The 3Cs of circuit protection

Circuit breaker technology advancements are emerging that enable engineers to strike a better balance of safety and uptime. Here are some tips from a circuit breaker expert.

10/17/2013


A Montana refinery was recently cited for a “willful” violation after an arc explosion. According to the local news affiliate, the citation involved exposing workers to electrical hazards as a result of bypassing a motor-circuit protector switch. Circumstances like these are rarely black and white and beg the question, Could this happen to me? While I can’t address the specifics of this specific incident, I do understand the inverse relationship between safety and uptime that system designers and facilities managers are challenged to contend with every day. 

Design an electrical system based solely on safety and you’ll be plagued with nuisance tripping and costly unplanned downtime. Likewise, design a system singularly focused on uptime and you’ll be putting your people, plant, and equipment at risk. Thankfully, circuit breaker technology advancements are emerging that enable you to strike a better balance of safety and uptime so you are not forced to compromise.

Making power systems safer from arc flash is about more than Ohm’s Law (E = I R, where E is the potential difference, I is the current, and R is the resistance), lockout/tagout, and following code because the electrical systems we design, maintain, and improve are parts of a bigger picture. Critical power applications in hospitals and data centers simply can’t go down. Uptime is also critical in industrial systems and commercial applications, where no power means no revenue or—even worse—danger to workers and damage to expensive equipment. We’ve got to keep the lights on.

The Three Cs of safer uptime

The good news is that there are many innovative ways to keep the lights on. The bad news is that many of them involve trade-offs that may not be all that palatable. The designer’s job is to find the right technologies to keep systems as safe and as productive as possible. By integrating the three Cs of safer uptime—modern circuit protection, coordination selectivity, and communications—we can keep our systems up and running safely. I call it the three Cs of safer uptime. 

Circuit protection. At the most basic level, circuit breakers break the circuit if there is an overload or short circuit problem. They’re not inherently very smart, but they offer basic wire and equipment protection. You size the breaker to the load and you’re done. You have circuit protection, but little protection from significant arc flash dangers, unless you de-energize the equipment. 

Circuit protection has certainly evolved from its most basic roots. Today there are technologies that report whether an overload or a fault opened a breaker, provide insights on power quality, measure harmonics, alarm certain events like ground fault and more. That’s all good information, but it doesn’t do much to help you prevent arc flash without sacrificing uptime.

Coordination of selectivity. Selectivity is a sound principle, and as good practice, designers factor it into every project to some degree. Based on application, type of facility, and risk factors, we use experience and judgment to optimize the inherent trade-off for reliability (selective coordination) and safety (arc flash mitigation). The 2005 edition of NFPA 70: National Electrical Code (NEC) brought this issue to the forefront. It effectively mandated coordination but inadvertently increased arc flash risk at the same time. It’s that sticky trade-off problem again—increase system reliability but at the expense of system safety. 

Since then manufacturers have responded with technologies to lessen that danger. Zone selective interlocking (ZSI) systems, for example, coordinate protection between upstream and downstream breakers. Energy reducing maintenance switches (ERMS) are another technology that compensates for increased arc flash risk from selective coordination requirements. 

While these solutions became part of the 2011 NEC code, they are far from complete answers. ZSI still allows significant incident energy from an arc flash event at higher fault levels because there is a trade-off between selectivity and protection. ERMS involves human, manual intervention to switch breakers to an instantaneous-trip setting to improve safety while working on the equipment and, just as importantly, additional human, manual intervention to reset the equipment for coordination when the job is done or risk more power interruptions and more downtime. 

Communications. Communicating circuit breakers aren’t new, but the latest generation of smarter circuit breakers communicates in new and better ways. That enables the final piece of the puzzle for both arc flash safety and selectivity. 

Figure 1: As the curve overlap illustrates (lower right), zone selective interlocking (ZSI) is effective but has limitations. With no ZSI, selectivity is limited to <6 kA. With ZSI, it improves to >10 kA. With instantaneous ZSI (upper right), 86 kA selectTime current curves tell us which breakers are going to trip when, though sometimes we need more than that. This is where ZSI assists. It allows a set of breakers to communicate and automatically change response time to a mid-level (short-time) or ground fault. The 2011 NEC recognized the merits of ZSI by identifying it as one of three technologies that can be used to reduce arc flash risk when circuit breakers without instantaneous (INST) protection are used in the interest of coordination, per article 240.87. Using that premise, a system coordinated to 10kA level could limit the arc flash incident energy while maintaining optimized selectivity (see Figure 1). 

