What is the PPE's Purpose?
There appears to be a bit of debate on the need for thermal personal protective equipment on circuits with a driving voltage equal to or less than 240-volt AC. Before endeavoring to answer this question, perhaps we need to define what it is we are trying to accomplish with the PPE. For our purposes in this article, let’s use the following:
“Thermally-rated PPE shall be used to cover all parts of the human body exposed to an incident energy level more than that which would cause a curable burn on bare skin. The thermally-rated PPE shall limit body thermal absorption to a curable burn.”
If you wish to have a more objective, quantifiable, and less accurate statement, you could use “1.2 cal/cm2” in that statement.
What is the Incident Energy Potential of the Equipment?
Now that we have an objective for the PPE, we must decide the likelihood of a particular piece of equipment experiencing an event capable of producing incident energy greater than 1.2 cal/cm2 at a distance equal to the distance from the arc to the torso/face of a worker performing the task.
We will assume that distance to be 18 inches. Keep in mind, the breakdown voltage is much higher than the voltage required to sustain. Using Paschen’s law, the breakdown voltage for standard air varies from 20 to 75 kV per inch of thickness, depending on the variables.
Sixty hertz, line-ground and line-line arcs will self-extinguish 120 times per second. The voltage required for re-ignition will depend on how much gas is de-ionized while the arc is extinguished. It will vary between the breakdown voltage and the voltage required to sustain. In a L-L-L-(G), there are typically multiple arcs, and the plasma is continuously provided with ionizing energy, making it easier to sustain or re-ignite an arc.
Incident energy is defined as a heat flux integrated over time. Therefore, all of the conditions shown in a, b, or c (below) will produce the same results, 1.2 cal/cm2. Keep in mind, only a small portion of the arc energy is radiated into the external environment. Most is used to maintain the ionization of the plasma and the magnetic field of the arc channel.
(a) 1.2 cal/cm2 per sec for a period of 1 second
(b) 0.6 cal/cm2 per sec for a period of 2 seconds
(c) 2.4 cal/cm2 per sec for a period of .5 seconds
The likelihood of a piece of equipment with a single-phase voltage source of 120 volt, rms, 60 hz, having a line to ground/neutral arc creating 1.2 cal/cm2 at 18” is nearly impossible due to the inability of the circuit to sustain the arc for an adequate time and the low levels of available fault current.
The likelihood of a piece of equipment with a single-phase voltage source of 208 or 240 volt, rms, 60 hz, having a L-L-(G) arc creating 1.2 cal/cm2 at 18” is highly improbable due to the inability of the circuit to sustain the arc for an adequate time and the low levels of available fault current.
The likelihood of a piece of equipment with a three-phase voltage source of 208 volt, rms, 60 hz, having a L-L-L-(G) having arc(s) creating 1.2 cal/cm2 at 18” is highly improbable when the available fault current is less than 2000 amps due to high source impedance.