Optimizing Arc-Flash Protection Strategies

05/18/2005


Editor’s Note: The following article was submitted to CSE in response to a report on arc flash that appeared earlier this year and arose from a discussion during the National Manufacturing Week in Chicago in March.

Serious workplace injuries and fatalities from electrical arc-flash incidents have been occurring ever since electrical energy was first generated and distributed for productive applications. Arc-flash accidents that result in a serious injury or fatality occur five to 10 times a day in the United States. Recently, in an effort to improve workplace safety, the industry has begun to focus on the arc flash hazard that is present when workers must perform work on energized electrical equipment.

One of the results of that focus has been the development and publication of NFPA 70E-2004 Standard for Electrical Safety in the Workplace . NFPA 70E is an industry consensus standard that defines the requirements for safely working on or around electrical equipment. OSHA now recognizes the NFPA 70E standard as a written, published standard, available to the industry, and they cite the requirements of this standard for employers under their general duty clause.

A short-circuit or fault occurs when the insulation between energized electrical phase conductors, or between a phase conductor and ground, is somehow compromised. During a so-called “bolted” fault, the fault current flows over a conductive path. While such faults can be damaging, little energy is released into the surrounding environment during the fault. During an arcing fault, however, the fault current instead flows through the air rather than through a conductor or busbar and a great deal of thermal energy is released into the environment. This sudden release of thermal energy, similar to that seen in an electrical arc furnace, is referred to as an arc-flash event.

The degree of arc-flash hazard is measured by the available incident energy, expressed in calories or joules per square centimeter. This incident energy defines the thermal exposure that a worker standing at a certain distance from the source of the arc (the “working distance”) would expect to receive on the head and torso. In addition to the thermal release, there are other hazards produced by these events, including arc blast (a high-pressure wave), sound levels that can lead to permanent hearing damage and often a ballistic threat from flying particles and objects.

The incident energy level at a given location in an electrical system is dependent on many factors, such as system voltage, available fault current and the arcing fault duration. The faster an arcing fault is detected and cleared from the system, the less energy it releases into the air, so the action of the over-current protective device—specifically, how quickly it can detect and clear the fault—is a critical parameter in determining the level of arc flash hazard in a given system. In fact, in most cases the fault clearing time is the only variable in the equation that can feasibly be controlled in order to limit the incident energy that will be produced by an arcing fault.

Most overcurrent protective devices, such as circuit breakers, fuses and protective relays, have somewhat inverse time / current characteristics. That is, the higher the current flowing through the device, the faster the device will act to clear a fault. This is an oversimplification, but it is useful in understanding the action of the overcurrent protective device when it encounters an arcing fault.

Electrical engineers are accustomed to performing device coordination studies and developing breaker or relay settings and selecting appropriately sized fuses in order to provide both overload protection and selectively coordinated protection against high-level bolted faults. To ensure that the selected overcurrent devices and/or settings also provide adequate protection against arc flash, the response of the protective devices to arcing fault current must also be taken into account.

In low-voltage systems (& 1kV), the resistance of the arcing fault itself means that the arcing fault current will always be less than the available bolted fault current at any given point in the electrical system. In a typical 480V system, for example, the arcing current ranges from 40% to 60% of available fault current. (The reduction in fault current occurs for arcing faults in medium-voltage systems as well, but the change is not as pronounced—for example, the arcing fault current can be as high as 95-97% of the bolted fault current at 12.47 kV.) The clearing time of the arcing fault is consequently determined based on the response of the overcurrent protective device at this lower arcing fault level, rather than on the full available bolted fault current.

To provide maximum personnel protection during an arcing fault, the objective is to achieve the fastest possible fault clearing time in order to reduce the incident energy levels in the system. The ideal overcurrent device would not only accomplish this, but would also respond correctly to overloads, motor or transformer inrush currents, and high-level bolted faults. Current limiting devices (fuses and circuit breakers) can be useful in a strategy to provide the best protection from all fault conditions, as they both can respond quickly to clear arcing faults. They can also be sized or set to achieve selective coordination, which is the key to preventing nuisance tripping. Maximum protection and maximum selectivity among protective devices are sometimes mutually exclusive goals, and engineers must sometimes walk a fine line between selective coordination and optimal arc flash protection.

Current limiting fuses are known to operate very quickly (within one-half cycle or about 8 milliseconds), as long as they are operating in their current limiting region . There is a level of fault current for each current limiting fuse type and size, which could be called the “threshold of current limitation,” above which the fuse operates in the half-cycle current limiting mode. Bolted fault current levels will usually be sufficient to cause a properly applied current limiting fuse to operate in this region, except when the available fault current is unusually low. Relatively low fault-current levels can be seen during an arc flash event, when the resistance of the arc can reduce the arcing fault current below the fuse’s current-limiting threshold, thus increasing both its fault clearing time and the incident energy released.

