The basic types of circuit faults in electrical system are:-
- Short circuit faults
- Earth fault
- Open circuit fault (High resistance fault)
The circuit faults occurs due to following one or more reasons(factors).
Moisture, dirt, dust, vibrations, heavy currents, surrounding heat, loose connection, heating cycles. Any electrical equipment cannot exist without insulation. The above factors adversely affect(impact) the property of insulating materials.
The short circuit fault is nothing but the failure of insulation. As we know, the current follows the shortest path, when the insulation is broken the current leaves the intended path and follows shortest path. The impedance value of the shortest path is negligibly small so heavy current passes through it.
The short circuit fault can be subdivided as:-
- Dead short circuit fault
- Partial short circuit
1. Dead short circuit fault – In this fault, the insulation is totally damaged. The other possibility of dead short circuit is a low resistance material completing the path of current.
This may take place unexpectedly. The dead short circuit is the extension of partial short circuit. The partial short circuit (PSC) when not rectified it leads to dead short circuit. The excess current in partial short circuit destroy the insulation gradually.
2. Partial short circuit – Any electrical equipment have three basic parts i.e. magnetic core, winding and insulation. Heat loss takes place in iron material as well as in electrical conducting material (winding).
The insulation on the winding and magnetic core dissipates heat to atmosphere. The heat continuously affects(impact) the insulation. As some place where the insulation is weak. At some place where the insulation is weak the two conductor in vicinity(near) come in contact and a short path is available for current. The reduction in number of turns increases the current, further increasing the heat and reducing insulating property. This is the start of partial short circuit.
Earth Fault – This is kind of short circuit fault. The short circuit fault is in between live conductors. The earth fault takes place between the outer body(earthed) of the equipment and the live conductor. The current follows the low resistance path and flows towards earth. As discussed above, the insulation is broken due to any of above factors and a low resistance fault is established. In this case, it is between the metal body of the equipment and the live conductor. The current follows the resistance path and flows towards earth.
Open circuit fault – In this fault, the current cannot complete its path due to an open in the conductor. The open in conductor is created mainly due to vibrations, loose contacts, necking of conductor, impurities in the winding material.
The rotating as well as the stationary machines experiences vibrations. The vibrations are produced due to rotating of the equipment shaft, magnetostriction forces between live conductors chattering of core laminations. This leads to loosening of the connection bolts at terminals. The overhang of the winding is tilted due to gravitation. This creates a tension on some conductors. The vibrations, tension create small breaks in the conductors.
The looseness at contacts creates high contact resistance. The I2R losses at contact point are increased and excessive local heat is produced. Oxidization of terminals takes place, they become more & more brittle & at one moment convert into open.
The localized heating takes place at winding turns due to small breaks in the conductor and due to impurities in the conductor material. The heating cycle (Heating & cooling of the conductors during loading condition & off-loading condition). The effectiveness of ventilation is another crucial factor causing open circuit or short circuit fault.
While removing the insulation over the conductor for connection, some strands of the conductor are broken, so the number of parallel paths is reduced and local heating takes place.
In case of necking, as shown in Fig.1 the conductor surface is removed. While removing insulation at that place, the cross section of conductor is reduced due to which the resistance is increased and I2R loss heating is increased, heating cycle takes place (heating cycle causes conductor expansion and contraction of conductors) heading to break at necking point.
The other types of short circuit fault are:-
- Flash over fault
- Creepage fault
1. Flash over fault – This fault takes place at high voltage equipment. The common places of this fault are high voltage transmission lines, high voltage terminal of output transformer and high voltage busbars at substations.
The high voltage lines or terminals, bus bars are displaced from each other with safe distance between them. The high voltage supply has high potential gradient. The air in between and surrounding the high voltage conductor/bushings busbar is charged. That air acts as dielectric medium. The high voltage charges the air molecules. The air becomes conductive in nature. In rainy season the rain, moisture is present in the air so the dielectric strength of the air is very much reduced and an arc is flashed between the live conductors.
The unbalanced in the terminal voltage of the transformer also causes flash over. Safe distance, indoor arrangement, balance voltage will reduce this fault.
2. creepage Fault – This fault is taking place due to high moisture, deposition of salt particles deposition of dirt, dust layer.
The deposition layer of these conductive particles reduce the dielectric strength of insulation system. Thus provide a path for current from conductor to earth, in case of transmission line towers, from one circuit on ships, electric pcbs.
This fault creating factors are present in coastal areas, on ships, oceans, tidal and wave power plant.
By cleaning and maintenance these faults can be avoided.