Earth leakage

More commonly known as earth leakage circuit breakers (ELCBs) or residual current circuit breakers (RCCBs). They operate on the principle of residual current. 

Cable sheath. Additional overhead line conductor Protective multiple earthing fP.M.E.) Buried Strip/rod/plate Earth-leakage circuit breaker, Voltage-operated Earth-leakage circuit breaker, Current-operated... 

The total earth loop impedance for ready operation of the largest rated excess current protective device relied upon for earth leakage protection. 

Earth leakage protection Current Operated Voltage Operated... 

Catastrophic corrosion damage to buried structures may occasionally occur as a result of earth leakage faults on d.c. equipment or on traction systems using metallic posts to support overhead catenary wires. Care should be taken when making earth potential measurements because of the high potential gradients that may be present. 

A systematic study by Herzer [44] on the effect of Si content on the field induced Ku in the nanocrystalline Fe-Si-B-Nb-Cu alloys indicated that Ku is mainly induced in the crystallites. By appropriate choice of alloy composition and annealing conditions, transverse field annealing of nanocrystalline Fe-Si-B-Nb-Cu alloys allows to induce anisotropies in the range of Au 5 - 100 J/m3. This corresponds to initial permeabilities of about fii 104 - 2 x 105. They perfectly cover the needs of applications like common mode chokes or earth leakage circuit breakers [45] which require high permeabilities. 

Find the most appropriate cable and fuses for the motor. Determine whether or not an earth leakage current relay should be used at the motor control centre. Assume a TN earthing arrangement. 

Hence an earth leakage circuit breaker should be used in the MCC to protect the circuit against electric shock hazard. 

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. 

Note In most practical power systems of the TN or TT types it is found that an earth leakage core-balance relay is recommended for all LV motors above approximately 18.5 to 30 kW. 

Note Some oil companies specify a lower disconnection time tdis than 5.0 seconds, e.g., 1.0 second. This significantly increases the disconnection current by a factor of about 3.0 times. This ensures a much lower permissible limit to Zioopf, and thereby making it more necessary to use an earth leakage circuit breaker. Indirectly this reduction in time should be accompanied by ensuring that the earth return impedance Z r (and Zer) is kept very low i.e. as far below... 

It has become standard practice in recent years to use sensitive earth leakage current detectors in circuit breakers to further protect against electric shock. The current sensitivity can be chosen from a range of standard current values, e.g. 30, 100, 500 and 1000 mA. The 30 mA sensitivity is used at individual consumer sub-circuits, e.g. feeders to domestic and small power socket outlets, feeders to luminairs. The higher sensitivities are used in the upstream circuit breakers so that protection discrimination is achieved. 

Short circuits often develop from faults of a leakage nature. It is therefore advisable to provide each sub-circuit with an earth leakage current relay or alarm unit, which has a sensitivity that adequately coordinates with other devices. Indeed this is a necessary requirement for sub-circuits that feed power to hazardous area equipment. The use of these earth leakage current relays and detectors will greatly increase the confidence that can be placed on the overall performance of the system of protective devices in the UPS. 

The Electrical Installation Equipment Manufacturer s Association (UK). Earth Leakage Circuit Breaker. 

Good grounding (earthing) is necessary for satisfactory results. For safety reasons, earth leakage resistances of less than 1 MQ (at 5 kV) are required. Good grounding can also be obtained via contactless, electrostatic processes. 

Earth leakage. Measures any current that leaks onto the equipment enclosure and to a person or ground (3.5 mA maximum for movable products). 

Earth leakage 500 1000 500 1000 500 1000 Higher values (e.g., for stationary equipment)... 

An earth leakage test measures the leakage current to earth through the insulation. It is a useful test to ensure that the portable appliance is not deteriorating and liable to become unsafe. It also ensures that the tested appliances are not responsible for nuisance tripping of RCDs (RCDs - see Chapter 3). A satisfactory reading is typically less than 3mA. 

A circuit breaker, although more expensive than a fuse, has several advantages for excess current circuit protection. The principle of operation is that excess current flow is detected electromagnetically and the mechanism of the breaker automatically trips and cuts off electricity supply to the circuit it protects. Circuit breakers are also available to detect earth leakage current and, indeed, units are available that detect both over-current and earth leakage currents and thereby give very good circuit protection. 

The majority of electric shock injuries occur when the body acts as conductor between line and earth. A general level of protection against such shocks is provided by the inclusion of a current sensitive earth leakage circuit breaker in the supply line. A typical example is shown in Figure 28.2. 

Figure 28.2 Cut-away illustration of a 30 mA current operated earth leakage circuit breaker. (Courtesy Crabtree Electrical Industries Ltd)...

Some portable equipment requires substantial power to operate and may require voltages higher than those usually used for portable tools, so that the current is kept down to reasonable levels. In these cases, power leads with a separate earth conductor and earth screen must be used. Earth leakage relays and earth monitoring equipment must also be used, together with substantial plugs and sockets designed for this type of system. 

If electrical equipment must operate at mains voltage, the best form of protection against electric shock is the residual current device (RCD). RCDs, also known as earth leakage circuit breakers, monitor and compare the current flowing in the live and neutral conductors supplying the protected equipment. Such devices are very sensitive to differences of current between the live and neutral power lines and will cut the supply to the equipment in a very short period of time when a difference of only a few milliamps occurs. It is the speed of the reaction which offers the protection against electric shock. 

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