Code Change Summary: Two new methods of arc-energy reduction were added.

This code section is all about large frame circuit breakers where the highest continuous current trip setting for which the actual overcurrent device installed in a circuit breaker is rated or can be adjusted is 1200 A or higher.

The idea behind arc-energy reduction is to place the energized electrical equipment in a state where there will be less arc-energy in the event that an accident occurs while the servicing electrician is working in the live gear.

Certain conditions may warrant the need to perform work on energized electrical equipment. Arc-energy reduction is a way to reduce the amount of potential arc-energy that may occur while a worker is servicing energized equipment.

Often, large frame circuit breakers are equipped with an instantaneous trip adjustment. Workers performing maintenance on live electrical equipment downstream of this breaker might adjust the sensitivity of the instantaneous trip feature in hopes that the breaker will trip faster during a fault, thus reducing the arc energy since the faster the breaker eliminates the fault condition, the less exposure a worker has to the arc energy. A temporary adjustment during servicing is not the intent of 240.87. The idea is to ensure that an instantaneous feature on a circuit breaker is less than the available arcing current regardless of how it is temporarily set in the field which is what triggered the statement “less than the available arcing current” to be added to items 5 and 6 in the 2017 NEC^®.

The two new arc-energy reduction methods in 240.87(B)(4 & 5) make it clear that an instantaneous trip setting or instantaneous override setting must be less than the available arcing current.

Below is a preview of Article 240. See the actual NEC^® text at NFPA.ORG for the complete code section. Once there, click on their link to free access to the 2017 NEC^® edition of NFPA 70.