Commercial and Industrial Switchgear Power Systems

All switchgear power systems are subjected to the potential damaging effect of high magnitude currents flowing from short-circuits occurring in the system components. To ensure that circuit protective devices are capable of quickly isolating faults to minimize equipment damage and personal hazard, it is essential that a short-circuit study be performed on the electrical design of new and existing facilities.

Purpose of Short-Circuit Study on a Switchgear Power System

  • To calculate the system fault currents and compare these values to the first cycle (momentary) and interrupting short-circuit current ratings of circuit protective devices such as circuit breakers and fuses.
  • To compare the calculated system fault currents to short-time, or withstand ratings of electrical components such as cables, transformers, and reactors.
  • To provide the system fault current data needed to perform a protective device coordination study for switchgear power systems.

Protective Device Coordination for switchgear power systems

There are three major aspects to consider when performing a protective device coordination study for switchgear power systems: 1) Safety, 2) Equipment protection, 3) Protective device selectivity. Safety requirements are satisfied if the protective devices are rated to carry and interrupt maximum load currents and withstand interrupt maximum fault currents. Protection goals are fulfilled if protective devices are set to operate above normal load conditions and below equipment damage curves. Selectivity objectives are satisfied if during a switchgear power system fault or overload condition, a minimal amount of electrical equipment is removed from the switchgear power system. The purpose, therefore, of a protective device coordination study is:

  • To specify the proper protective devices (relays, fuses, and Circuit Breakers) to protect distribution equipment.
  • To calculate the proper circuit breaker trip unit and protective relay settings to minimize damage to the faulted component and selectively remove the faulted device from the switchgear power system.
  • To ensure that the protective devices do not operate under normal load conditions.
  • To generate the time-current curves (TCCs) needed to perform an arc-flash hazard study on the power system.

Arc-Flash Hazard and switchgear power systems

An arcing fault is the flow of current through the air between phase conductors, or phase conductors and neutral or ground. An arcing fault can release a tremendous amount of concentrated radiant energy in just a small fraction of a second resulting in extremely high temperatures, a significant pressure blast, shrapnel hurling at high velocities (sometimes in excess of 700 mph), and severe or fatal injuries to personnel. The purpose of an arc-flash hazard study on switchgear power systems is:

  • To determine and label the maximum incident energy available at every location along the distribution network likely to require examination, adjustment, service, or maintenance.
  • To specify the protective clothing and personal protective equipment required for the application.
  • To determine the arc-flash boundary distance (safe working distance) that must be maintained by unprotected plant personnel.