Q&A: How Do You Evaluate Arc Flash Risk in Your Facilities?

1. From your experience, what are the most common causes of arc events in industrial pumping systems?

A hazardous arc flash condition may arise from various causes, and often occurs during maintenance or troubleshooting. Ideally, work on an electrical system can be 100 percent safe only if that system is totally de-energized while work is being performed. In many instances, such as in the process industries where facilities are required to operate continuously, a totally de-energized system may not always be possible. The electrical system may need to be energized, and the process may need to remain running while maintenance or troubleshooting tasks are performed. The steps involved in confirming that an electrical circuit is indeed de-energized may also put workers at risk. Employees who work in the electrical inspection industry find that they may be exposed to shock and arc flash hazards while conducting the necessary inspections of electrical systems. Other maintenance tasks are performed at times when the facility or its processes are not fully operational. Although the power system is energized, some of the contributing motor loads may be shut down. Therefore, during maintenance operations, when the propensity for arc flash conditions is high, the available fault current may be significantly lower than the calculated maximum. In many cases, lower fault currents can result in higher arc flash energy, since the overcurrent protective device may not clear the fault as quickly.

2. What steps can I take to evaluate arc flash risk in my facilities?

The most important first step is to complete an arc flash hazard analysis. During this facility evaluation, power systems engineers perform a short circuit, selective coordination and arc flash hazard analysis that quantifies the release of thermal energy associated with a potential arc flash event. They will then describe safety recommendations to protect workers and equipment based on the analysis. The results of the arc flash calculations are based on the calculated values of fault current magnitudes found in the short circuit study and the associated clearing times of overcurrent protection devices as determined by the coordination study. The purpose of this analysis is to determine the incident energy (measured in calories/centimeter-squared) potentially present during an arc flash event. The magnitude of the incident energy is calculated on the basis of the available fault current, the clearing time of associated system protection and the physical parameters of the system location. Associated with this calculation is the determination of a working distance within which the incident energy level does not exceed 1.2 cal/cm². Appropriate personal protective equipment (PPE) should be used when working on or near energized equipment above the 1.2 cal/cm² threshold. The results of the working distance and incident energy calculations should be displayed in labels on equipment enclosures to inform and direct facility personnel with respect to the potential arc flash hazard and appropriate PPE.

3. What are the important arc flash standards to know?

A number of established guidelines for arc flash prevention are intended to help prevent injury and equipment damage. The overall goal of the arc flash hazard analysis performed by power systems engineers is to help facilities meet or exceed these standards. National Fire Protection Association (NFPA) 70E-2018 requires facility personnel to wear PPE when performing various tasks in locations susceptible to potential arc flash hazards. For instance, if the arc flash hazard analysis determines that the incident energy at a given electrical panel is 6.8 cals/cm², then the worker might wear an 8.0 cals/cm² suit before performing energized work. The heat energy hazard is addressed by the PPE. The NFPA 70E also addresses the risk of injury, both in terms of likelihood and severity. The risk is work task based. For instance, operating a circuit breaker from behind a closed door would pose a lower risk than using a test instrument to measure voltage at a circuit breaker’s terminals with the door open. In 2002, the Institute of Electrical and Electronics Engineers (IEEE) reported the results of extensive testing during arc flash events and created a model that is used to calculate incident energy based on electrical system parameters. IEEE Standard 1584-2002 describes the test procedures and provides a calculator that develops an accurate means of determining a safe arc flash boundary, working distance and incident energy level. The basis for this method is experimental data recorded from simulated arcs corresponding to bolted, three-phase fault currents measured at the terminals of an experimental enclosure. IEEE Standard C37.20.7-2017 is a guide for testing switchgear rated up to 52 kilovolts (kV) for internal arcing faults. Equipment tested to IEEE Std. C37.20.7 is engineered to safeguard against the impact of abnormal internal pressure or arc flash, as long as all doors and access areas are closed and appropriately secured.

4. I’ve seen the term “arc resistant” used when discussing motor control centers, drives and switchgear. What does it mean?

Arc resistant switchgear protects operating and maintenance personnel from dangerous arcing faults by redirecting or channeling the arc energy out of the top of the switchgear. This equipment will also incorporate sealed joints, top-mounted pressure relief vents, reinforced hinges or latches and “through-the-door racking” to minimize exposure to harmful gases and significantly reduce the risk of injury to facility personnel in the event of an arc flash event. IEEE C37.20.7 defines switchgear arc resistance in two basic categories:

• Type 1—switchgear with arc resistant designs or features at the freely accessible front of the equipment only
• Type 2—switchgear with arc resistant designs or features at the freely accessible exterior (front, back and sides) of the equipment only

A suffix may be added to either of these two types to further define the type of protection provided:

• Suffix A is the base rating. It indicates the equipment has met the requirements of the Type assigned with no additional performance features.
• Suffix B is applicable to any type of equipment containing a compartment designated as low-voltage control or instrumentation. The suffix B designation indicates the equipment meets the requirements of Type 1 or Type 2 testing for that design with the indicated control compartment door or cover open.
• Suffix C is designated for equipment where isolation from the effects of an internal arcing fault is desired between all adjacent compartments within a switchgear assembly.
• Suffix D is applicable to equipment specifically designed for installation where there is restricted access to specific sides or surfaces created by the installation. Equipment with surfaces located against a wall or equipment mounted at an inaccessible height such that exhaust gases are not released into an area designated by the Accessibility Type 2 requirements are typical examples where suffix D may be applied.

