4.1 Enclosure and conduit integrity: the protection you can undo with a screwdriver

1 The approval is conditional, and the condition is you

29 CFR 1910.303(b)(2) is eleven words long and it governs this entire part of the course: "Listed or labeled equipment shall be installed and used in accordance with any instructions included in the listing or labeling."

A laboratory tested that enclosure with every bolt in place, the correct gasket, the specified thread engagement and the cover machined face clean. That is the object that was approved. An enclosure missing two bolts was never tested and is not the object on the certificate. This is not a technicality about paperwork — the flame path from Part 3 is a physical dimension, and a cover held by eight bolts instead of twelve does not close to the dimension it was tested at.

2 The flame path is a machined surface, and it is fragile

The joint between an explosionproof body and its cover is a precision surface. It has to be, because that gap is what cools escaping gases below ignition temperature. Three things ruin it, and all three look harmless.

• Damage. A nick from a dropped cover, a burr from a screwdriver used as a lever, corrosion pitting under a coat of paint. The flame path is a dimension, and a gouge across it is a hole.
• Contamination. Grit, old sealant, paint or hardened grease on the mating faces stops the cover from seating and changes the gap. Clean the faces the way the manufacturer says, with the material the manufacturer names.
• Substitution. A different bolt grade, a shorter bolt, a washer that was not there before, a gasket from the general stores. The fasteners are part of a pressure-containing assembly and they are specified in the installation instructions.

The same logic applies to threaded entries. An explosionproof fitting screwed in three turns instead of five has a flame path a fraction of its designed length. There is no way to see this once the assembly is together, which is why the number of threads engaged is something you check while you are doing it and record if your procedure asks you to.

3 Conduit: threaded, wrench-tight, and why the standard says so

29 CFR 1910.307(d) is one sentence: "All conduits shall be threaded and shall be made wrench-tight. Where it is impractical to make a threaded joint tight, a bonding jumper shall be utilized."

The zone requirements at (g)(2)(iii) give the same instruction with the specification and, unusually, with the reasoning: all threaded conduit "shall be threaded with an NPT (National (American) Standard Pipe Taper) standard conduit cutting die that provides 3/4-in. taper per foot. The conduit shall be made wrench tight to prevent sparking when fault current flows through the conduit system and to ensure the explosionproof or flameproof integrity of the conduit system where applicable."

Two separate jobs in one requirement. A loose conduit joint is a resistive connection in what is supposed to be a continuous grounding path, and a fault current crossing it makes a spark in the exact place you least want one. And a loose joint is a flame path that no longer meets its dimensions. Hand-tight is not a state a classified conduit system is allowed to be in.

The standard also anticipates metric equipment, at (g)(2)(iv): for equipment with NPT threaded entries, listed conduit, conduit fittings or cable fittings must be used; for equipment with metric threaded entries, those entries must be identified as metric or listed adaptors must be provided with the equipment, and adaptors must be used for connection to conduit or NPT-threaded fittings. Forcing an NPT fitting into a metric entry is a documented way to destroy both.

4 Seals, and why a conduit is a route

Part 2 quoted a line from the Class I, Division 2 note that most people read past: "Electrical conduits and their associated enclosures separated from process fluids by a single seal or barrier are classed as a Division 2 location if the outside of the conduit and enclosures is a nonhazardous location."

The standard is telling you that the inside of the conduit is a place with a classification of its own. Gas that enters at a fitting in a classified area travels along the conduit and arrives wherever the conduit ends — the control room, the motor control center, the panel on the other side of the wall. Conduit seals are what stop that migration and stop a flame propagating along the run.

Where seals go, how they are packed and how much compound they take is detailed installation design and it lives in NFPA 70 and in the fitting manufacturer's instructions, not in this course. What everyone in the field needs to know is shorter and it is enough:

• A seal fitting that has not been poured is not a seal. An empty seal is a hole with a fitting around it, and it is a common finding on installations that were commissioned in a hurry.
• A seal that has been drilled, cracked or opened to pull an extra conductor is no longer a seal, and it does not become one again because the cover went back on.
• Drains and breathers exist because conduit collects water. Removing one to stop a drip defeats it; blocking one is the same thing more slowly.
• If you add a device, you may have added a seal requirement. That is a design question, and the answer comes from the drawing, not from the truck.

5 Openings, and the most cited habit in the trade

29 CFR 1910.305(b)(1)(ii) applies everywhere, classified or not: "Unused openings in cabinets, boxes, and fittings shall be effectively closed." The paragraph before it requires that conductors entering boxes, cabinets or fittings be protected from abrasion and that "openings through which conductors enter shall be effectively closed."

In an ordinary room, an open knockout is an untidy installation and a route for a finger. In a classified location it is the difference between an enclosure and an ornament. Everything the previous four zones described — the flame path, the containment, the exclusion of dust — assumes a closed volume.

The field version of this rule is a short list of things to look for on any walk-through:

6 The temporary fix that becomes permanent

Almost every degraded classified installation got that way through a sequence that seemed reasonable at each step. A light fitting fails at 2 a.m. and the only spare on the shelf is general-purpose, so it goes in "until Monday". A gasket is torn and the store is closed. A junction box is opened for troubleshooting and gets covered with a plate that fits. A cable is added and there is no time for a seal.

None of those decisions is made by someone careless. All of them are made by someone trying to keep a plant running, in the dark, alone, under pressure. Which is why the countermeasure is not a lecture about diligence — it is a system: correct spares stocked before they are needed, a defect tag that goes somewhere, a work order that cannot be closed with a temporary in place, and a supervisor who would rather run short than run wrong.

If you take one habit from this chapter, take this one: when you cannot restore the installation to its approved condition, do not leave it looking finished. Tag it, log it, tell somebody, and make the next person see what you saw.