Circuit breakers 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... 

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. 

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 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... 

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

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)...

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. 

In rural areas there are still some TT supply systems where consumers have to supply their own earth, in which case it is usually necessary to protect the installation by means of a current-operated earth leakage circuit breaker because of the difficulty of providing an earth of sufficiently low resistance to ensure that the protection operates within the prescribed time in the event of an earth fault. Note that voltage-operated earth leakage circuit breakers are no longer acceptable. 

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. 

In the UK, the electricity supply is connected to earth. It is this system that enables earth faults on electrical equipment to be detected and the electrical supply to be cut off automatically. This automatic cut-off is performed by fuses or automatic circuit breakers if a fault occurs the fuse will blow and break the circuit. Although they do not eliminate the risk of electric shock, danger may be reduced by the use of a residual current device (RCD) designed to operate rapidly at small leakage currents. RCDs should only be considered as providing a second line of defence. It is essential to regularly operate the test trip button to maintain their effectiveness. 

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. 

Circuit breakers and fuses are installed in circuits to operate in the event of excess current arising from overload conditions and faults. The most common type of fault is an earth fault, but it is frequently the case that the current flowing due to earth faults is too low to operate the overcurrent protection devices. In addition, the overcurrent protective devices will not operate in the event of somebody making direct contact with a live conductor the current which flows through the body to earth will be too low to operate the devices but will often be high enough to cause fatal electric shocks. These two problems can be obviated by the use of earth leakage protection devices. 

There are two generic types of device used for earth leakage detection those that are voltage-operated and those that are current-operated. The voltage-operated devices are no longer used but, for completeness, they consisted of a coil connected in series in the earthing conductor or between the metalwork of the installation and an auxiliary earth electrode. The device sensed a voltage rise in the metalwork with respect to earth and, when this occurred, tripped the circuit breaker. 

Neither is the technique of earth leakage detection restricted to low voltage systems. The technique is employed on high voltage systems although the core balance method is not the only one used. For example, another way to detect earth fault current is to monitor the amount of current that flows in the earthing conductor at the point of supply, using a current transformer. If the amount of current exceeds a particular value, a circuit breaker will operate to cut off the supply. 

If it is not possible to establish an equipotential zone because, for example, there is a conducting floor (such as a concrete floor) or because equipment is being used outdoors, supplementary or alternative measures must be taken. The use of residual current circuit breakers to provide sensitive earth leakage protection, in addition to the overcurrent protection, is one acceptable option the RCD would detect earth fault currents, including shock currents flowing to earth, and rapidly interrupt the circuit. Indeed, it is general practice to ensure that socket outlets that will foreseeably be used to supply external equipment should have RCD-protection fitted. 

Part II addresses protection and earthing arrangements. The protection requirements are non-prescriptive and relate to precautions, such as the provision of fuses and circuit breakers, against excess current and earth leakage currents causing damage. 

To cater for the supply company s needs, the standard describes the requirements for an incoming supply assembly (ISA). It has two compartments with lockable access - one for the incoming cable termination, service fuses, neutral link, current transformers and meters, and another for the consumer s main switchfuse or circuit breaker fitted with excess current protection and, where appropriate, earth leakage protection. 

The basic difference between the circuit breaker and the GFI is the amount of current to which each is designed to respond. Basically, the GFI is an extremely rapidly acting device sensitive to very low current—approximately 5 to 20 milliamperes. The GFI is used where a live conductor may develop a high-resistance short to a direct earth ground for example, where faulty insulation may develop in wet areas or a spill becomes electrically hot. Conventional breakers ordinarily will not respond to current leakage in such cases. In the U.S., electrical codes require that all outlets that are newly installed in bathrooms, kitchens near sinks, and outdoors be protected by GFI devices. Their purpose is to protect... 

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