The Arc Flash Protection Versus System Reliability Challenge -
A No-Compromise Approach for Electrical Panel Design
January 2019 | By Brian Schmalberger
Brian Schmalberger is an OEM segment market leader for ABB.
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There always seem to be trade-offs in any design process. Automotive engineers swap speed and acceleration for fuel efficiency. Mobile device designers balance screen size or blazing-fast graphics for battery life. For large, facility-level electrical distribution and protection design, the trade-offs are often providing protection against catastrophic arc flashes in lieu of overall reliability and system uptime.
An arc flash event – when accidental contact or faulty equipment causes a connection between two power contacts – expels tremendous energy when the air between electrical contacts is ionized, creating an explosive flash with temperatures as high as 35,000 degrees Fahrenheit.
Yet, mitigating the risk of arc flashes comes at the cost of lowering power protection sensitivity, resulting in yet another trade-off – less reliability due to increased levels of false or nuisance trips and system downtime.
Balancing System Reliability and Arc Flash Protection
In recent years, many power protection technologies have come together to improve that balance between system reliability and arc flash protection. These combined technologies reduce the potential for an arc flash event by either shortening the duration of a crossed or shorted circuit, or by limiting the available current generated by the event.
Two core technologies can be employed separately or together, in various combinations, to impede the potential for arc flash; “instantaneous zone selective interlocking,” and “wave form recognition.” Both do separate but related and supporting functions.
- Instantaneous zone selective interlocking (I-ZSI) deploys multiple layers of large circuit breakers that operate as a system to protect against either small overloads or large faults. Each circuit breaker only operates when needed and acts as back-up only when necessary. I-ZSI capability provides virtually instantaneous protection selectively, regardless of the size of a power system or the main circuit breaker.
- Wave Form Recognition (WFR), developed by GE, is an algorithm that instantly allows a feeder, or downstream, circuit breaker (above a panel or motor control center [MCC] that has current limiting fuses or circuit breakers) to be set at very sensitive levels while still providing selectivity. This technique provides a high level of virtually instantaneous protection where and when needed.
Technologies that Address the “Protection Vs. Reliability” Dilemma
Let’s look at these two technologies and the design and protective issues they address. Modern designs increasingly call for sensitive and fast-acting circuit breakers to be selective. That is, only the circuit breaker closest to the fault responds, resulting in strong uptime performance of the larger electrical system, while also inherently addressing arc flash risks.
Selectivity, as in “selectively-coordinated breakers,” is a technique where the circuit breaker closest to the fault trips first, thereby minimizing the affected area of an interruption. These new techniques use signaling between intelligent circuit breakers to automatically coordinate protection among these circuit breakers.
If protection isn’t selective, short circuits can result in equipment outages, affecting much more of the facility than required. Selectivity can be compromised if, to achieve lower arc flash incident energy, the circuit breaker trip points are set too sensitively. Thus, the traditional safety-versus-uptime dilemma can be, and often is, referred to as “protection versus reliability.”
Making Both Selectivity and Arc Flash Safety Possible in Your Facility
So, what are the options that provide both selectivity and safety for arc flash hazard mitigation solutions? Innovative electrical systems can now communicate and be coordinated to optimize their responses to electrical events. Zone Selective Interlocking (ZSI) systems, for example, allow a set of breakers to coordinate protection between upstream and downstream breakers. They can change response time to different events, like a short-time event or a ground fault, thus reducing the arc flash energy potential from the lower settings used on the upstream circuit breaker, and allowing the main circuit breaker to respond faster to arcing faults.
When instantaneous response is added to traditional ZSI, I-ZSI breakers communicate fully to adapt and coordinate their response to small overloads or large faults, only operating when needed. I-ZSI provides arc flash protection, since clearing arcing current at instantaneous speed results in lower arc flash incident energy.
I-ZSI and waveform recognition (WFR) intelligent protection systems use the same simple voltage signal and coordination capabilities to deliver both mandated selectivity and improved safety simultaneously.
Waveform recognition (WFR) technology is employed in trip units. The upstream circuit breaker must have a WFR-enabled trip unit and the downstream circuit breakers must be current-limiting. When a fault is below the downstream circuit breaker and within the current-limiting range of the downstream circuit breaker, the upstream circuit breaker with WFR senses the fault current signature caused by the downstream current-limiting circuit breaker responding to the fault. This can prevent a trip—even where overlap exists—in the instantaneous parts of the time current curves.
Innovative circuit breaker communications systems, enabled by I-ZSI and WFR technology, allow electrical designers to deploy flexible, safe and reliable power for both new or upgraded industrial and commercial systems.
Additional Reference and Sources
- Basics of Arc Flash - https://www.geindustrial.com/sites/geis/files/gallery/The-Basics-of-Arc-...
- Arc Flash Q&A -- http://apps.geindustrial.com/publibrary/checkout/DEQ-208?TNR=FAQs%7CDEQ-208%7Cgeneric
- GE Arc Flash Resources Page -- http://go.ge-energy.com/arcwatch.html