However, new technologies introduced since the 2011 code greatly expand the communications of ZSI. By automatically adapting not only short-time and ground fault delays (ZSI) but also instantaneous response (I-ZSI), the system maintains the same level of arc flash mitigation but at a range of up to 85kA, continuously without manual intervention(see Figure 1). Where ZSI brings protection capabilities beyond what can be achieved in a “traditional” static curve based study, I-ZSI adds protection and coordination beyond previous limitations 

I-ZSI allows multiple layers of circuit breakers and all of their protections to operate as a system. With small overloads or large faults, breakers communicate with one another, adapting and responding accordingly. Then, each circuit breaker operates only when needed and acts as a backup only when necessary.

Here’s the math:

At 65 kA available fault current

1) With I-ZSI:

A 2000 A circuit breaker with upstream 3000 A (or any size) I-ZSI networked circuit breaker

Fully selective INST setting = 8.5x (8.5x rating plug) = 25.5 ka

Incident energy potential is <8 cal/cm2

2) Without I-ZSI:

A 2000 A circuit breaker with upstream 3000 A

Fully selective INST setting = “off” (or above the maximum available current)

No instantaneous protection from arcing faults = potentially huge arc flash event.

With I-ZSI each breaker has line of sight to what’s happening throughout the system. As a result, the system gives a facility protection and selectivity, helping operators meet demanding uptime business objectives without increasing risk. It’s a “language” that speaks uptime and safety in the same breath.

The three Cs in the real world

Let’s say I’m putting together an electrical plan for an oil refinery facilities manager who is concerned with safety, certainly, but also the pressure of maintaining uptime. 

Option 1: Super safe. I could give him super-high safety with INST pickups all set to their minimums. He’d have top safety, but a benign, unanticipated motor start-up might result in a trip and bring the whole operation to a screeching halt. Ultimately, his business would suffer because of my compromise. After he’s past the headache of getting the operation up and running again, I’m his first phone call to get this fixed. 

Option 2: Always running. From that experience, I change my approach and design for ultimate uptime instead. There is no way this system is going down because of something (seemingly small) added to the load plan. I’m using all non-INST air circuit breakers and dialing the INST protections to the maximum. I’ve got it perfectly configured for selectivity, but now if there is an arc flash event, my response times aren’t sensitive enough to mitigate it. Not only did I put people operating the system at risk, but an accident also includes extensive damage to the equipment and facility, requiring even more downtime to fix. Ironically, by planning for uptime I’m still putting uptime at risk, not to mention employee safety. 

Option 3: The sweet spot. Robust circuit breaker technologies like ZSI and I-ZSI deliver a more balanced solution. An electrical system design that maximizes both safety and uptime is now possible. My customer benefits from increased uptime and productivity, helping his business meet or exceed operating objectives. At the same time, we are protecting the business from a catastrophic, high-powered arc flash event. The previous scenarios were exaggerated to prove a point, but they do illustrate the challenges system designers face in balancing uptimes, safety, and compliance. 

You can see how uptime decisions can influence a system design, perhaps to a fault. There’s no doubt that arc flash protection is critically important, but it can run contrary to meeting operating objectives in an increasingly competitive business world. No business can afford to trade protection for greater uptime or even perceived operational flexibility, as I’m sure that Montana refinery has learned. Fortunately, today's technologies are smart enough to deliver mandated selectivity and protection simultaneously. Engineers who investigate and apply these new technologies can design systems that help their customers reap the business benefits they want, while also lowering risk and providing safer operations.


Tim Ford is global product manager, molded case circuit breakers at GE Industrial Solutions. He manages a broad portfolio of circuit breakers products covering all nonresidential 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.



MEYNARDO , GU, United States, 11/08/13 08:05 AM:

Short circuit calculations generally figures the maximum available fault current based on the known parameters of the electrical components along the path of the short circuit current. The assumptions are all based on the minimum resistance (cold) of the electrical components and the minimum resistance at the faulted point (contact resistance = 0), but how many of the faults, realistically have zero contact resistance for example, bolted fault? Factually most of the faults are line to ground faults, almost 75 %. The reality of a bolted fault happens most of the time when somebody is working on an energized circuit and accidentally make the fault like accidentally letting some parts of a conductive material makes a contact with an energized bus. Even this kind fault will not produce the theoretical let thru current calculated on a bolted fault. Working in the equipment regularly in a maintenance environment will give you more correct feedback on what the real field conditions are than reading the report and making conclusions based on that report. In my more than 45 years in the electrical engineering environment of maintenance, operation, design, and studies, many of the faults are human errors, faulty workmanship, oversight in inspections, carelessness in working in the energized environment. I have seen several of these faults and short circuits in medium voltages, overhead primary circuits, 480 volt motor control centers, 208 volt panelboards, and many many of the failures can be traced to human errors and faulty workmanship left un-noticed that suddenly blows into a significant short circuit or arc flash. Making the circuitry more complex by add ons will lead into more problems than a smooth system operation.
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