Earth fault

Lythall, R.T., AC Motor Control (on earth fault protection and thermistor protection). 

General Electric Co. Ltd, Protective Relays and Application Guide, GEC Measurements, St, Leonards Works. Stafford, UK Taylor, H. and Lackey, C.H., Earth fault protection in mines, The Mining, Electrical and Mechanical Engineer, June (1961)... 

Overload devices in current use are typically thermal overload relays to BS 4941, motor starters for voltages up to and including 1000 V A.C. and 1200 V D.C. or BS 142 electrical protection relays. Relays to BS 4941 generally provide overload and single-phasing protection. Those complying with BS 142 are also frequently fitted with instantaneous earth fault and over-current trips. 

Charger failures Rectifier failures Battery earth fault Battery boost charge Battery low volts... 

Electrical supply systems and equipment are grounded in order to maintain the voltage at any part of the system at a known potential relative to true earth and to provide a path for current flow under earth fault conditions so that protective devices operate correctly. The connection to earth should be such that the flow of fault current to earth does not cause voltages or voltage gradients to be of sufficient magnitude or duration likely to cause danger. 

An electrical earth system comprises the provision on the supply system of an earth connection to facilitate earth fault current flow, the connecting of all exposed metalwork within the installation to a common grounding terminal and the connection of this terminal to earth. For a complete installation the principal factors which need to be considered, are ... 

In the event of an earth fault the difference in potential between any point which may be touched and the ground (touch potential) should not reach dangerous levels. 

The grounding conductors must be capable of carrying maximum earth fault current without overheating or causing mechanical damage. 

The earth electrode system must be designed to be capable of carrying without damage to the full earth fault current of the supply system. 

The resistance of the system to earth must be low enough to ensure that earth fault protection equipment operates correctly. 

If one of the structures to be bonded is the sheath or metallic armouring of an electric supply cable, special precautions will be necessary to ensure that the voltage rise at the bond in the event of an instantaneous earth fault on the power-supply system does not endanger personnel or equipment associated with other buried structures. The bond and any associated current-limiting device should be suitably insulated and of adequate current-carrying capacity. 

Moulded-in busbars 6 Isolator blades with moving contacts 7 Hinged front door 8 Door interlocked with isolator 9 Flameproof window for instruments, indicator lights and operation counter 10 Flameproof windows for visual isolation 11 Fault reset and/or earth fault test push-buttons under covers 12 Static protection unit ... 

Dropdown slide rails for circuit breaker withdrawal 16 SF6 circuit breaker chamber 17 Auxiliary wiring harness multi-pin plug and socket 18 Integral skid underbase 19 Voltage transformer 20 Earth terminal 21 Fuses 22 Split clamp connectors 23 Earth fault current transformer 24 Incoming adaptor and half coupler. 

L1, L2, L3 phase conductors 1/1, 1/2, 1/3 voltages phase to neutral C1, C2, C3 capacitances phase to earth PE protective earthed conductor RF earth fault resistance UN, PE voltage between neutral N and PE in case of an earth fault... 

With the exception of terminal compartments in e or d or within an enclosure in case of direct cable entries into d or p, such an IT system shows totally screened phase conductors which are not subject to an immediate short-circuit condition in case of cable damage or an internal insulation fault. Even in the earth fault condition the network may remain operational. And this is the great advantage of these systems compared with, e.g., a TN system... 

With method B, the PE and UL conductors are at identical PE potential, i.e. the protection method is reduced solely to earth fault monitoring. An insulated auxiliary conductor is used as a monitoring conductor to ensure a protective earthed conductor correctly connected all over the total system length. 

When high voltage motors are being considered, it is usually found that the minimum conductor size of the cable is determined by the let-through fault withstand capability rather than the full-load or starting current. Cable manufacturers provide graphical data for fault withstand capabilities of their cables, which are based on practical tests. These aspects are also associated with the protection system used for the motor, e.g. a contactor-fuse combination, a circuit breaker, the protective relay characteristics (thermal, inverse time with or without instantaneous or earth fault elements). 

Earth fault protection, settings, status, alarms. 

Moulded case circuit breakers are also available for incoming and busbar section purposes, with ratings up to 6000 A and service voltages between 220 V and 660 V. (At 415 V a 4000 A circuit breaker would satisfy the duty of a 2500 kVA feeder transformer with about 15% spare capacity.) These are also available as 4-pole units. Circuit breakers having ratings of 800 A and above are often provided with several adjustments that widely modify the shape of the complete protection curve, as described in Chapter 12. This enables the curve to coordinate with almost any other protective device or equipment that is immediately upstream or downstream of the circuit breaker. Some circuit breakers with the higher rated currents are also provided with integral earth fault protection facilities. 

When an earth fault occurs at the far end of a cable it is possible that the armouring, cable gland and the frame of the consumer equipment can be raised to a dangerous potential with respect to electric shock exposure to human operators. This subject has been given considerable attention over the last 20 years, and is well documented in for example IEC60364. The international documentation concentrates on low voltage fixed and portable equipment protected by fuses and miniature circuit breakers. See also Chapter 13. 

BS7430 (1998), sub-section 3.13, defines the earth fault loop impedance Zioop in relation to the various types of earthing systems, as follows. 

The star-point connection to earth is provided with a current transformer and a sensitive relay. The relay (51 G) is of a definite time delay or inverse time delay type so that it can be graded as back-up protection to earth fault relays at downstream feeders and consumers. 

Earth fault relay settings of downstream relays. 

The thermal withstand time for the neutral earthing resistors are usually specified as 10 seconds for the duration of the fault current. This allows adequate time for main and back-up protection relays to operate and clear the fault. The standby earth fault relay (51 G) time-current characteristic must be chosen so that its Pt curve is lower than that for the neutral earth resistor and the connecting cables. (The curve is derived directly from the I-t data, and not by integrating the curve.)... 

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