450 Transformers and transformer vaults.440 Air conditioning and refrigerating equipment.430 Motors, motor circuits, and controllers.427 Fixed electric heating for pipelines and vessels.Sections for articles pertaining to equipment typically found in commercial buildings include: NEC Table 240.3 provides a list of the applicable sections. The requirements for overcurrent protection of equipment can be found in the NEC article that addresses that specific equipment. How to protect a circuit from dangerous overloads and short circuits Again, if the fault current persists, the insulation will melt and the conductors will eventually be damaged. As in the case with the short circuit, the circuit breaker will allow the fault current to flow for only a short time. The fault current is approximately 5,000 amps. 012 ohms of resistance in parallel with the load resistance, resulting in a much lower circuit resistance. In this example, the ground-fault path adds approximately. If the fault current persists, the insulation will melt and the conductors themselves will be damaged.įigure 5 shows a ground-fault condition. The circuit breaker will allow the short circuit current to flow for only a short time. The fault current is approximately 10,000 amps. The conductors will begin to heat up, but will not be damaged.įigure 4 shows the result of a short circuit condition. The circuit breaker will allow the overload condition to continue for approximately 2.5 minutes before opening the circuit. In the overloaded circuit, the load current is nearly 20 amps. The circuit breaker does not open the load current flows and the conductors do not overheat.įigure 3 illustrates the result of an overload condition. The circuit is protected by a 15-amp circuit breaker. In this case, the load current is 10 amps. What happens during an overload or fault condition? Figure 2 depicts a simple single-phase circuit operating in a normal configuration. The different types of faults are shown in Figure 1 to illustrate the concept of overcurrent protection. The single line-to ground fault is the most common type of fault. Ground faults include a single line-to-ground fault and multiple-line-to-ground faults (Figure 1). Ground faultĪ ground fault is a specific type of short circuit involving at least one of the phase conductors encountering a grounded conductor or surface. Unsymmetrical faults are more common and usually result in less fault current than a symmetrical 3-phase fault. Symmetrical 3-phase faults rarely occur, but their analysis is useful in understanding a system’s response to a fault and usually results in the worst-case fault levels. Unsymmetrical faults have different fault currents in each phase. Symmetrical faults result in the same current flow in each phase during the fault condition. In a 3-phase circuit, two types of short circuits are possible: symmetrical 3-phase faults and unsymmetrical single-phase faults (Figure 1). Short circuitĪ short circuit is defined as flow of current outside the intended current path. However, some equipment-motors, transformers, and conductors, for example-have overload-protection requirements set by the NEC. Information regarding equipment-overload capability often comes from the manufacturer. Electrical equipment can usually withstand some level of load current over its rating for a length of time. Overload conditions are usually not as time-critical as short circuits and ground faults.
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