UFC 3 -520-01
June 10, 2002
ELECTRICAL SYSTEM PROTECTION AND COORDINATION
PROTECTION SYSTEM DESIGN .
Electrical protection initiates the prompt removal from service of any
component of a power system when it suffers a short circuit or operates abnormally in a
manner that might interfere with the effective operation of the rest of the system.
When a fault occurs within a system, the primary protection closest to the
fault should act promptly to isolate the fault. Depending on the design and function,
backup protection farther from the fault might begin to operate but should not actually
initiate any tripping functions as long as the primary protection functions properly. If the
primary protection fails to isolate the faulted condition, the backup protection should
complete its operation to isolate the fault.
Refer to IEEE 242, Protection and Coordination of Industrial and Commercial
Power Systems (IEEE Buff Book), for guidance regarding electrical protection. These
documents provide specific guidance regarding the protection needed for each
Not all installations offer the same level of reliability and protection. Three
basic system design approaches are available for low voltage systems:
Fully rated--all installations must be fully rated.
Selectively coordinated--selectively coordinate critical installations down to the first
panelboard as a minimum.
Series-combination rated--do not use series-combination ratings. This type of
design guarantees loss of entire panelboards or load centers in response to a short
circuit. Also, inappropriate breaker replacements will invalidate the rating and create
the potential for equipment or personnel damage when a short circuit occurs.
ELECTRICAL C OORDINATION ANALYSIS CRITERIA.
9-2.1.1 Most facilities tend to have fully rated systems with some level of selective
coordination available. Determine if electrical coordination is a design requirement for
the electrical system or some portion of the electrical system. If coordination is a design
requirement, a coordination study must be completed.
9-2.1.2 Selectively coordinate critical installations down to the first panelboard as a
minimum. Coordinate circuit breaker performance with upstream and downstream
circuit breakers and protective devices to the maximum extent possible. Often,