Hazards of Moisture Ingress in Explosion-Proof Enclosures
Why a compromised seal on an explosion-proof housing creates a ticking time bomb in hazardous locations.
1. Introduction & Context
In Class I, Division 1 hazardous locations—where explosive gases or vapors are normally present—electrical equipment is often housed in massive, cast-metal “explosion-proof” (NEMA 7) enclosures. A common misconception is that these enclosures are airtight or gas-tight. They are not. They are designed to allow gas to enter, ignite inside, and safely cool the escaping exhaust gases so they don’t ignite the surrounding atmosphere. However, this non-airtight nature introduces a critical vulnerability: moisture ingress.
2. The Core Issue
Because an explosion-proof enclosure “breathes” as temperatures fluctuate (drawing in humid air during cooling cycles), condensation can rapidly accumulate inside if properly designed breathers and drains are not installed.
- Corrosion of Flame Paths: The most critical safety feature of an explosion-proof enclosure is the “flame path”—the precisely machined gap between the cover and the box where hot exhaust gases cool. If moisture pools and corrodes these machined surfaces, the gap widens. In the event of an internal explosion, hot plasma can escape through the rusted gaps and ignite the facility.
- Short Circuits and Arcing: Accumulated water can submerge terminal blocks, creating low-resistance paths for ground faults or phase-to-phase shorts. The resulting arc provides the ignition source for the explosive gases trapped inside.
- Compromised Seals: Sometimes, to stop water ingress, a well-meaning maintenance technician will apply silicone sealant or RTV around the flange of an explosion-proof enclosure. This is a catastrophic mistake. Sealing the flange blocks the flame path, meaning the enclosure will shatter like a grenade during an internal explosion rather than safely venting the pressure.
3. Actionable Takeaways
- Install Breathers and Drains: Explosion-proof enclosures in high-humidity or outdoor environments must be fitted with listed explosion-proof breathers (top) and drains (bottom) to allow condensation to safely escape without compromising the enclosure’s pressure rating.
- Inspect Flame Paths: During maintenance, meticulously inspect the machined flanges for pitting, scratches, or rust. Never use abrasive tools (like wire wheels or grinders) to clean a flame path.
- Never Seal the Flange: Never apply gaskets, tape, or silicone to the flange of an explosion-proof enclosure unless explicitly supplied and certified by the manufacturer.