Earthing supply system

Electrical supply systems and equipment are grounded in order to maintain the voltage at any part of the system at a known potential relative to true earth and to provide a path for current flow under earth fault conditions so that protective devices operate correctly. The connection to earth should be such that the flow of fault current to earth does not cause voltages or voltage gradients to be of sufficient magnitude or duration likely to cause danger. 

An electrical earth system comprises the provision on the supply system of an earth connection to facilitate earth fault current flow, the connecting of all exposed metalwork within the installation to a common grounding terminal and the connection of this terminal to earth. For a complete installation the principal factors which need to be considered, are ... 

The earth electrode Neutral grounding Substation grounding Supply system grounding Bonding... 

The earth electrode system must be designed to be capable of carrying without damage to the full earth fault current of the supply system. 

Portable electrical hand tools and equipment shall be properly grounded and wound to operate on llOV a.c. center tapped to earth supply, and shall only be connected to the system by permanent joints or proper connections. 

If one of the structures to be bonded is the sheath or metallic armouring of an electric supply cable, special precautions will be necessary to ensure that the voltage rise at the bond in the event of an instantaneous earth fault on the power-supply system does not endanger personnel or equipment associated with other buried structures. The bond and any associated current-limiting device should be suitably insulated and of adequate current-carrying capacity. 

Diatomaceous earth filtration systems have been successfully adopted by the US Army for both potable water supply and wastewater treatment in the battle field since 1970s (7-9,15,16). 

Figure 3.34 Cable sheath earth supplies (TN-S system) showing earthing and bonding arrangements...

The principle, as illustrated in Fig. 3.1, is that anybody simultaneously touching one pole of the unreferenced supply and earth will not experience an electric shock because there is no complete circuit back to the point of supply. However, if there were to be an insulation failure that inadvertently connected one pole to earth, the system would become an earth referenced supply and anybody touching the live pole and earth would be at risk, as illustrated in Fig. 3.1. For that reason, it is very important that unreferenced systems are routinely tested for such insulation failures. Note that Fig. 3.1... 

The IT supply system is the same as a TT system (a TT installation is one in which the earth is derived from a local earth electrode rather than from the suppher s supply cable - see Chapter 10) but either without a source earth or with earthing through a high impedance. It is not legal for public supplies but may be used in private installations. The system is monitored to provide visual or audible warning of an earth fault. The applications include continuous processing plants and medical intensive care apparatus where the supply needs to be maintained even if an earth fault should occur. It is occasionally used to supply test facilities as it affords protection against direct shocks. As described for electrical separation , which is a form of IT system, a direct shock hazard arises only if there is an earth fault, and an... 

The basic circuit of a circulating current earth monitoring system is shown in Fig. 3.2. It entails an extra core in the flexible cable. The step-down transformer s secondary winding circulates a current of a few milliamps at PELV through a contactor coil, the pilot core in the flexible cable, the protected apparatus and back to the transformer via the protective conductor. Any break in this circuit causes the contactor to open and to switch off the supply. The system is apphcable to multiphase systems also. If a braided armoured flexible cable is used, the armour can be employed as the... 

As the public supply system employs neutral earthing, the most common method of compliance with this Regulation is to connect together any earth exposed and extraneous conductive parts by means of low impedance protective and bonding conductors. This is the EEBADS technique outlined in Chapter 3. The technique creates an equipotential zone so that when an earth fault occurs these conductive parts are raised to substantially the same potential with respect to the ground (earth). This condition persists until the protective device, be it a fuse or a circuit breaker, interrupts the circuit and clears the fault. Anyone in simultaneous contact with more than one of the conductive parts should not experience a shock because the parts are at about the same potential. 

For special applications, compliance may be achieved by other means. For example, the insertion of a 1 1 safety transformer in a mains voltage system would isolate the system on the secondary side from the earthed neutral system on the primary. Earthing one pole of the secondary side through an impedance so as to limit the potential earth fault current to no more than about 5 mA would create a system that would prevent eleetric shock injuries for a phase-to-earth fault. It is usual to employ a circuit that detects the flow of fault current and trips a circuit breaker controlling the supply. This type of system is often used in test areas. 

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. 

Connect electricity supply systems to earth as close as possible to the source of voltage and meet other general earthing requirements set out in Regulation 5. 

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