Calculating DC Arc-Flash Hazards in PV Systems

In large commercial and utility-scale PV systems, hundreds or thousands of PV modules are connected in series and parallel. The combined dc output power potential at aggregation points, including combiner boxes, recombiner boxes or inverter-input combiner compartments, is often higher than 1 MWdc. In addition, dc voltages may approach 1,000 V and current levels may exceed 1 kA. While a system is energized, failures or human error during commissioning, testing and troubleshooting can cause arc-flash events that severely injure or even kill technicians.

In this article, we propose a method to calculate the incident energy on the dc side of a PV system. You can use the results to identify and label arc-flash hazards and to select personal protective equipment (PPE). We also discuss practical considerations for protecting workers from dc arc-flash hazards.

Arc-Flash Events

An arc-flash event is the release of energy caused by a line-to-line or line-to-ground fault. During an arcing fault, electrical energy is transformed into other forms of energy such as heat and pressure. Arc flash refers to the intense light created by the electrical arc. An arc blast is the explosive release of energy—characterized by intense sound pressure and vaporized metal—that normally accompanies an arc flash.

The risks. Common testing and commissioning procedures require technicians to clamp or measure energized circuits at multiple locations on the dc side of the system, including combiner and recombiner boxes, disconnect switches and inverters, putting them at risk for arc-flash events. DC arc flashes can expose workers to temperatures as high as 35,000°F. Due to the rapid release of large amounts of energy, individuals who have had the misfortune of being close to an arc-flash event have lost their lives or suffered severe physical trauma including hearing loss, blindness and high-degree burns. According to the Informative Annex K of the 2012 NFPA 70E: Standard for Electrical Safety in the Workplace (NFPA 70E), “the majority of hospital admissions due to electrical accidents are from arc-flash burns, not from shocks, and arc flashes can and do kill at distances of 3 m (10 ft).” Informative Annex K also notes that arc blasts can expel material and molten metal “away from the arc at speeds exceeding 1,600 km/hr, fast enough for shrapnel to completely penetrate the human body.”

Possible causes. Several conditions can cause an arc-flash event, including accidental contact within or across an electrical system made by a tool or individual; buildup of conductive dust; corrosion; improper use of equipment; or loose splices or terminations. Under any of these conditions, a line-to-line or line-to-ground fault can result in a dangerous arc-flash event.

Understanding the Hazard

NEC Section 110.16 requires that electrical equipment “likely to require examination, adjustment, servicing, or maintenance while energized shall be field marked to warn qualified persons of potential electric arc-flash hazards.” Informational Note No. 1 refers users to NFPA 70E for assistance in “determining severity of potential exposure, planning safe work practices, and selecting personal protective equipment.”

NFPA 70E Article 100 defines arc-flash hazard as “a dangerous condition associated with the possible release of energy caused by an electric arc.” Section 110.3(F) requires that “an electrical safety program shall include a hazard identification and a risk assessment procedure to be used before work is started within the limited approach boundary or within the arc-flash boundary of energized electrical conductors and circuit parts operating at 50 V or more or where an electrical hazard exists.”

Arc-flash hazard analysis. NFPA 70E Section 130.3(B)(1) requires that an arc-flash hazard analysis be conducted prior to exposing workers to energized electrical parts or circuits operating at or above 50 V if workers cannot be placed into an electrically safe working condition. Section 130.5 states that an arc-flash hazard analysis must determine “the arc-flash boundary, the incident energy at the working distance and the personal protective equipment (PPE) that people within the arc-flash boundary shall use.”

Incident energy is the energy measured on a surface at a specified distance from the arc-flash location. In particular, it is the heat energy per unit area that an individual could be exposed to. You calculate incident energy values for a specified distance from the blast. IEEE Standard 1584 lists a standard distance of 455 mm (45.5 cm or approximately 18 inches) for these calculations. Per NFPA 70E, the arc-flash boundary for systems operating at or above 50 V is the distance at which the incident energy equals 5 J/cm2 (1.2 cal/cm2). Exposure to incident energy equal to or greater than this value can cause second-degree burns, and a person entering the arc-flash boundary needs to wear appropriate PPE.

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