Arc vs. Spark: Definitions

The following definitions for an arc and an electrical spark are given in NFPA 921 as follows:

• 3.3.7       Arc. A high-temperature luminous electric discharge across a gap.
• 3.3.42    Electric Spark. A small, incandescent particle created by some arcs.

This convention is not universal however; the term “spark” may be used by authoritative sources to mean “arc” (as defined by NFPA 921). This is especially true in the context of certain automotive components such as spark plugs were a “spark” may jump the gap or an electric motor may be described as “sparking” during normal operation as motor brushes break contact with the commutator. In NFPA 77, static electrical discharges are referred to as “sparks,” inconsistent with the definitions in NFPA 921. As such, caution must be used when interpreting or using these terms.

In this course, we will sometimes refer to electrical arcs and sparks interchangeably.