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Safety by Design: A Technique That Incorporates Hazard Analysis and Risk Assessment

Most electrical power systems are considered “legacy” systems, meaning they are older and may not utilize the latest technologies. In contrast, newer systems often adopt a design strategy that prioritizes electrical safety. This approach, known as Safety by Design, involves incorporating hazard analysis and risk assessment techniques right from the initial design phase. As a result, alternative system configurations and equipment choices may be made to enhance electrical safety and reduce the risk and exposure for electrical workers. Below are a few examples of these strategies.

Alternative System Configuration

Eliminating hazards is the top priority in the hierarchy of risk control methods. This involves creating an electrically safe work condition as per NFPA 70E 120.6. However, operational and production requirements can make this challenging. A design that includes redundancy may offer greater flexibility in operating configurations and improve the ability to de-energize systems while minimizing production disruptions.

Arc Duration and Isolation

The duration of an arc flash significantly impacts the incident energy. Arc duration is typically defined by the clearing time of an upstream protective device located outside the same enclosure and unaffected by the arc flash. If the main protective device shares an enclosure with 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 escalate to a line-side fault. Consequently, the main protective device might not clear the arc flash. An alternative design would isolate the main protective device sufficiently to ensure it remains unaffected by downstream arc flashes.

Reliability — No Instantaneous

To enhance reliability, equipment without instantaneous protection on the main protective device might be selected. This allows feeders to trip first, minimizing the extent of outages. However, the potentially long delay of the main protective device could result in significant incident energy. To temporarily reduce arc duration, methods such as arc energy reduction maintenance switches can be used. This is addressed by National Electrical Code 240.87, Arc Energy Reduction, applicable to devices with continuous current trip ratings of 1,200A and above.

Impedance Grounding

Technical references suggest that around 80% or more of all faults involve only one phase. This high percentage can be likened to the analogy of a flat tire—although a car has four tires, typically only one fails at a time. Similarly, a phase-to-ground fault is more likely than a three-phase fault. Incident energy calculations using IEEE 1584 consider arc flashes as three-phase events. Even if an arc flash starts as a single-phase event, it can escalate to a three-phase event due to conducting plasma reaching other phases. Using neutral grounding resistors, where permitted by the National Electrical Code and appropriate for the design, can eliminate the likelihood of phase-to-ground arc flashes.

Arc-Resistant Equipment

Arc-resistant equipment is designed to contain and redirect arc flash energy away from electrical workers. Although it does not eliminate arc flashes, it protects workers if the doors are properly closed. I once witnessed the testing of 15-kilovolt arc-resistant switchgear in a high-power laboratory. Despite a massive explosion and smoke, the test was considered a success. If a worker had been standing in front of the equipment, the energy would have been redirected away from them, potentially saving their life.

Back to the Future

Having been involved in electrical training for over four decades, I have observed the evolution of electrical safety. In the early days of arc flash training, I predicted that more arc-resistant equipment and alternative design techniques would be adopted in the future. Although there was initial resistance due to cost concerns, I argued that if an alternative design or equipment could prevent an injury or fatality, the cost is justified. Explaining such a decision to a jury after an incident would be far more challenging.