A precise calculation

First, let’s examine the role current plays in arc flash hazards. Most of us have had experience with labels affixed to equipment after completing an arc flash study. These labels provide critical information on arc flash such as incident energy and the arc flash boundary. 

However, between these two numbers, it is the incident energy that really gives us an idea of how bad the arc flash-related injury might be. The arc flash boundary is just another function of incident energy, since it is the distance from the source of the arc flash where incident energy is equal to 1.2 cal/cm².

Many variables must be known to determine the incident energy. The basic values that make up incident energy are arcing current and how long it will take to interrupt the fault, either from an upstream overcurrent protective device or the arc burning away enough material that it can no longer jump the gap. Here we can see that electrical current flowing through an arcing fault is the main culprit again, just like in an electrical shock.

So, where does voltage fit into all of this? Going back to the basics of electricity, remember Ohm’s Law—essentially, the amount of electrical current that flows in a circuit with a given resistance is directly proportional to the applied voltage. This means that while current is the main factor in estimating the incident energy, how much current is flowing in an arcing fault will be determined by the voltage applied when all other parameters are kept constant. 

Voltage also plays a role in the ability of an arc flash to become self-sustaining. If the voltage is not high enough to jump the gap between conductors, an arc is less likely to evolve into an arc flash. However, keep in mind that there have been many documented arc flash events that have started at 120V. 

So, it would seem that voltage is just as big of a culprit as current since it determines how much current will flow through a given resistance. 

Understanding the current

However, we have not addressed resistance yet. A key principle of stepping voltage up and down is that power in equals power out. Therefore, to deliver the same power at higher voltages, the current goes down. Since conductor sizes are based on circuit current, the result of lower current values means smaller wire sizes. Smaller wire sizes mean more resistance and lower fault current values. 

Another factor in arcing fault resistance is that equipment built for higher voltages incorporates larger spaces between conductors and often has features meant to increase resistance in an arc flash. For instance, the spacing between phases in a 480V switchgear is typically far less than what is found in 15-kilovolt (kV) switchgear. The spacing in a 15-kV gear is far less than bus spacing in a 245-kV switchyard.

A surprising find

So, while higher-voltage systems tend to have much greater danger from a shock standpoint, because the body’s resistance is the same at 120V as it is at 13.3 kV, the labels for these higher-voltage systems tend to have lower incident energy levels due to the design of the equipment and system having higher resistance values in an arcing fault, meaning less arcing current. In a facility with a mix of voltage systems from 120V up to 34.5 kV, it is common to see the highest incident energy values on the 480V three-phase equipment as the fault currents are higher, and the arcing fault resistance is lower. 

So, while voltage plays a part, it is really the current and time that increases the severity of injury during an arc flash. 

Until next time, stay safe and always remember to test before you touch!