2017 NEC 240.87 Instantaneous Trip

The NEC requires that where a circuit breaker’s highest continuous current trip setting is rated or can be adjusted to 1200 Amps or higher, a method for reducing the arc energy must be provided.

The 2014 Edition of the NEC provided a list of methods which included: zone-selective interlocking, differential relaying, energy-reducing maintenance switching and energy-reducing active mitigation systems or an approved equivalent means. The device’s instantaneous trip function was excluded from the list.

The 2017 Edition of the NEC now permits the use of the device’s instantaneous trip unit or instantaneous override as long as there is sufficient arcing short circuit current for it to trip. IEEE 1584 – IEEE Guide for Performing Arc-Flash Hazard Calculations is referenced in this section as a method for calculating the arcing short circuit current.

Since the National Electrical Code (NEC) is a U.S. based standard, this requirement may not be applicable to those in other countries. It would be interesting to hear the different views.

With that long introduction, here is this week’s question:

Will you/your clients begin using the instantaneous to reduce arc flash energy for breakers 1200A and greater?

Yes
No
Doesn’t apply
We were using it anyway/already
ANSWER QUESTION

 

About Jim Phillips: Electrical Power and Arc Flash Training Programs – For over 30 years, Jim Phillips has been helping tens of thousands of people around the world, understand electrical power system design, analysis, arc flash and electrical safety.

Jim is Vice Chair of IEEE 1584, International Chairman of IEC TC78 Live Working and Steering Committee Member – IEEE/NFPA Arc Flash Collaborative Research Project. He has developed a reputation for being one of the best trainers in the electric power industry.  Learn More

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