Applying “Selective” Technology to Limit Arc Flash Risks
April 2018 %7C By Brian Schmalberger
Brian Schmalberger is an OEM segment market leader for ABB.
Electrical engineering textbooks and standards guides describe an arc flash event as occurring during an electrical fault or short circuit that passes through a physical arc gap, or bridge, between two electrodes.
But ask anyone who’s witnessed an arc flash incident and they’ll tell you it’s like standing next to a bolt of lightning. The sound it creates is 64 times louder than a passing freight train.( And, not to compare worker safety to dollars and cents, but an average arc flash incident can cost millions of dollars in employee claims, increased insurance costs, equipment replacement and lost productivity.
An arc flash can be cause by an accidental contact across conductors, the use of underrated equipment, contamination or tracking over insulated surfaces, corroded cables or equipment and parts – even the occasional ill-fated rodent.
An arc flash event ionizes the air, resulting in arc flash temperatures as high as 35,000 degrees Fahrenheit – hotter than the surface of the sun. In addition to worker injury and death, arc flash temperatures can liquefy or vaporize copper or aluminum conductors, or steel equipment parts. Material rapidly expands in volume as it changes state from a solid to vapor resulting in explosive pressure and ear-deafening sound waves. Molten metal can be sprayed by the blast throughout the vicinity. Solid metal debris and other loose objects, such as tools, are turned into deadly projectiles resulting in costly equipment and facility damage and catastrophic injury and death.
In response, engineers and equipment manufacturers use a variety of approaches and technologies to mitigate these conditions and potential hazards. As a result, the facility manager, consulting engineer and ultimately power panel manufacturers are faced with a complex set of choices when developing mitigation strategies for new or upgraded power distribution systems.
Facility electrical engineers use a variety of basic techniques to reduce the potential damage from arc fault events. They can keep employees further away from potential event locations to limit the human risk of the event by using remote accessories or remote diagnostic tools. Employees working or doing maintenance in these areas, are also provided with personal protective equipment (PPE). Or typically, when conducting upgrades or maintenance, companies can power-down the system completely.
Power system engineers can also design distribution systems that reduce the amount of incident energy available by selecting and coordinating the appropriate circuit breakers.
Power Protection Selectivity
Power equipment manufacturers play a role in designing their systems with an additional mitigation approach; limiting the amount of the current employed for a particular system or shortening the duration of the fault or short circuit event. Shortening a fault event time can be achieved by setting up circuit breakers to operate “selectively” in coordination. For example, ensuring that the circuit breaker closest to the fault is tripped first, or delaying or desensitizing an upstream breaker to give it time for the first breaker to clear the fault. This is critical to ensuring that the upstream circuit breaker doesn’t trip and shut down an entire electrical service powering, for example, a manufacturing line or wing of a hospital. But these selective methods for a typical 1500 kVA service may have up to five layers of coordinated breakers, creating longer delays and increasing the risk of arc flash on the main breaker.
“Smarter” power distribution systems and breakers, with advance communications and monitoring capability, are now being deployed to control selectivity while providing higher performance, reliability and better protections against arc flash incidents. Modern trip units, widely used in new construction and upgrade of older systems, are embedded with communication capabilities to better signal their responses to electrical events.
For example, using our waveform recognition (WFR) in an upstream circuit breaker, the system automatically senses the fault current signature caused by a downstream current-limiting circuit breaker responding to a fault. This cuts back on unnecessary circuit breaker trips and improves arc flash protection through intelligent selective coordination.
Zone selective interlocking (ZSI) is another smart power protection approach that communicates with upstream and downstream breakers to automatically coordinate protection between breakers. Response time is automatically changed for different events, such as a short-time event versus a ground fault. This reduces the arc flash energy potential and allows the main circuit breaker to respond faster to arcing faults. Instantaneous ZSI (I-ZSI) breakers communicate to adapt and coordinate their response to small overloads or large faults, only operating when needed. Additionally, clearing arcing current at instantaneous speeds results in lower arc flash incident energy.
Using smarter, automatically selective power protection technology significantly raises the “intelligence” of any power distribution system to help reduce arc flash events, increase worker safeguards and improves maintenance and system downtime.
Additional Reference and Sources
- The Basics of Arc Flash
- A Practical Methodology to Arch Flash Mitigation
- Modern Power Distribution Systems – Best Practice Guide
- Guide to Low Voltage System Design and Selectivity