Arc Energy Reduction Considerations
Protecting workers from the hazards of electric shock has been understood for decades. Recently though, more attention has been given to the role electrical equipment can play in minimizing arc flash hazards. The energy exposure of an arc flash incident can be significantly reduced when attention is given to the type of equipment specified and where it is installed in an electrical system.
The 2014 edition of the National Electrical Code® (NEC®) addresses arc energy reduction directly in Section 240.87. Previously, the 2011 edition of the NEC® required arc energy reduction whenever a circuit breaker did not have an instantaneous trip function. This would apply to a circuit breaker with no instantaneous trip function at all, but questions lingered about other applications.
This was cleared up in the 2014 edition of the NEC®. The updated code requires arc energy reduction where the highest continuous current trip setting for which the actual overcurrent device installed in a circuit breaker is rated or can be adjusted is 1200 A or higher (see Figure 1 at right). This means that even though an electronic trip circuit breaker with a 1200 A sensor has its current rating switch set to, for example, 0.5 (600 A), it will still need an arc energy reduction means (see Figure 2 below). If the overcurrent device in the circuit breaker meets this criteria, then documentation and a method to reduce the clearing time must be provided.
APPROACHES FOR LOWERING CLEARING TIME
To see the connection between clearing time and arc energy reduction, we need to understand incident energy. Incident energy, as defined in NFPA 70E, Standard for Electrical Safety in the Workplace®, is “the amount of energy impressed on a surface, a certain distance from the source, generated during an electrical arc event. One of the units used to measure incident energy is calories per centimeter squared (cal/cm2).” A method used to calculate incident energy may be found in IEEE 1584, Guide for Performing Arc Flash Hazard Calculations.
Incident energy is a function of current and time. If the arcing time is reduced, then the incident energy will be reduced. Clearing time refers to the length of time necessary for an overcurrent protective device to completely extinguish the arc.
There are five clearing time reduction means:
1. Zone Selective Interlocking
Zone selective interlocking (ZSI) preserves the desired coordination between main, tie, and feeder protective devices, and it allows fast tripping for faults within the protected zone – the conductors between the interlocked devices. This is accomplished through wired connections between circuit breaker electronic trip units, ground fault relays, or protective relays. If a feeder device detects an overcurrent condition, it sends a restraining signal to the upstream device(s). The upstream device(s) then follows its normal time-current characteristic and serves as a backup. However, if the upstream device(s) detects an overcurrent condition above its short time – or ground fault – pickup setting but the downstream device(s) do not (e.g. due to a main bus fault), then the main circuit breaker will not receive a restraint signal and trip with no intentional time delay. In this way, ZSI offers the “best of both worlds” – fast clearing of fault currents without sacrificing coordination. ZSI is also available on both low- and medium-voltage equipment and can be applied for both phase faults and ground fault protection.
2. Differential Relaying
The concept of this protection method (Figure 3 at right) is that current flowing into the protected zone must equal the current flowing out of the zone. If these two currents are not equal, then a fault must exist within the zone, causing the relay to operate. Differential relaying uses current transformers located on the line and load sides of the protected equipment, a fast-acting relay, and a shunt trip on a circuit breaker or switch. They are very sensitive to faults inside their zone of protection but immune to load inrushes or pass-through faults.
Differential relaying protection is more common at medium voltage versus low voltage due to the increased space requirements for relay class current transformers, differential protective relays, and additional wiring complexity. The costs associated with low-voltage differential relaying protection are also substantial when compared to the cost of the base equipment. Saturation of the current transformers at high fault currents is also a concern.
3. Energy-Reducing Maintenance Switching With Local Status Indicator
This switch (Figure 4 below) allows a worker to set a circuit breaker electronic trip unit or protective relay to operate faster should an arc fault occur while the worker is working within an arc flash boundary, as defined in NFPA 70E. After completing the task, the worker would set the maintenance switch back to its normal setting. The switch temporarily reduces the pickup and/or time delay settings, or it causes an alternate setting to become effective. It may even enable a faster acting instantaneous trip function. To be effective, the “maintenance mode” settings must result in a faster tripping time based on the actual prospective fault current levels at the location being protected.
