Earthing High impedance

If a 50 mm cable and a fuse rating of 125 amps are chosen as recommended in g) then the circuit earth loop impedance is still too high by a ratio of about 1.65 1. Hence an earth leakage circuit breaker should still be used for this motor circuit. The hazardous shock voltage is still too high. 

In order to restrict the detection of earth faults to those within the stator winding, and those from the stator terminals to the switchgear current transformers, a sensitive relay (64) is used. Three current transformers are used in the stator live lines and one in the star to NER connection. All four current transformers are connected in parallel such that any unbalance in the currents due to an internal fault is detected by the restricted earth fault relay (64). A sensitive high impedance relay is used to achieve an instantaneous response. However, if a high impedance is connected across a current transformer it is possible that very high voltages will appear across the impedance. This is due to the action of... 

Note For small motors, e.g. 22 kW and below, the earth loop impedance inclnding the feeder cable armouring may be too high. When this is the situation a risk of electric shock exists dnring a short circnit at or near to the motor. To reduce the exposure to the risk it is necessary to nse a 51 N or a 50 N core balance current transformer and relay at the motor control centre. The choice of a 50 N is preferred subject to the contactor being properly coordinated with its upstream fuses. 

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. 

The high impedance is usually a resistance chosen to limit the earth current to about 20 mA. A current detector is used in conjunction with the resistance to raise an alarm if a line-to-earth fault occurs. A zig-zag transformer, or reactor, is sometimes used with three-wire supplies such as used in drilling rigs and emergency systems. It is specially designed and internally connected to create a very low zero sequence impedance to earth currents. Therefore, the current is limited only by the resistance of the neutral earthing resistor. Some special purpose earth current alarm systems are available that inject a small DC current into the three-phase system, which is used to identify the actual location of the fault. 

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. 

The phase voltage at the substation transformer will be a little higher than 230 V to allow for the inevitable voltage drop in the distribution cables. In urban areas, the line/neutral and the line/earth loop impedances will be comparable and will probably be only a small fraction of an ohm, whereas the victim s hand-to-hand impedance will be in the order of 2000 ohms. Under these circumstances the effects of the circuit impedances can be ignored. The victim s touch voltage will be about 230 V and, for a total body impedance of 2000 ohms, the shock current would be 230/2000 = 0.11 A. This is high enough to cause ventricular fibrillation in many people should the current flow for about 0.5 s. 

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... 

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. 

If the earth loop impedance is too high for overcurrent protection to operate within the prescribed time, Regulation 413-02-04(i) permits this time to be exceeded provided local supplementary equipotential bonding is used to meet the requirements of Regulations 413-02-27 and 28 and the fault persistence does not cause damage. Again, the intention is to minimise the potential differences between touchable metalwork items on the occurrence of an earth fault. As an alternative. Regulation 413-02-04(ii) allows the use of RCD protection. 

If the supply system is PME, the supply company is precluded from providing an earthing terminal for outdoor sites because there is no equipotential zone, so the contractor has to provide his own earth. He may well then find that the earth loop impedance is too high for operation of the excess current protection, on the occurrence of an earth fault, within the specified time and he will have to resort to earth leakage protection using an RCD. 

Limitation of energy is a method used in electrostatic spraying where a high impedance is inserted in the HV circuit to reduce the potential earth fault current and thus the spark discharge energy to a sufficiently low value to avoid ignition. 

Solar cells have been used extensively and successfully to power sateUites in space since the late 1950s, where their high power-to-weight ratio and demonstrated rehabiUty are especially desirable characteristics. On earth, where electrical systems typically provide large amounts of power at reasonable costs, three principal technical limitations have thus far impeded the widespread use of photovoltaic products solar cells are expensive, sunlight has a relatively low power density, and commercially available solar cells convert sunlight to electricity with limited efficiency. Clearly, terrestrial solar cells must be reasonably efficient, affordable, and durable. International efforts are dedicated to obtaining such devices, and a number of these activities have been reviewed (1). 

As an example. Figure 54a shows the zero-bias impedance of LSC electrodes on rare-earth-doped ceria in air at 750 °C measured using a symmetric cell incorporating a traditional reference electrode. Although the two screen-printed electrodes (1 and 2) were processed identically and aligned to an accuracy of 0.1 mm, the cell response is highly asymmetric... 

---