Section 3: Appendices F: Short Circuit Remediation
Short circuit problems and their solutions remain a struggle for many sites since the solutions can be both time consuming and expensive. This appendix addresses the general methodology in more detail as well as provides some background on the ratings and the impact of underrated equipment on safety.
What are short circuit ratings
Short circuit ratings address the problem of the physical forces that are applied in the event of a fault and the operation of equipment under fault conditions. When applied to enclosures, this rating is referred to as a short circuit rating (kASCR) or bus bracing rating. When applied to protective devices such as fuses and circuit breakers it is referred to as an interrupting current rating (kAIC). In effect, the short circuit rating of gear is ensuring that the equipment can withstand a fault without sustaining permanent damage or tearing itself apart and that all the components will operate properly in the event of a fault. It is required by code to have the short circuit rating listed on equipment. With older and custom equipment, it is common to have this labeling missing which can lead to the necessity of assumptions on equipment ratings. The rating of gear by code is based on the lowest rated component within the equipment or a series rating. Series ratings are provided through laboratory testing which ensure the gear can operate to its rated kASCR even with components which are rated for less. Series ratings require a special label detailing that the equipment is using a series rating, what the rating is, and what components are required to maintain the series rating. With series ratings, even replacing a breaker can invalidate the kASCR.
Implications of underrated equipment
Equipment is considered underrated if the calculated fault current exceeds the kASCR or kAIC of the equipment. If the equipment is underrated, it cannot be considered “enclosed” for the purposes of arc flash boundaries and operation. This means that the equipment may not be able to contain an arc and must be considered “open”. This has a large impact on personnel safety; operators must be fully protected by appropriate PPE to operate or even approach the gear. A strict reading of the code also indicates that operating this equipment would require a live work permit by a qualified person. Underrated equipment also runs a high risk of needing replacement in the event of a fault, which adds cost and downtime. The resulting physical forces from a short circuit could tear the enclosure open and damage internal components.
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