4. Energy-Reducing Active Arc Flash Mitigation System
The arcing duration can be reduced with this system by causing the upstream circuit breaker to open more rapidly or by creating a low impedance current path. The former approach may utilize relays, which sense light, current, and/ or other parameters. The latter is most commonly achieved through a “crow bar” switch. The closing of this switch, located within a controlled compartment, causes the arc fault current to transfer to a new current path while an upstream circuit breaker clears the fault. The system works without compromising existing selective coordination in the electrical distribution system.
5. An Approved Equivalent Means
Approved equivalent means was included in recognition that technology to reduce arc flash hazards will continue to evolve. Since ZSI, differential relaying, and energy-reducing maintenance switches all cause a circuit breaker or switch to open instantaneously should an arc flash occur, it seems obvious that the instantaneous trip function on a circuit breaker should be considered an approved equivalent means. An instantaneous trip function, whether it is field adjustable or a nonadjustable override type, can reduce the arc energy if its pickup point is set below the prospective arc fault current. (Note: Text proposed to be added to the 2017 edition of the NEC® will list field adjustable and nonadjustable override type instantaneous trip functions as approved means.)
GOING BEYOND CODE COMPLIANCE FOR IMPROVED SAFETY
While well intended, it is quite easy to achieve the requirements outlined in Section 240.87 without meeting the spirit of the requirement, namely an improvement in worker safety or even a reduction in arc energy. Simply supplying one of the listed means to reduce the clearing time in the event of an arc fault may or may not improve safety. Factor in the following:
- The sensing function, whatever it is, needs to be set to activate (pickup) at a point below the prospective arcing fault current.
- Each one of the means listed establishes a zone of protection, meaning the clearing time will be reduced for faults within the zone but not for those outside it. For example, the line side of a main circuit breaker would be outside the protected zone for any means provided within the equipment where the main circuit breaker is located.
- The means may not reduce the level of incident energy listed on the arc flash label to be applied to the equipment per NFPA 70E and thus may not reduce the type of personal protective equipment (PPE) that must be worn.
- The means may not reduce the incident energy enough to allow workers to use a lower level of PPE.
Consider the following measures to achieve enhanced safety:
- Arc flash study: Conduct a study as described in IEEE 1584 and involving the determination of the level of the prospective arcing fault current.
- Energy-reducing maintenance switching and instantaneous tripping: Ensure trip levels are below the prospective arcing fault current level. Conduct a coordination study to ensure that unwanted tripping will not occur due to high inrush currents that may occur when large motors are started or transformers are energized.
- Consider the protected zone: Each of the energy reduction means listed in 240.87 effectively establishes a protected zone. However, the work to be done may not be in the protected zone or may expose the worker to energized conductors outside the protected zone, such as the line side terminals on a main circuit breaker.
- Arc flash labeling: When determining the proper labeling of equipment, all potential energized conductor exposures must be considered, not just those within the protected zone.
- Safety by design: Consider techniques such as compartmentalization, protective barriers, insulated bus, and remote mounting of main disconnects.
Conducting maintenance on de-energized equipment is without a doubt the best arc flash mitigation action that can be taken. Following the NFPA 70E guidelines to protect workers as well as having trained personnel, labeled equipment, and a well-established maintenance process are essential elements of an effective and comprehensive arc flash mitigation solution.
Ed Larsen is an industry standards manager for Schneider Electric USA and is based in Cedar Rapids, IA. He is responsible for overcurrent protective device and motor control product standards and holds a bachelor’s degree and master’s degree from the Milwaukee School of Engineering. He has been an employee of Square D/Schneider Electric for 41 years as an application engineer, marketing manager, product manager, and engineering manager for control products and circuit breakers. Ed has authored articles and papers and frequently speaks on various product application and code compliance topics. He is a member of numerous industry technical committees and NEC® Code Making Panels 2 and 11.