NEC 2017 240.67.B.2 Arc Energy Reduction (Fuses)

[iceworm]

New section to the NEC. It is not effective until 2020, so lots of time. The particular section I am looking at is "Energy-reducing maintenance switching".

240.67 Arc Energy Reduction. Where fuses rated 1200 A or higher are installed, 240.67(A) and (B) shall apply. This
requirement shall become effective January 1, 2020.
(A) Documentation. Documentation shall be available to those authorized to design, install, operate, or inspect the
installation as to the location of the fuses.
(B) Method to Reduce Clearing Time. A fuse shall have a clearing time of 0.07 seconds or less at the available arcing
current, or one of the following shall be provided:
(1) Differential relaying
(2) Energy-reducing maintenance switching with local status indicator
(3) Energy-reducing active arc flash mitigation system
(4) An approved equivalent means

Informational Note No. 1: An energy-reducing maintenance switch allows a worker to set a
disconnect switch to reduce the clearing time while the worker is working within an arc-flash
boundary as defined in NFPA 70E -2015, Standard for Electrical Safety in the Workplace, and then to
set the disconnect switch back to a normal setting after the potentially hazardous work is
complete.

Informational Note No. 2: An energy-reducing active arc flash mitigation system helps in reducing
arcing duration in the electrical distribution system. No change in the disconnect switch or the
settings of other devices is required during maintenance when a worker is working within an arc
flash boundary as defined in NFPA 70E -2015, Standard for Electrical Safety in the Workplace.

Informational Note No. 3: IEEE 1584, IEEE Guide for Performing
Arc Flash Hazard Calculations, is one of the available methods that
provides guidance in determining arcing current.

Bold emphasis is mine)

I'm at a complete loss as to the physics about how one might do this.

Anybody got any clues?

Iceworm



Harmless flakes working together can unleash an avalanche of destruction

 

[davidbeach]

No idea about the "disconnect" switch part, but a switch that turns on a normally blocked instantaneous trip is very common.

 

[iceworm]

Yes, that would be my understanding - If it were an electronic trip unit in a CB (240.87). But this new section, (240.67) is for fuses. Color me confused.

Harmless flakes working together can unleash an avalanche of destruction

 

[dpc]

Interesting. Seems to parallel requirement for arc reduction for circuit breakers 1200 A and above.

For most 480 V and 600 V systems, a 1200 A or larger fuse is unlikely to be in its current-limiting range for most arcing faults, since arcing current is going to be much less than bolted fault current. This will likely result in high incident energy downstream. So high incident energy is a reality in many cases with these large fuses. The physics is real.

I'm not sure sure how this would be implemented in practice. There has been some work done in developing new "smart" fuse-like devices that can have more flexible time-current characteristics

If I was more motivated, I'd review the Code Making Panel discussions on this. Perhaps this was proposed by a manufacturer of circuit breakers.

Cheers,

Dave

 

[davidbeach]

Maybe you install a current limiting fuse in series with the non-current limiting fuse and then bypassed with the disconnect switch. Operate normally with the disconnect closed and open it for arc flash limitation.

 

[iceworm]

If I was more motivated, I'd review the Code Making Panel discussions

Save you motivation for something that matters. I did check. Wasn't a CB guy, but an IBEW guy.

Thanks anyway guys. Later.

iceworm

Harmless flakes working together can unleash an avalanche of destruction

 

[iceworm]

David -
Two sets of fuses, two disconnects - yep, thought of that. Hurts my soul, but possible.

thanks for looking in

ice

Harmless flakes working together can unleash an avalanche of destruction

 

[HCBFlash]

Now, don't forget that the primary purpose of the ridiculously bloated National Exception Catalog is to promote relatively new, patented gizmos by requiring their use. We'll find out soon enough who's behind this, like it or not.

Maybe we could pay the same fee to have them NOT release "new" versions. Once again it's environmental impact vs. employment stability.

.

(Me,,,wrong? ...aw, just fine-tuning my sarcasm!)