Check Your Arc-Flash Boundaries

Have you measured/checked your arc flash shock-protection boundaries lately?

By Vince Plank, CSP, Safety Management Group

Two common causes of electrical fatalities in the workplace are electrocution and arc flash/blast. To minimize injuries and fatalities, OSHA implemented electrical safety standards more than 40 years ago. In 2007, those rules were updated to provide consistency between OSHA (osha.org) and many state and local building codes, most of which adopted the updated National Fire Protection Association (NFPA, nfpa.org) and National Electrical Code provisions. 

OSHA’s General Industry and Construction standards regulate work “on and near” energized equipment and who is authorized to perform this work. The NFPA 70E “Standard for Electrical Safety in the Workplace” provides guidance for employers and employees on safe work practices, risk assessment, and other controls. While both require de-energized work when feasible, there are times when de-energization is not possible. According to NFPA 70E, work on energized systems by qualified persons is only allowed when the employer can show that de-energizing will introduce additional hazards or increased risk, such as interruption of life-support equipment.  

Defining arc flash

An arc flash is the light and heat produced as part of an arc fault, a type of electrical explosion or discharge that results from a low-impedance connection through air to ground or another voltage phase in an electrical system. The blast is the supersonic shockwave produced when the uncontrolled arc vaporizes the metal conductors. Both are part of the same arc fault and are often referred to as simply an arc flash. From a hazard standpoint they are often treated separately.

What causes the arc fault that leads an arc flash/blast? The most common cause is spark discharge that results from accidental contact with or dropping tools near energized equipment. Other causes include dust and impurities on insulating surfaces, corrosion of equipment that creates impurities on insulators, condensation or water-vapor drip that causes tracking on insulator surfaces, and failure of insulating materials.

Arc flash is incident energy, measured as calories/cm2 at the working distance. The 70E Standard states that it is the owner’s or host employer’s responsibility to perform an arc flash risk assessment/analysis. The assessment identifies arc-flash hazards, estimates the likelihood of occurrence and potential severity of injury or damage to health, and determines protective measures, including boundaries and personal protective equipment.

Often an engineering group is contracted to perform an incident-energy analysis. This method uses the available energy or fault current, the working distance, and the clearing times of the electrical distribution system over-current protective devices. The threshold, or arc flash boundary, required to provide protection is 1.2 cal./cm2. This amount of energy will cause a second-degree burn and likely require medical treatment, making it an OSHA recordable incident.

There are two shock-protection boundaries, limited and restricted. These are fixed and based on voltage and can be found in 70E table 130.4 (E)(a) for AC systems, and 130.4 (E)(b) for DC systems. This assessment leads to a report and electrical-equipment labels for switchgears, panelboards, motor-control centers, and other equipment. The 70E Standard requires an assessment update when changes are made to the electrical distribution and/or at least every five years. EP

Vince Plank is a Safety Advisor at Safety Management Group, Indianapolis, IN, (safetymanagementgroup.com). He is a Certified Safety Professional with almost 20 years of occupational safety and health experience in general industry and construction. Contact him at VincePlank@safetymanagementgroup.com.