For arc resistant motor control centers, an isolation switch design typically disconnects the starter from the low-voltage or medium-voltage source in the rear arc chamber, not in the front starter compartment—improving protection for workers in the event of an arc flash. And when it comes to medium-voltage drives systems, arc resistant enclosures provide the rigidity needed to withstand the forces of arc events up to 50 kiloamps (kA) and provide worker protection from the front, sides and rear of the enclosure, even with the control doors open. Additionally, enclosure controls help direct arc blast energy through safe exhaust locations while an embedded exhaust cooling system significantly reduces the temperature of exhaust gas.

5. What other solutions are available today to help enhance arc flash safety?

The NFPA70 or National Electrical Code (NEC) includes a list of solutions for addressing arc flash energy reduction in circuits that include low-voltage fuses and also circuit breakers rated 1,200 amperes and higher. With the publication of the 2017 NEC, Article 240.87, a list of six acceptable means (plus a seventh loosely defined as “an approved equivalent means”) have been defined for circuit breaker applications. These defined methods allow the use of various technologies focused on improving breaker clearing times as a means to reduce incident energy and the impact of arc flash events. The new list of allowable means in the 2017 NEC is as follows:

1. zone-selective interlocking
2. differential relaying
3. energy-reducing maintenance switching with local status indicator
4. energy-reducing active arc flash mitigation system
5. an instantaneous trip setting that is less than the available arcing current
6. an instantaneous override that is less than the available arcing current
7. an approved equivalent means

Most of these defined methods to reduce arc flash energy are passive in nature. These include differential relaying, zone-selective interlocking or instantaneous/maintenance settings to reduce the sensing time and thus the total clearing time of the circuit breaker tripping system. Although these are effective solutions, low-voltage power circuit breakers typically operate within a four to five cycle clearing time, which is about 65 to 80 milliseconds. The peak pressure and heat energy from an arc flash event occur in the first 10 milliseconds, so there is a limitation to significantly reduce the energy from an arc flash using these passive reduction methods. One new arc quenching solution has also been released this year for switchgear. This technology falls in the category of active arc flash mitigation systems as defined in NEC Article 240.87. In passive systems of the past, arc flash mitigation has been addressed from two angles: reducing the incident energy through faster clearing times or containing and redirecting the incident energy with arc resistant switchgear. Typical passive methods for reducing the incident energy, such as energy reducing maintenance switches, zone selective interlocking and bus differential relays can improve clearing times but oftentimes this is not fast enough to provide adequate protection. Traditional arc resistant switchgear, which consists of a heavily reinforced enclosure to contain the arc energy as well as ducts and plenums to direct the arc gases out of the room, can provide an increased level of safety. But equipment could also suffer catastrophic internal damage following an arc flash event, which can lead to extended downtime and high equipment replacement costs. Furthermore, traditional arc resistant equipment does not provide arc resistant protection when enclosure doors are open or panels are removed. New arc quenching switchgear, on the other hand, can quench an arc fault in as little as 4 milliseconds or about one-quarter of a cycle. This is approximately 10 times faster than the aforementioned technologies, which rely solely on a power circuit breaker to clear the fault. And arc quenching switchgear provides arc resistant protection compliant with IEEE Std. C37-20.7 even when doors are open or panels are removed, all while greatly reducing or completely eliminating damage to the switchgear.

6. Beyond a safer working environment, what ROI can I expect to achieve from modern arc flash safety solutions?

Enhancing personnel and equipment safety should always be the top priority, but modern arc flash safety solutions can also reduce the time and cost of recovering from an arc event. Typical methods for reducing the incident energy can take upwards of four to five cycles to clear the fault. Often, this is not fast enough to provide adequate protection. Arc flash events can damage circuit breakers, compartments, structures and even the entire lineup leading to costly downtime. Traditional arc resistant switchgear could also suffer catastrophic internal damage in the event of an arc fault event, which can lead to extended downtime and high equipment replacement costs. In comparison, arc quenching switchgear minimizes or eliminates damage from an arc flash, reducing downtime from weeks or months to minutes. Service can be quickly restored by returning the switchgear to normal operating condition and replacing a draw-out device. And when it comes to protecting equipment, new arc quenching technology can reduce peak stress on upstream equipment by at least 44 percent compared to the stresses developed from a similar prospective bolted fault current. In process environments where uptime is everything, investments such as these can quickly deliver return by not only helping enhance personnel safety, but also by significantly reducing equipment damage and downtime following an arc event.