Arc Flash Standards: NFPA 70E
The NFPA 70E standard requires work to be performed under electrically safe working conditions for electrical conductors and circuit parts operating at 50 V or more under one of the following conditions:
- The employee is within the limited approach boundary. The limited approach boundary is defined as the approach limit for exposed electrical conductors or circuit parts where a shock hazards exists. For voltages from 50-750 V, it is given as 1 meter for fixed circuit parts or 3 meters for movable conductors. The standard contains a table for other voltages.
- The employee interacts with equipment where conductors or circuit parts are not exposed but an increased likelihood of injury from an exposure to an arc flash hazard exists.
When a task is performed in potentially unsafe electrical working conditions, a risk assessment considering electrical arc hazards is required. The risk assessment should identify the hazards, estimate the likelihood of occurrence and severity of electrical arcs and lastly determine what control measures should be taken. The estimate of the likelihood and severity must consider the design of the electrical equipment, the operating conditions and the adequacy of maintenance of the installation.
To determine if additional control measures are needed, table 130.5(C) can be used. This table gives a ‘yes’ or ‘no’ for likelihood of occurrence. It is based on the task and equipment condition. When PPE is used as a control measure the following items need to be determined:
- Appropriate safety-related work practices
- The arc flash boundary
- What PPE is to be used while working within the arc flash boundary
To find the arc flash boundary and determine what PPE is to be used, two methods are given: the arc flash PPE category method and the incident energy analysis method. Using both or parts of these methods is not permitted.
For the incident energy analysis method an energy exposure level needs to be estimated at the working distance between the energy source and the face and chest areas. Arc flash PPE needs to be selected based on this estimation. For body parts that are expected to be closer than the working distance, additional PPE must be used.
The IEEE Standard 1584 is typically used for the incident energy analysis method. The incident energy is determined based on the available fault current and protective device characteristics. It requires a detailed model of the electrical distribution system and engineering judgement to properly interpret the resulting calculations.
The PPE category method can be used to determine the arc flash boundary and arc flash PPE based on the task performed and equipment involved. The equipment needs to satisfy certain parameters for the results to be valid. Once determined, the resulting arc flash PPE Category numbered 1 through 4 gives the required PPE.
One noteworthy difficulty in using the arc flash PPE Category method is ensuring the equipment parameters are applicable. Some of the parameters are easy to determine such as voltage level and the class of equipment. However available fault current and fault clearing time require a detailed analysis of the system.
The fault clearing time must be estimated without knowing the size of the arcing fault. From the incident energy analysis method’s calculations, we know that arcing fault is smaller than a short-circuit fault but by how much depends on a variety of factors. NFPA 70E does not provide guidance on how to determine the fault clearing time or what safety margins to use.
If one of the parameters are not satisfied, the incident energy analysis method must be used instead.
