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Don’t Reset That Device! Without determining the cause

By Jim Phillips | Sep 11, 2023
drill
The lights just went out and plunged the site into darkness.

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The lights just went out and plunged the site into darkness. After recognizing that there is no electric utility outage, thoughts turn to the possibility of a tripped circuit breaker or an interrupted fuse. Time to locate the panel and overcurrent device responsible for the outage and reset it or replace the fuses. Easy to resolve, right? Not so fast! It’s not the device that is the problem. Something happened that caused it to interrupt.

As a minimum, overcurrent devices generally have two tripping functions: overload and instantaneous. The overload function introduces a time delay before opening for lower-magnitude currents. However, if a higher-magnitude current flows, it can cause the device to trip instantaneously—usually in just a few electrical cycles. Larger currents are often due to a short circuit from electrical insulation failure, incorrect wiring or something similar, and can be quite serious.

Reclosing circuits after protective device operation

NFPA 70E 130.8(M) requires that a circuit protective device shall not be manually re-energized until a qualified person or persons determines the equipment and circuit can be safely energized. Manually reclosing or replacing fuses is prohibited until the fault has been cleared. However, an exception exists if it can be determined the interruption was from an overload.

We have all heard the stories. Just reset the circuit breaker and see what happens. If it trips again, reset it again, and again. Use a piece of wood to prop the handle closed, stick a soda can or pipe in the fuse clips. Of course, I am just kidding. There is a reason the device is interrupting, and it needs to be determined and corrected.

The device is doing its job

Back in the late 1970s, I was an engineering student working summers at a foundry. The department located next to our group was responsible for casting brass fittings. Brass fittings can be easily damaged if they fall and hit the concrete, so the floor was covered with wood blocks to absorb the impact and minimize this type of damage.

One afternoon, a maintenance crew in that area was attempting to mount a piece of equipment to the floor. This seemed simple enough, they just drilled into the wood and … poof! The lights went out. They were not aware of the conduit running under the wood-block floor that contained an electrical feeder. 

Before you remind me of everything that was done incorrectly, this was 45 years ago when electrical safety practices were, well, lacking. Thanks to standards like NFPA 70E and the National Electrical Code, there have been significant improvements.

Back then, the “normal” procedure after an outage would be to reset the tripped circuit breaker and see what happened. However, when we heard the commotion in the distance, we knew something was wrong. Had we simply reset the circuit breaker as a typical knee-jerk reaction, we would have closed into their mistake—a bolted fault caused by a drill bit through the conduit and conductors.

That is why NFPA 70E prohibits manually re-energizing until a qualified person determines the equipment and circuit can be safely energized.

Normal operation of electric equipment

Normal operation of electric equipment such as operating a switch or circuit breaker requires satisfying seven conditions listed in Exception No. 1 of NFPA 70E 110.2(B). This includes conditions such as: the equipment has been properly installed, properly maintained and is rated for the available fault current. The last of the seven conditions is “no evidence of impending failure.” Although it may require some judgment, if a device interrupts due to a fault, checking the “yes” box to indicate no evidence of impending failure may be difficult to justify.

What about arc flash? Table 130.5(C), Estimate of the Likelihood of Occurrence of an Arc Flash Incident for AC and DC Systems, lists various tasks and the operating condition determined from 110.2(B) Exception No. 1 above. The table rates the likelihood of occurrence of an arc flash as yes or no.

One task listed in the table is: “Operation of a circuit breaker or switch the first time after installation or completion of maintenance to the equipment.” This task is listed as “yes” for likelihood of occurrence of an arc flash.

What could possibly go wrong?

Although resetting a circuit breaker or replacing a fuse seems like a simple solution, you have to address the important question of “Why did the device trip?” . What could possibly go wrong by resetting the device—even if there is a drill bit stuck in the conduit and conductors?

About The Author

PHILLIPS, P.E., is founder of brainfiller.com and provides training globally.  He is Vice-Chair of IEEE 1584 Arc Flash Working Group, International Chair of IEC TC78 Live Working Standards and Technical Committee Member of NFPA 70E.  He can be reached at [email protected].

 

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