TABLE OF CONTENTS Several sections of the NEC relate to proper overcurrent protection. Safe and reliable application of overcurrent-protective devices based on these sections mandate that a short-circuit study and a selective-coordination study be conducted.
The protection for an electrical system should not only be safe under all service conditions but, to ensure continuity of service, it should be selectively-coordinated as well. A coordinated system is one in which only the faulted circuit is isolated without disturbing any other part of the system. Overcurrent protection devices should also provide short-circuit as well as overload protection for system components, such as bus, wire, motor controllers, and so forth.
To obtain reliable, coordinated operation and assure that system components are protected from damage, it is necessary to first calculate the available fault current at various critical points in the electrical system.
Once the short-circuit levels are determined, the electrical design professional can specify proper interrupting rating requirements, selectively coordinate the system, and provide component protection.
Short-circuit calculations should be done at all critical points in the electrical system, which would include the service entrance, panelboards, motor control centers, motor starters, transfer switches, and load centers.
Normally, short-circuit studies involve calculating a bolted three-phase fault condition. This can be characterized as all three phases "bolted" together to create a zero-impedance connection. This establishes a worstcase condition that results in maximum thermal and mechanical stress in the system. From this calculation, other types of fault conditions such as...
