How to perform arc flash risk assessment under NFPA 70e
NFPA code 70e helps firefighters calculate and maintain a safe boundary when dealing with flashovers
By Alex Bryant, FireRescue1 Contributor
Arc flash - also often called a flashover - is a type of strong electrical explosion that poses a serious threat to firefighters when dealing with powerful electrical components. An arc flash is caused by electrical current flowing through an air gap between conductors. The temperatures can reach in excess of 35,000°F (19,000°C), vaporizing the conductors and sending molten metal and plasma outward at significant force.
That’s why the NFPA 70E code has specific guidelines for how a firefighter should assess a potential arc flash situation. Below, we’ve outlined the general guidelines one should take when running an arc flash risk assessment. It’s important to understand that all potential arc flash hazards are required by law to be marked as such with clear labeling. Finding such a label will help a firefighter know that an arc flash risk assessment is required.
General safety requirements for NFPA 70e arc flash risk assessment
As part of general safety requirements, every fire department should implement risk assessment procedures that follow NFPA 70e guidelines. The specifics of these programs can vary slightly based on the resources available to each department, but include:
- Identifying the potential hazards and risk
- Establishing whether arc flash personal protective equipment (PPE) is required
- Establishing an appropriate arc flash boundary
- Determining how to control the risk
Identifying whether an arc flash risk is present.
NFPA 70e details many scenarios in which arc flash PPE is required. Any situation that requires arc flash PPE means that an arc flash risk is present to firefighters and workers in the area. The best rule is it is better to be safe than sorry. Subjecting yourself to a potential arc flash could result in serious and even life-threatening injury. Scenarios which always require arc flash PPE include:
- Improperly installed equipment
- Improperly maintained equipment
- Equipment doors are open or unsecured
- Equipment covers are off or unsecured
- Evidence of impending failure of equipment
The best rule is it is better to be safe than sorry. Subjecting yourself to a potential arc flash could result in serious and even life-threatening injury. If you are unsure, many risk assessment procedures will outline the procedure for getting a second opinion to confirm arc flash risk and appropriate actions. As an extra precaution, try to schedule any risk assessment and maintenance during a period of time when the circuit can be shut down.
Determining your appropriate PPE category for arc flash assessment
Arc flash PPE falls into one of four categories based on the power potential of an arc flash hazard.
Arc Flash PPE Category 1
- Panelboards or other equipment rated 240V and below
- Batteries, dc switchboards and other dc supply sources with a voltage between 100V and 250V and a short-circuit current less than 4 kA
- Batteries, dc switchboards and other dc supply sources with a voltage between 250V and 600V and a short-circuit current less than 1.5 kA
Arc Flash PPE Category 2
- Panelboards or other equipment rated between 241V and 600V
- 600-V class motor control centers with a maximum of 65kA short-circuit current available
- Other 600-V class equipment
- Batteries, dc switchboards and other dc supply sources with a voltage between 100V and 250V and a short-circuit current between 4kA and 7kA
- Storage batteries, dc switchboards, and other dc supply sources with a voltage between 250V and 600V a short-circuit current between 1.5kA and 3kA
Arc Flash PPE Category 3
- Storage batteries, dc switchboards, and other dc supply sources with a voltage between 250V and 600V a short-circuit current between 3kA and 7kA
- Batteries, dc switchboards and other dc supply sources with a voltage between 100V and 250V and a short-circuit current between 7kA and 15kA
Arc Flash PPE Category 4
- 600-V class motor control centers with a maximum of 42kA short-circuit current available
- 600-V class switchgear (with power circuit breakers or fused switches) and 600-V class switchboards
- NEMA E2 (fused contactor) motor starters, 2.3 kV through 7.2 kV
- Metal-clad switchgear, 1 kV through 15 kV
- Arc-resistant switchgear Type 1 or 2 with doors open
- Storage batteries, dc switchboards, and other dc supply sources with a voltage between 250V and 600V a short-circuit current between 7kA and 10kA
NFPA 70e Arc Flash PPE standards
After you understand which PPE category your arc flash hazard falls into, NFPA 70e outlines the specific items needed for each category.
Arc flash PPE category 1 equipment
- Arc-rated long-sleeve shirt and pants, or arc-rated coverall
- Arc-rated face shield or arc flash suit hood
- Arc-rated jacket, parka, rainwear, or hard hat liner
- Hard hat
- Safety glasses or safety goggles
- Hearing protection
- Heavy duty leather gloves
- Leather footwear
Arc flash PPE category 2 equipment
- All category 1 equipment
- Arc-rated balaclava
Arc flash PPE category 3 and 4 equipment
- All category 1 equipment
- Arc-rated flash suit jacket
- Arc-rated flash suit pants
- Arc-rated gloves
While there is some crossover between categories, all arc-rated equipment is rated for dependent categories. When selecting your equipment, make sure it’s rated for the PPE category of your specific arc flash hazard.
Calculating your arc flash boundary
After determining that an arc flash risk is present and gathering the appropriate PPE, the next step is to calculate the arc flash boundary according to NFPA code 70e. This can be accomplished through four different methods, each with their own limitations:
- Ralph Lee Calculation Method - for an arc in open air. Measurements become conservative over 600 volts and more conservative as voltage increases
- Doughty Neal Paper Method - for three-phase arc on systems 600 volts and below
- IEEE1584 Calculation Method - for 208 volt to 15 kilovolt systems
- Direct-Current Incident Energy Method - for dc systems rated up to 1000 volts dc
NFPA 70e goes into further detail on the various equations involved for each of the four methods. These equations lay out exactly how to determine the potential power an arc flash could generate, and exactly far away is a safe operating distance. Calculating this distance can make sure that all workers can stay safe. Even if the circuit is shut off, it is important to calculate an appropriate arc flash boundary as an extra layer of precaution.
NFPA 70e hierarchy for controlling arc flash risk
The most important part of the arc flash risk assessment, after ensuring the firefighter’s personal safety is eliminating or mitigating the risk going forward.NFPA 70e details a hierarchy of methods for controlling risk:
- Eliminate the risk
- Substitute an alternate method of operation
- Modify engineering controls
- Increasing awareness of the risk
- Administrative controls to mitigate risk
- Requiring personal protective equipment
The options at the top of the hierarchy are designed to fix the hazard. If those aren’t available, the top priority becomes informing workers of the potential dangers at the workplace, making sure they keep an appropriate distance or wear appropriate PPE if they can’t avoid the area.
Arc flash is an extremely serious, and potentially life-threatening hazard that takes careful consideration and precise measurements to analyze and resolve. By using NFPA code 70e, firefighters can understand the risks involved, calculate and maintain a safe boundary and determine the appropriate PPE for dealing with the risk.
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