Earth leakage current

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. 

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. 

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. 

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. 

Electrode boilers are covered by subsection 554-03. As some earth leakage is inevitable, earthing is important, so the boiler shell has to be connected to the proteetive eonductor and to the metallie armour and sheath, if any, of the incoming supply cable and also to the supply neutral. Earthing the neutral will affeet the operation of an RCD as the earth leakage current will be shared by the neutral and protective conduetors. Regulation 554-03-04 refers to a three-phase electrode boiler supplied direetly from an HV supply. In this case there is no neutral connection, but an RCD is a requirement. The neutral does not have to be connected to the boiler shell for boilers not piped to the water supply see section 554-03-07. 

Section 607 - Earthing of high earth leakage current equipment... 

Inadequate earthing Carelessness and complacency Overheated apparatus Earth leakage currents Loose contacts and connectors Inadequate ratings of circuit components Unprotected connectors Poor maintenance and testing... 

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... 

Safety Bond a bond connecting the metallic carcase of a piece of electrical apparatus with earth, in order to limit the rise in potential of the carcase above earth caused by the passage of any fault current or excessive leakage current, and so reduce the risk of electric shock to anyone touching the carcase. 

It has often been the practice in conductance work to ground certain parts of the bridge network for the purpose of improving the sharpness of the sound minimum in the detector at the balance point unless this is done with care it is liable to introduce errors because of the existence of leakage currents to earth. The telephone earpiece must, however, be at ground potential, otherwise the capacity between the telephone coils and the observer will result in a leakage of current. Other sources of leakage are introduced by the use of vacuum-tube oscillator and amplifier, and by various unbalanced capacities to earth, etc. 

We also know that in steady state the average current through a capacitor must be zero. So there is no way that a constant leakage current can keep flowing into the earth. It must be going back and forth so as to keep the average voltage across the parasitic capacitance a constant. 

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... 

Electrical leakage currents, cathodic corrosion protection In electrically conductive equipment stray or leakage currents can flow as reverse currents to electric generators (in particular in the area of electric trains and major welding equipment), if, for example, conductors like rails, pipes and cable liners laid on or in the earth reduce the resistance of the reverse path, due to earth contact in faulty electrical equipment, due to induction (e.g. in the vicinity of electrical equipment with high currents or high frequencies), due to lightning. Potential equalization, respectively additional electrical override required... 

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. 

CAUTION. High leakage current Earth connection essential before connecting supply, (for stationary equipment)... 

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

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. 

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)...

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