Earth fault detection

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... 

It is worth noting that the stator differential relays (87) will not normally be sensitive enough to detect the low earth fault currents that are limited by a high resistance NER. With the modem practice being to limit these currents to typically 20 amps, it is necessary to install the restricted earth fault relays (64). 

It is therefore necessary to provide a sensitive method for detecting earth fault currents. The most common method is to provide a core balance current transformer at the circuit breaker or contactor. This current transformer has a current or turns ratio, which is independent of the ratios used by the transformers connected in the three-phase conductors. This is because a particular level of current is to be detected rather than a fraction or multiple of the stator load current. The switchgear manufactnrer will normally recommend the ratio of the core balance transformer and the matching relay. The relay will be either instantaneous 50 N or an inverse time 51 N type depending upon whether the motor is controlled by a circuit breaker or a contactor. 

The sensitivity of the scheme should allow an earth fault current in the order of 30 mA to be detected and reliably tripped. 

A restricted earth fault relay is used with generators and transformers to detect internal faults. The relay will usually be of the voltage operated instantaneous type. 

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. 

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. 

It may seem at first sight that this type of indirect contact scenario is rather unlikely to occur. However, it is in fact a surprisingly common occurrence. What tends to happen is that a break in the earthing circuit will occur first, maybe as a result of the earth connection at a terminal becoming loose, or the earth connection not being made correctly in the first place, or an open circuit fault occurring in the protective conductor. This type of open circuit earth fault will not be detected unless the circuit is tested and the fault... 

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. 

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. 

The requirements for electrical separation are in subsections 413-06 and 471-12. Apart from shaver unit installations in bathrooms, the most common application of the method is in specialist applications such as some types of electrical test areas. Care must be taken in its use because of the fact that an earth fault, in the separated system, could occur and remain undetected, thus reducing the safety of the system to that of a TN system. It is therefore important that separated systems are routinely tested for earth faults, or a monitoring system is incorporated to detect such faults. 

Residual current devices - detect earth faults and cut off the supply to the circuit. Their use should be considered a requirement on all temporary power circuits, because they will offer improved protection where, for example, connectors and cables downstream have been damaged and not detected. 

Short circuits that do not involve earth, and which are within the length of the cable, can be detected by setting the instantaneous elements of the overcnrrent relays to a value of current calculated at the receiving end of the cable that flows into a zero-impedance fault. Customarily this fault is a three-phase fault for which the calculations are straightforward. If the fault is beyond the cable for example in a consumer then the fault current will be less and shonld be cleared by the consnmer protective device. The feeder cable relays will then act as a back np to the consumer relays. 

An RCD is a type of circuit-breaker that continuousiy compares the current in the iine and neutrai conductors of the circuit. The currents in a healthy circuit wiii be equai, but in a circuit that deveiops a fault, some current will flow to earth and the line and neutral cun ents will no longer balance. The RCD detects the imbalance and disconnects the circuit. Figure 3.33 shows an RCD s construction. 

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