DC CIRCUIT FAULTSBasic DC TheoryShortCircuit(Series)In a DC circuit, the only current limit is the circuit resistance. If there is no resistance in acircuit, or if the resistance suddenly becomes zero, a very large current will flow. This conditionof very low resistance and high current flow is known as a "short circuit" (Figure 56).Figure 56 Shorted DC CircuitA short circuit is said to exist if the circuit resistance is so low that current increases to a pointwhere damage can occur to circuit components. With an increase in circuit current flow, theterminal voltage of the energy source will decrease. This occurs due to the internal resistanceof the energy source causing an increased voltage drop within the energy source. The increasedcurrent flow resulting from a short circuit can damage power sources, burn insulation, and startfires. Fuses are provided in circuits to protect against short circuits.ShortCircuit(Parallel)When a parallel circuit becomes short circuited, the same effect occurs as in a series circuit: thereis a sudden and very large increase in circuit current (Figure 57).ES-02 Page 68 Rev. 0
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