Outsmarting arc flash

Have you ever seen an arc flash incident? Feel the unbelievable force and potential destructive power firsthand? I have. It was in a controlled, protected setting, from a safe distance, but I never met anyone who experienced one live and in person until I met an electrical contractor after one of my recent arc flash information presentations.

I’d been sharing information on arc flash safety, selective coordination and minimising arc flash dangers in talks around the US at every opportunity I could find. I was confident with my delivery, which was polished and clean. I explained the US standard for electrical safety, NFPA 70E, hazard risk category (HRC) levels and compared 10 calories to 20 calories, etc. After one customer conference session, a veteran, experienced electrical contractor came up and startled me, saying, “Tim, I loved your presentation. I’m impressed with the technology, but I thought I was going to have to leave the room. Talking in depth about arc flash brings me back, like it was yesterday, to a terrible experience I had on a job site long ago.”

He captured my attention and I invited him to tell me more. My new friend described what he saw firsthand at a hospital retrofit job many years earlier. The team was almost done with their work on energised gear when disaster struck. A piece of sheet metal slipped out of the switchboard top hat and across conductors. The system went down and so did his crew, thrown across the room, bloodied and battered by the arc flash energy. To this day you could see the terror in his eyes as he remembered the human toll. I realised that the arc flash experiments I’d seen in test labs were nothing compared to the real-world arc flash incidents that occur all too often. In fact, in the US alone, as many as 5-10 arc flashes occur every day according to a report by the Center for Disease Control and Prevention, and when you consider incidents globally, no doubt multiples of that figure.

As he continued to talk, I understood how his experience paralleled the very situations I’d been lecturing about. Since it was in a critical hospital power application, the main breaker was set to be as insensitive as possible. So the arc was very powerful, resulting in extensive equipment damage and, worse yet, significant injury. He recalled tending to the injured and working to save their lives before moving on to assess the extensive equipment damage and restoring hospital power that affected hundreds more lives. It was more than 20 years after the incident and I could still sense his pain. This arc flash accident had changed his life.

Fortunately, the degree of destruction he saw doesn’t need to happen any more. Recent arc flash strategy improvements can reduce the effects of arc flash while keeping operations up and running.

Arc flash can be dangerous, expensive and lethal

When bridges across conductors cause an arc flash, the world changes in a millisecond. Temperatures hotter than the surface of the sun, molten shrapnel flying at supersonic speeds and intense pressure waves can injure and kill people. According to the Chicago Electrical Trauma Institute, there are 320 deaths and 4000 work-loss injuries from electrical incidents in the US each year. The institute also cites electrical accidents as the second leading cause of construction industry fatalities.

The two most important factors in limiting the incident energy that powers the destructive force of an arc flash are energy and time. Since the voltage is fixed per the application, the only practical way to protect from arc flash destruction is with strategies that can reduce the duration of an event and therefore limit incident energy. In addition to reducing worker injuries, lower incident energy also reduces damage to equipment and facilities. That means faster incident recovery times and lower restoration costs.

Selective coordination and its unintended arc flash consequences

Every professional, experienced electrical system designer will tell you selective coordination is a sound strategy to maximise uptime, or system reliability, throughout a location. And that reliability, of course, delivers its own measure of safety for a facility, especially when protecting emergency circuits. It localises an overcurrent condition, so the circuit breaker closest to a fault is the only one that trips. For an extreme example, in a critical application, such as a hospital, a trip-worthy event in a patient room does not affect power in operating rooms.

On the surface, this sounds like a perfect solution for electrical reliability throughout a facility. Indeed, in some markets, like the US, codes have been changed to require selectively coordinated systems. Specifically in 2005, the US National Electrical Code (NEC) mandated selective coordination for emergency circuits and legally required standby systems. And unfortunately, selective coordination, despite its many benefits, can increase the risk for arc flash injury since it increases arc flash incident energy by increasing time.

