An unfortunate truth about Arc Flash incidents is that they can occur regardless of training. Without proper protective measures, workers are left exposed to dangerous conditions and have to suffer the consequences. Every year, more than 2,000 people are admitted to burn centers with severe arc-flash burns.
While arc-flash cannot be completely eliminated, there are steps we can take to prevent it. The single most important measure is placing the equipment in a "Safe Working Conditions" If it is more dangerous to turn the equipment off, workers should wear appropriate Personal Protective Equipment (PPE).
Arc flash PPE is required by NFPA 70E (2018) as per 130.7:
"Employees exposed to electrical hazards when the risk associated with that hazard is not adequately reduced by the applicable electrical installation requirements shall be provided with, and shall use, protective equipment that is designed and constructed for the specific part of the body to be protected and for the work to be performed."
NFPA 70E (2018) has identified various Flame Resistant Hazardous Risk Category levels. These are broken down into a total of four categories, ranging in severity from 1 to 4.
For example, working with low-voltage wiring with minimal likelihood of an arc flash occurring would not require extensive PPE. On the other hand, working on a high-voltage machine involves a very serious risk of an arc flash, so a category 4 rated PPE is likely going to be needed in this case.
Each category includes the minimum Arc Rating value for the required PPE, which represents the amount of incident energy (in cal/cm^2, or calories of heat energy per square centimeter) on a fabric or material that carries a 50% probability of causing a second degree skin burn injury.
In other words, you have a 50% chance of being burned if exposed to an electric arc with the same number of calories of heat. The arc rating indicates the level of protection provided by Flame Resistant (FR) clothing measured in cal/cm^2. The higher the value the greater the protection.
A brief overview of each of these four PPE Categories is explained below:
The first category represents the lowest level of Arc Rated PPE, requiring only a single layer of arc-rated PPE.
Workers need the following clothing:
The second category can also be fulfilled with a single layer of Arc Rated PPE.
In PPE Category 2, workers need the following clothing:
PPE Category 3 and 4 require additional layers of PPE. Arc flash suit hoods are required and rubber insulating gloves and leather protectors, or arc-rated gloves are required.
For PPE Category 3, workers need the following clothing:
The final PPE Category requires AR clothing with a minimum rating of 40 cal/cm^2:
The categories of PPE as described in NFPA 70E are summarized as follows:
|HAZARD RISK CATEGORY||
(Typical number of clothing Layers given in parentheses)
|Required Minimum Arc Rating of PPE (cal/Cm2)|
|1||FR shirt and FR pants or FR coveralls (1 layer)||4|
|2||Cotton underwear plus FR shirt and FR pants (1 or 2 layers)||8|
|3||Cotton underwear plus FR shirt and FR pants plus FR coveralls, or Cotton underwear plus two FR coveralls (2 or 3 layers)||25|
|4||Cotton underwear plus FR shirt and FR pants plus multilayer flash suit (3 or more layers)||40|
About The Author
Abdur Rehman is a professional electrical engineer with more than eight years of experience working with equipment from 208V to 115kV in both the Utility and Industrial & Commercial space. He has a particular focus on Power Systems Protection & Engineering Studies.
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