A careful examination of the time-current curves for current limiting fuses will reveal that the current limiting threshold is lowest for the smaller ratings of fuses (400A and below), but increases as the fuse rating increases (up to 4000A). For example, a typical 600V Class L current limiting fuse will have current limiting threshold values as shown in the table below.

Fuse Size

Approximate Current-Limiting Threshold (kA)

Bolted Fault Current Required for
Arcing Fault at C-L Threshold (kA)

For each size fuse, the table also shows the minimum bolted fault current that will result in an arcing fault current that exceeds the current-limiting threshold (assuming a 480V system with 25 mm bus gap and an “in-box” arcing fault). For example, at a location protected by a 2000A Class L fuse, the available bolted fault current must exceed 52kA before an arcing fault would cause the fuse to operate within its current-limiting region. Significant reduction in incident energy levels may not be seen until fault currents are somewhat higher than even this level. The equations in IEEE Std. 1584-2002, IEEE Guide for Performing Arc-Flash Hazard Calculations , are applicable for bolted fault currents ranging from 700A-106kA. For the largest Class L fuses (4000A-6000A), the required bolted fault level to produce an arcing fault current sufficient to cause the fuse to operate in its current-limiting region is above this 106kA limit.

Electrical distribution system designers should take the current limiting characteristics of fuses into account when designing equipment that can respond properly to high level faults and yet still provide optimal protection from high impedance, lower magnitude arcing faults. Because fuses are static devices, i.e., there is no adjustment feature to alter the time current characteristics of the fuse, their application must be carefully considered by the designer. Particular attention should be paid to the threshold of current limitation for the fuse class and size and the prospective fault current levels expected at that point in the system.

Current limiting circuit breakers can also be very useful in responding to and mitigating arc flash events when properly applied. In most cases, modern circuit breakers will perform comparable to or better than current limiting fuses. From molded case circuit breakers to larger frame power circuit breakers, these devices can be selected to provide optimal arc flash incident energy control while still allowing selective coordination in the system. Circuit breaker trip units with the instantaneous function installed and activated can be particularly useful, since lowering the instantaneous setting can cause the breaker to clear a fault quickly even for relatively low fault current levels, significantly reducing the incident energy from an arcing fault. Smaller frame modern molded case circuit breakers typically outperform IEEE 1584 calculations for arc flash incident energy over a wide range of typically encountered conditions.

However, it should be noted that as with fuses, it is critical that system designers and operators correctly apply circuit breakers if arc flash protection is their goal. Blindly turning circuit breaker trip settings to their maximum level to prevent nuisance tripping—particularly for larger-frame breakers—can result in trip times even longer than fuses, providing little or no benefit where arc flash protection is concerned. Turning a breaker’s instantaneous protection off in order to improve selective coordination of devices can also lead to an increase in available arc flash incident energy levels. To ensure that optimal device coordination and arc flash protection is provided in a given facility, the overcurrent protective system should be reviewed by an engineer familiar with both device coordination and arc flash calculations.

Schneider Electric has been engaged for a number of years, along with other industry manufacturers, in helping to create industry standards that help engineers understand how to calculate the prospective incident energy that will be produced by an arcing fault. Extensive laboratory testing of arcing faults has been performed by these manufacturers as part of this effort. Based on data recorded during this testing, equations have been developed that allow for calculation of incident energy levels and flash protection boundary distances based on the characteristics of the power system under study. This work formed the basis for IEEE Std 1584Ô-2002, the IEEE Guide for Performing Arc Flash Hazard Calculations . IEEE 1584 calculation methods are widely recognized today as the most accurate methods available to calculate incident energy from arcing faults.

Schneider Electric manufactures many Square D fusible devices, such as fused bolted pressure switches, fused metal-enclosed load interrupter switchgear, fused disconnect (safety) switches, as well as circuit breakers of every conceivable type and size. In fact, the very first products manufactured in 1902 by the McBride Manufacturing Company, which subsequently became the Square D Company, were electrical fuses. Square D invented the first electrical safety switch within the first decade of the company’s existence.

As a result of this familiarity with the complete range of electrical protective devices available and the extensive knowledge of how they operate in every condition, from high level bolted faults to low level over-current conditions and every condition in between, Schneider Electric engineers are uniquely able to offer unbiased recommendations to electrical system designers to help them optimize their strategies for circuit protection and people protection, without compromise.

Given the current focus on arc flash safety, it is critical that electrical distribution system designers understand the characteristics of all available over-current protective devices and select those that will provide the best combination of protection for both the electrical system and the worker.





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