Despite the great strides that have been made over the years to get workers into safer clothing, researchers still want to know what level of thermal protection is safe enough.
The last 15 years have seen tremendous progress in protecting workers against the heat energy associated with arc fl ash. One major area of improvement has been the steps taken to get workers into safer clothing. The arc rating system developed by ASTM and the development of the predictive equations identifi ed in NFPA70E and IEEE1584 have been instrumental in this effort.
Arc fl ash testing has been at the center of these developments. The arc thermal performance value (ATPV) of electrical personal protective equipment (PPE) relies on arc fl ash tests performed in a high power test lab. The IEEE 1584 equations were developed empirically from arc fl ash tests performed in North American test labs from the late 1990s through 2002.
Recent research into arc fl ash phenomena, however, indicates that workers could be under-protected against the heat generated during an arc fl ash event. Test results presented at IEEE conferences [Ref. 1, 2, 3] and at the 2007 IEEE Electrical Safety Workshop show that different confi gurations of electrodes (conductors) yielded heat energy higher than current predictions due to the directional nature of the arc development. Additionally, initial tests of PPE, when placed within this directional plasma fl ow, did not provide the level of thermal protection predicted by its APTV.
Directional nature of arc development
Unrestricted high-current arcs move according to magnetic forces to increase the area of the current loop. Currents fl owing in the opposite direction in parallel conductors give rise to forces that drive the arc away from the source to the end of the conductors where they typically burn off the tips of electrodes (busbars).
The behavior of a 3-phase arcing fault in equipment is very chaotic, involving rapid and irregular changes in arc geometry due to convection, plasma jets and electromagnetic forces. Arc extinction and re-ignition, changes in arc paths due to restriking and reconnection across electrodes and plasma parts and many other effects add to this chaotic nature and make it diffi cult to create equations for accurate predictions of its properties (e.g. impedance). Although it does not capture this chaotic behavior, Fig. 1 demonstrates an arc’s general directional nature. The alternating 3-phase current creates successive attractive and repulsive magnetic forces, dramatically moving the plasma jets which feed an expanding plasma cloud. The cloud is driven outward, away from the tips, creating “plasma dust” as the highly energized molecules in the plasma cool, then recombine into various materials. The molten electrode material ejected off the tips also is in this fl ow.
Arc fl ash hazards
When the arc is being established, current begins passing through ionized air, generating massive quantities of heat. Large volumes of ionized gases, along with metal from the vaporized conductors, are explosively expelled. As the arc runs its course, electrical energy continues to be converted into extremely hazardous energy forms. Hazards include the immense heat of the plasma, radiated heat, large volumes of toxic smoke, molten droplets of conductor material, shrapnel, extremely intense light and a pressure wave from the rapidly expanding gases.
Recent tests have shown that an object in the expanding plasma cloud (refer to the red object in Fig. 1) is directly exposed to the highest heat of the event. Temperatures greater than 15,000 C have been cited for this area. In addition to the convective heat transfer from the plasma, this object is directly exposed to the molten metal ejected from the electrode tips and radiated heat from surrounding plasma.
Objects close to the arc but outside of the plasma jets (refer to the green object in Fig. 1) are not likely subjected to as high a quantity of heat. Exposure is predominately radiant heat, but includes convective fl ow from the thermal expansion of the gases. Objects in line with the electrodes but distant from the plasma jets (refer to the blue object in Fig. 1) receive lower convective heating and less radiant heat and molten metal spray.
The amount of heat absorbed varies with the method of heat transfer and receiving surface properties. For example, the amount of heat transferred from a mass of molten copper to a surface area would be greater if it adhered to the object instead of contacting it for a brief time.
Test setups currently used for standards
Although the overriding principle of electrical safety is to de-energize equipment and place it into an electrically safe condition prior to work, there are numerous cases where companies put workers in PPE to perform tasks on energized equipment. The standards typically utilized to predict the magnitude of heat exposure and the protective ability of fl ame resistant (FR) fabric worn by exposed workers are based upon two unique electrode confi gurations in their test procedures. Heat transferred during tests with these orientations is most likely dominated by radiant heat (see Sidebar page 36).
Effects on heat measurements with alternate test confi gurations
Research performed at Ferraz Shawmut’s High Power Test Laboratory has uncovered electrode confi gurations that project signifi cantly more heat energy out of enclosures toward worker locations than currently predicted by the standards. To simulate components found in low-voltage electrical equipment, various setups were created for controlled testing. Heat was measured and compared with results obtained with the standard confi guration shown in the Sidebar fi gure on page 36. Results of these comparisons were published in two recent IEEE papers. [Ref. 2, 3] Confi gurations that forced the arc’s plasma jets outward toward the worker produced heat measurements nearly twice those predicted by current IEEE 1584 equations when studied at typical working distances of 18 inches.
In the barrier confi guration setup, the electrodes are “terminated” into a block of insulating material (barrier) as shown on the left in Fig. 2. This setup represents conductors connected to equipment from the top, such as the component shown on right in Fig. 2.
With the barrier in place, the arc’s downward motion is halted and plasma jets are formed along the plane of the barrier top surface (i.e. perpendicular to the plane of the electrode). This signifi cant fi nding is demonstrated in Fig. 3. The photo on the top shows a side view of arc development along the plane of the barrier in a setup without side panels. This test shows the possibility of higher convective heat transfer toward workers than the open vertical setup, shown from the front, on the bottom in Fig. 3. The barrier confi guration also ejected signifi – cantly more molten electrode material. [Ref. 3]
Chart 1 compares heat measurements (made with copper calorimeters) with the barrier setup to standard predictions. The black line represents predictions of IEEE 1584 equations for switchgear (20” cubic box) for the available fault currents with a fi xed 6-cycle clearing time. Alarmingly, the barrier test results almost always rose above the line—sometimes more than twice the prediction. All tests with the vertical confi guration at this voltage were at or below the prediction.
Another confi guration that deserves serious consideration is the “horizontal electrode confi guration.” This setup simulates equipment where bussing is open-ended, but pointing toward the front of the enclosure, like that in the equipment shown on the left in Fig. 4. The arc development, very similar to that described for Fig. 1, is shown on the right in Fig. 4. Like the barrier confi guration, all tests resulted in heat measurements signifi cantly above the predicted levels.