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: