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Safety by Design: A technique that incorporates hazard analysis and risk assessment

By Jim Phillips | May 14, 2024
Safety by Design: A technique that incorporates hazard analysis and risk assessment
The strategy known as Safety by Design incorporates hazard analysis and risk assessment techniques during the initial design process. 

While most electrical power systems fall under the “legacy” category, meaning they are older and may not use the latest technologies, newer systems often employ a design strategy that places greater emphasis on electrical safety.

Known as Safety by Design, this strategy incorporates hazard analysis and risk assessment techniques during the initial design process. This may lead to the use of alternative system configurations and equipment selection to improve electrical safety and reduce an electrical worker’s exposure and risk. A few examples of these strategies are listed below.

Alternate system configuration

Hazard elimination is first in the hierarchy of risk control methods. This requires establishing an electrically safe work condition in accordance with NFPA 70E 120.6. However, system operation and production demands can make this a challenge. A design that includes redundancy may provide greater flexibility with operating configurations and improve the ability to de-energize while minimizing disruption of production.

Arc duration and isolation

Arc duration can have a significant effect on arc flash incident energy. The duration is normally defined by the clearing time of an upstream protective device that is not in the same enclosure and would not be affected by the arc flash. If the main protective device is in the same enclosure as the branch or feeder devices, an arc flash in the enclosure could occur on the line side of the main or a load-side arcing fault could potentially escalate to a line-side fault. The result is the main would not clear the arc flash. An alternative design would isolate the main sufficiently so it would be unaffected by an arc flash downstream.

Reliability—no instantaneous

To improve reliability, selecting equipment with no instantaneous on the main might be used. This allows the feeders to trip first and minimize the extent of the outage. However, the main’s potentially long time delay could result in a significant amount of incident energy. It is necessary to temporarily reduce the arc duration, using one of many methods such as arc energy reduction maintenance switches. This is addressed by National Electrical Code 240.87, Arc Energy Reduction, which applies to devices with continuous current trip ratings 1,200A and greater.

Impedance grounding

Many technical references suggest approximately 80% or more of all faults involve only one phase. Why is this such a large percentage? I like to use the analogy of a flat tire (you read that correctly). Although a car has four tires, usually only one fails at a time. The same logic holds true for a phase-to-ground fault. It is more likely to have a fault on one phase than all three phases together.

Incident energy calculations using IEEE 1584 consider the arc flash as a three-phase event. Even if the arc flash does not begin as three-phase, there is a possibility of a phase-to-ground arc flash quickly escalating into a three-phase event due to the conducting plasma that can reach the other phases. Eliminating the likelihood of a phase-to-ground fault can eliminate these cases.

The use of neutral grounding resistors where permitted by the National Electrical Code, and as appropriate for the design, can eliminate the likelihood of a phase-to-ground arc flash.

Arc-resistant equipment

Arc-resistant equipment is specially designed to contain and redirect the arc flash energy away from the electrical worker. Although arc-resistant equipment does not eliminate the arc flash, it is designed to protect the electrical worker if the doors are properly closed.

I had the opportunity to “accidentally” witness testing of 15-kilovolt arc-resistant switchgear at a high-power laboratory while conducting my own independent testing. I heard a massive explosion at the other end of the lab and went running toward the area to see what happened. With smoke still billowing out of the end of the equipment, I was told it was a success. Putting it into perspective, although a terrifying event, if a worker was standing in front of the equipment, instead of the energy aiming right at them, it would be redirected away—and they would have a good story to tell.

Back to the future

I have been involved with electrical training for more than four decades, and it has been interesting to watch how electrical safety has evolved. In the early years of arc flash training, I often made the claim that in the future, more arc-resistant equipment and alternative design techniques would be used.

Of course, I received immediate pushback about the increased cost. Without skipping a beat, I would answer, if there is an electrical incident where an alternate design method or equipment could have prevented an injury or fatality, how do you explain that to a jury? You get the idea.

stock.adobe.com / fontriel

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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