![]() ![]() Shock and arc flash are the electrical hazards addressed by the NFPA 70E electrical safety standard in the workplace and relevant OSHA regulations. Although it’s an advantage to evaluate systems while energized, the disadvantage is the potential exposure to the two primary electrical hazards – shock and arc flash. Some 70 years later, thermal scanning of electrical systems was capable of testing electrical systems and equipment in an energized, operating condition and detect hot spots and identify areas that require corrective action to prevent failures. Therefore, it often cannot detect conditions that can cause insulation to deteriorate or fail prematurely. ![]() The problem is that a megohmmeter is only effective in evaluating insulation condition in a deenergized state. ARCAD Arc Flash Analytic software automatically selects either IEEE empirical model or Lee method for arc flash calculations based on input system parameters.The first instrument invented for testing electrical systems was the Megger megohmmeter patented in 1905 for evaluating insulation, arguably the most important component in an electrical system. For cases where voltage is over 15 kV, or gap is outside the range of the model, the theoretically derived Lee method can be applied. The IEEE procedure is valid for voltages ranging from 208V volts to 15kV with gap ranges between 3 mm. The equations are used to calculate the incident energy and arc flash boundary. IEEE Standard 1584 details the procedure and needed equations for arc flash calculations. The boundary is defined by NFPA 70E as the distance at which the worker is exposed to 1.2 cal/cm 2. In addition, a qualified person must accompany unqualified persons. Persons crossing into the arc flash boundary are required to wear the appropriate Personal Protective Equipment (PPE) as determined by calculating methods contained in NFPA 70E. ![]() In some instances, calculations may decrease the boundary distance. NFPA 70E also allows the AFB to be calculated. NFPA 70E establishes the default arc flash boundary at 4 feet for low voltage (< 600V) systems where the total fault exposure is less than 5000 amperes-seconds (fault current in amperes multiplied by the upstream device clearing time in seconds). The AFB is a safe approach distance from energized equipment or parts. The arc flash boundary is based on voltage, the available fault current and the time it takes for the upstream protective device to operate and clear the fault. The arc flash hazard analysis should determine the arc flash boundary (AFB) and level of personal protective equipment (PPE) that the worker must wear. It is important to note that conductors and equipment are considered live when checking for voltage while putting equipment in a safe work condition. Until equipment is placed in a safe work condition (NFPA 70E 2000 Part II 2-1.1.3), it is considered live. Also, NFPA 70E 2000 requires that before a worker approaches exposed electric conductors or circuit parts that have not been placed in a safe work condition, a flash hazard assessment must be performed. The limited, restricted and prohibited approach boundaries are based on the voltage of the energized equipment. There are three shock approach boundaries (limited, restricted and prohibited) required to be observed in NFPA 70E 2000. NFPA 70E has developed requirements to reduce the risk of injury to workers due to shock and arc flash hazards.
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