Quantcast Short-Circuit Currents

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4.2 SHORT-CIRCUIT CURRENTS. This section provides a general discussion of
short-circuit currents, focusing on circuit impedances, fault currents, and the analysis of
short-circuit currents and interrupting ratings to coordinate protective devices.
4.2.1 Circuit Impedances. The determination of short-circuit current is dependent principally
upon the reactance (X) of the elements from the source (or sources) to the fault point. This holds
true for all elements except cable, open-wire lines, and buses. When the ratio of reactance to
resistance (X/R ratio) of the entire system from the sources to the fault is greater than 4,
negligible error will result from neglecting resistance. Neglecting R introduces an error that
always makes the calculated short-circuit current slightly larger than the actual short-circuit
current. It is common practice to refer to reactances (X), even when they represent
impedances (Z).
4.2.2 Fault Currents. Certain simplifying assumptions are customarily made when
calculating fault current. An important assumption is that the fault is bolted; that is, it has zero
impedance and is sustained (not intermittent). This assumption not only simplifies calculation,
but also applies a safety factor, since the calculated values are a maximum and equipment
selected on this basis is rarely stressed beyond its full rating. Three-phase and single-phase fault
currents are customarily assumed for the purpose of calculation because one of these two currents
will define the maximum short-circuit current available in a circuit, and because these current
values will be needed to properly coordinate phase and ground overcurrent protective devices.
4.2.2.1 Actual Fault Currents. Actual fault currents are usually less than the calculated
values. Bolted line-to-line currents are about 87 percent of the three-phase value, while bolted
line-to-ground currents can range from a few percent to possibly 125 percent of the three-phase
value, depending on system parameters. Line-to-ground currents of more than the three-phase
value are rare but may occur in industrial systems. Actual faults, especially line-to-ground faults,
usually involve arcing. Ground-fault currents, particularly in low-voltage systems, are often less
than normal load currents. These currents, however, can be extremely destructive because they
may build up voltage 3 to 8 times more than normal.
4.2.2.2 Sources of Fault Current. Basic sources of fault current are: the utility supply
system, local generators, synchronous motors, induction motors, and capacitors.
(a) A typical modern electric utility supply system represents a large and complex
interconnection of generating plants. The individual generators in a typical system are not
affected by a maximum short circuit in an industrial plant. Transmission and distribution lines
and transformers introduce impedance between the utility generators and the industrial customer.
Were it not for this impedance, the utility system would be an infinite source of fault current.
(b) In-plant generators react to system short circuits in a predictable way. Fault current
from a generator decreases exponentially from a relatively high initial value to a lower
4-3





 


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