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A ground fault (earth fault) is any failure that allows unintended connection of power circuit conductors with the earth. [citation needed] Such faults can cause objectionable circulating currents, or may energize the housings of equipment at a dangerous voltage. Some special power distribution systems may be designed to tolerate a single ...
While the fault current from a distribution or transmission system can usually be calculated or estimated with precision, calculation of the earth grid resistance is more complicated. Difficulties in calculation arise from the extended and irregular shape of practical ground grids, and the varying resistivity of soil at different depths.
In polyphase electrical systems, generally phase-to-phase, phase-to-ground (earth), and phase-to-neutral faults are examined, as well as a case where all three phases are short-circuited. Because impedances of cables or devices varies between phases, the prospective short-circuit current varies depending on the type of fault.
The two halves of each limb have an equal number of turns and are wound in opposite directions. With the neutral grounded, during a phase-to-ground short fault, a third of the current returns to the fault current, and the remainder must go through two of the three phases when used to derive a grounding point from a delta source. [5]
The connection between neutral and earth allows any phase-to-earth fault to develop enough current flow to "trip" the circuit overcurrent protection device. In some jurisdictions, calculations are required to ensure the fault loop impedance is low enough so that fault current will trip the protection (In Australia, this is referred to in AS3000 ...
The similar Varley loop uses fixed resistors for RB1 and RB2, and inserts a variable resistor in the faulted leg. Test sets for cable testing can be connected for either bridge technique. If the fault resistance is high, the sensitivity of the Murray bridge is reduced and the Varley loop may be more suitable.
The ELCB detects fault currents from live to the Earth (ground) wire within the installation it protects. If sufficient voltage appears across the ELCB's sense coil, it will switch off the power, and remain off until manually reset. A voltage-sensing ELCB does not sense fault currents from live to any other Earthed body.
These factors can be estimated for an existing fault to determine the magnitude of past earthquakes, or what might be anticipated for the future. [ 49 ] An earthquake's seismic moment can be estimated in various ways, which are the bases of the M wb , M wr , M wc , M ww , M wp , M i , and M wpd scales, all subtypes of the generic M w scale.