most power systems use the solidly earthed star points. High impedance earthing may be preferred for ships, occasionally for offshore platforms, and frequently for emergency and uninterruptible supplies in all locations. 

It is possible to design three-wire and four-wire systems that do not need to be solidly earthed. Instead a high impedance is inserted between the star point connection and earth, as shown in Figure 13.3. Alternatively an artificial star point is created and again a high resistance is connected to earth as shown in Figure 13.4. 

If a circuit conductor is connected to earth or to any other reference point nothing which could break electrical continuity or introduce high impedance, for example, fuse, thyristor or transistor, is allowed in the conductor unless suitable precautions are taken. Permitted devices would include a joint or bolted link, but not a removable link or manually operated knife switch without bonding of all exposed metal work and multiple earthing. 

This Regulation aims to prevent open circuits or high impedances arising in the referenced conductors, which could cause hazardous potential differences between them and the reference point, and to prevent the flow of fault current in systems employing automatic disconnection protection techniques. In most cases, earth is the referencing point for supply systems but there are exceptions, such as motor vehicle wiring systems which are not earthed and which use the chassis as both a reference point and a common return. Other non-earthed systems connect the protective conductor to one of the supply poles. 

High voltage generators used in the oil industry are usually earthed by connecting an impedance, which is invariably a resistor, between its star point and the ground . Occasionally generators are... 

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