Venting of Exhaust Gases from Arc-Resistant Equipment

Understanding Arc Events and Exhaust Gases

When an arcing event occurs inside switchgear, extremely high temperatures (around 20,000 K at the arc terminals) can vaporize materials in the vicinity, including steel, copper, aluminum, and insulating materials. The resulting hot vapor combines with air oxygen to produce metal-oxides. As these particles cool, they become minute airborne particles, appearing as black smoke for copper and iron, and gray smoke for aluminum.

These materials are considered hazardous. Personnel in the vicinity should wear special filtered breathing apparatus and appropriate personal protective equipment (PPE).

Siemens strongly recommends that gases from an internal arcing event be exhausted to the outside environment, not within the building or enclosure housing the switchgear.

Hazardous Materials: These metal-oxide minute particles expelled from the arc vicinity are hazardous. Personnel should not be allowed in the area without special filtered breathing apparatus and appropriate PPE.

Arc-Resistant Equipment and Safety Standards

ANSI/IEEE C37.20.7 defines arc-resistant equipment as designed to provide additional protection to personnel operating in close proximity during normal conditions. However, it does not ensure total personnel protection under all possible internal arcing fault circumstances.

Users must take appropriate actions to protect personnel from internal arcing hazards.

Siemens' type GM-SG-AR medium-voltage metal-clad switchgear and type SIMOVAC-AR medium-voltage controllers are classified as arc-resistant and have been tested for internal arcing resistance, qualifying for a type 2B accessibility rating per IEEE C37.20.7.

Equipment instruction manuals specify the need for a plenum system to exhaust hot gases, burning particles, and other arc byproducts away from the equipment, preferably to the outside environment. This arrangement enhances personnel protection by isolating them from elevated pressures, temperatures, and potentially toxic gases generated during an arcing event.

Indoor Exhaust Considerations

Siemens acknowledges that site conditions may necessitate indoor exhaust placement. In such cases, the following conditions must be met to maintain arc-resistant qualification:

1. Exhaust plenums must extend a minimum of two (2) meters from the switchgear or motor controller periphery before exhausting into the room.
2. Each exhaust plenum outlet must be at least 2.4 meters above the floor. The plenum can exit the equipment in any direction, but the outlet must be oriented horizontally.
3. Outlets must exhaust into a restricted access area that is inaccessible to personnel when the equipment is energized and free of flammable or combustible materials. This area should extend a minimum of three meters in front of and 1.5 meters to each side of the exhaust outlet.
4. Building design must incorporate ventilation to mitigate hazards from potentially toxic gases, including vaporized metals and gases from burning insulation materials.
5. ANSI/IEEE C37.20.7 reinforces the restricted area philosophy, stating that personnel must not be permitted in areas near pressure-relief vents or exhaust gas vents.

Diagrammatic Explanations

Figure 1: Restricted Area in Vicinity of Exhaust Plenum Outlet illustrates the required restricted area around the exhaust plenum outlet. This area must be kept clear of personnel and combustible materials when the equipment is energized and should not be used for storage.

Figures 2 & 3: SIMOVAC-AR Exhaust Plenum Arrangement show the pressure relief channel on top of the equipment and the exhaust plenum directing gases away. Figure 2 depicts SIMOVAC-AR controllers with the exhaust gas path, where gases are directed to the top-mounted pressure relief channel (PRC) and then to the exhaust plenum. Figure 3 shows a SIMOVAC-AR controller lineup connected to GM-SG-AR switchgear, utilizing a shared plenum exhaust duct.

The exhaust plenum can exit the equipment assembly from the top, front, rear, or side. The outlet must be horizontal and extend at least two meters beyond the equipment periphery.

Siemens strongly recommends exhausting gases outside the building. If indoor exhaust is necessary, users must take appropriate actions to protect personnel from hazards associated with gases and byproducts from arcing events.

Contact Information

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