"Normal" and "overload" conditions ... "fault current"..."interrupt" versus "withstand" ratings..."current-limiting." Knowing what these terms mean and applying them correctly is fundamental to designing safe, reliable electrical distribution systems. This is especially true in light of more stringent code enforcement and the current design trend to deliver energy savings by selecting low-impedance transformers. Why? Lower transformer impedances mean higher short-circuit currents.

Simply choosing a circuit breaker with a high interrupt rating won't assure adequate protection under short-circuit conditions. With an "ounce of prevention," you can avoid the code official's "red tag" at your next system start-up. Let's review the meaning of the terms that opened this article, define some of the issues related specifically to HVAC motor starter applications and identify practical, effective solutions.

Table of Contents

• Normal Operation, n.
• Overload Operation, n.
• Fault Current, n.
• Interrupt Rating, n.
• Withstand Rating, n.
• Current Limiting, n.
• Summary