Code Change Summary: Revised code language on Arc energy reduction for circuit breakers.

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

Certain conditions may warrant the need to perform work on energized electrical equipment. The idea behind arc-energy reduction is to provide a means for energized electrical equipment to have a reduced level of arc energy should an accident occur while the servicing electrician is working in live equipment.

The amount of potential arc energy is based on several factors, one of which is the fault duration or time the fault condition persists before an overcurrent device ahead of the faulted circuit opens to clear the fault condition. This code section was put in place to ensure an extra measure of safety for the worker. Using one of the seven permitted methods to reduce potential arc energy ensures additional worker safety should a mishap occur triggering an arc flash event.

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 relocation of the statement “less than the available arcing current” from items 5 and 6 to the parent text so that it applies to all seven permitted methods to reduce the arc energy.

Another change is a new statement that prohibits temporarily adjusting the instantaneous trip setting in hopes to achieve arc energy reduction. According to the committee statement from CMP 10, “it is the final setting of the instantaneous trip that determines whether or not additional arc energy reduction techniques are required. It is not intended that the minimum setting of the instantaneous trip, as is typically shipped from the factory, be the determining factor of whether or not additional arc energy reduction is necessary, but rather the final setting as determined by the electrical system requirements such as inrush characteristics or selective coordination. Nor is it the intent that the instantaneous trip be adjusted to a lower setting while a worker is working on the equipment, and then adjusted back to the desired setting after work is complete”.

In the 2020 NEC^®, new documentation requirements were also added to demonstrate that whatever method chosen to reduce clearing time is set to operate at a value below the available arcing current. This is much needed code language since the NEC^® does not currently require that the available arcing current be calculated, yet it must be known in order to ensure compliance with 240.87(B).

Below is a preview of the NEC^®. See the actual NEC^® text at NFPA.ORG for the complete code section. Once there, click on their link to free access to the 2020 NEC^® edition of NFPA 70.