While Australia doesn’t have the same selectivity and arc flash standards mandated, many designers use the US standards as a guide, which can create incompatibilities. However, regardless of various standards around the globe, we can all agree that arc flash hazards are a significant risk to people and property deserving of our attention and focus to minimise. Modern electrical systems need a smarter design that can manage both arc flash mitigation and system reliability, simultaneously.

The impossible compromise

Selective coordination improves system reliability, but at the risk of more powerful arc flash incidents. Sensitivity settings and tripping schemes force engineers to choose between protection and reliability. Since working on energised systems is a fact of life, engineers and electrical workers need a better alternative.

I-ZSI: a new strategy without compromise

GE is recommending a totally new approach based on a proven technology for safety and reliability without compromise. Instantaneous Zone Selective Interlocking (I-ZSI) is a breakthrough that delivers safety and reliability at the same time.

Traditional Zone Selective Interlocking (ZSI) has been used to coordinate upstream and downstream protection for years. Connecting downstream feeder circuit breakers to upstream main breakers, the trip unit of a feeder circuit breaker sends a restraint signal to the trip unit of the main circuit breaker and then trips to clear the fault within the programmed short time delay.

For a fault between the main and feeder circuit breakers, there would be no restraint signal from any of the trip units on those branch circuit breakers thus the main circuit breaker would automatically trip at the minimum delay. This ZSI technology will give you some measure of selectivity. But the reduction in arc flash energy is minimal because the instantaneous protection must be disabled for complete selectivity, and consequently, the duration of the event is longer.

I-ZSI, which is embedded into GE’s EntelliGuard family of global trip units, takes ZSI to the next level. It’s a system-wide rethinking that redefines selectivity. Instantaneous protection is enabled all the time by interlocking the circuit breaker’s protection from overloads and its response to short circuits. So trip unit connection schemes slash arc flash energy levels everywhere, every time - delivering safety and selectivity. You no longer need to hinder performance to ensure protection or hinder protection to ensure performance.

Every circuit is always available because I-ZSI doesn’t require disabling instantaneous protection for the zone to work. Therefore, you always have the capability to reduce arc flash energy.

Seamless upstream and downstream coordination - regardless of distance

Engineers considering I-ZSI should also consider that it’s an easily implemented strategy. Simple twisted-pair wiring makes the safety connection between trip units.

For downstream protection, Waveform Recognition (WFR) provides end-to-end system protection, by enabling an intelligent response to a fault for smaller load side breakers. Trip units sense trouble from the branch circuit breakers and analyse the waveform to determine when to trip. Smartly, the trip units use this WFR to see the operation of the downstream current limiting breakers and trip only when required.

With I-ZSI and WFR implementations, engineers have a complete, proven solution for slashing arc flash risk throughout a facility. And it’s available today.

The smarter arc flash approach: minimise without compromise

Now when I speak with engineers about arc flash dangers, I always share what I learned from that contractor that day last autumn. He was a typical guy on a typical day working on what he thought was a typical job - until a small slip-up created a huge problem. The right arc flash strategy could have lessened the effects of that tragic day that are burned into his memory. Because, with the right strategy in place, such as I-ZSI and WFR, an arc flash incident doesn’t need to be a prescription for disaster.

*Tim Ford is the Global Product Manager for Molded Case Circuit Breakers (MCCBs) for GE’s Industrial Solutions business, a GE heritage business, which is leading the future of electrification with advanced technologies that protect and control the distribution of electricity throughout a facility’s infrastructure. In this role, Tim leads product strategy and execution for MCCBs globally. He manages a broad portfolio of circuit breaker products covering all non-residential NEMA/UL and IEC MCCBs including the Spectra and Record Plus branded product and helped develop the industry-first Instantaneous Zone Selective Interlocking (I-ZSI) technology that delivers safety and reliability. He has over 15 years’ experience in the electrical industry with product development and management roles at both GE and ABB. He holds a Bachelor of Science degree in Mechanical Engineering from West Virginia University and is a licensed Professional Engineer (PE).