When it comes to arc flash safety, nothing may be more important to qualified electrical workers than their ability to interpret the information found on Arc Flash Warning labels.
It is employer’s responsibility to provide employees with the information and training they need to safely perform electrical tasks. Part of this responsibility is ensuring that arc flash hazard equipment warning labels are in place, and that employees have the information necessary to select the correct level of PPE, and set-up appropriate arc flash boundaries.
The warning labels are required to display specific information such as the nominal voltage of the equipment, the arc flash boundary distance, and the available incident energy at the prescribed working distance.
One of the more easily confused concepts is “working distance”. Working distance is described in IEEE 1584, a guide commonly used for performing arc flash incident energy and boundary calculations. IEEE 1584 defines working distance as “the dimension between the possible arc point and the head and body of the worker positioned in place to perform the assigned task”.
To be clear on how to properly interpret the working distance, let’s take a closer look at the IEEE 1584 definition
- The possible arc point could vary based on the equipment and work being performed, but is generally considered to be the point at which the MCC bucket or breaker attaches to the bus bars at, or near, the back of the equipment.
- The position of the worker in place to perform the task is the distance from the electrical panel needed to perform the necessary work task. This distance includes the arms extended a comfortable length to use hand tools or test equipment.
Why is it important to understand the working distance?
Let’s say we are looking at the pictured arc flash equipment warning label which has an incident energy of 6.8 cal/cm2 @ 18 inches. This simply means that if a person were standing in front of the panel (normal position to perform work), and an arc flash were to occur, the person would receive 6.8 cal/cm2 of thermal energy on their head and body.
Here is where the problem comes in.
As we get closer to the point of the arc the incident energy increases by the square of the distance. If we don’t plan for this the PPE our electrical worker is wearing could be insufficient. For instance, if we decrease the distance to the potential arc source by half, the incident energy increases four times the original level. (Example: For the same arc hazard, 6.8 cal/cm2 @ 18 inches becomes 27.2 cal/cm2 @ 9 inches. A significant difference in the PPE required.)
For many years, I worked in a nuclear power plant as an electrical maintenance supervisor. This plant has large 15 kV breaker cubicles. To troubleshoot a control switch, or the male stab of a secondary disconnect we had to get inside the cubicle to access it. In this case, we had to recalculate the available incident energy because the working distance was now considerably less than what was printed on the door mounted arc flash warning label. Once we determined this, we could change the PPE requirements accordingly.
The working distance printed on an arc flash warning label was determined by the arc flash incident energy analysis performed by an electrical engineer. They selected the working distance based on the voltage class of the equipment.