3.4 The protection techniques, and what each one actually does
There are only three ways to stop electrical equipment from igniting an atmosphere: contain the explosion so it cannot get out, exclude the atmosphere so it never reaches the spark, or limit the energy so that a spark cannot ignite anything even in the worst mixture. Every protection technique named in 29 CFR 1910.307 is one of those three ideas, engineered. This chapter names them all and explains what each one physically does.
1 Three strategies, and why there are only three
Go back to the fire triangle from Part 1. In a classified area the fuel and the oxygen belong to the process, so the equipment designer's whole problem is the third leg. There are three logical places to break the chain, and the entire catalogue of techniques falls into them.
Containment is heavy, machined and expensive, and it works with any amount of power — that is why motors and switchgear use it. Exclusion needs a supply of clean air or gas, or a genuinely sealed volume, and it fails when the supply fails. Energy limitation is cheap, light and inherently safe, and it only works on circuits that need very little power — instruments, sensors, communications.
2 The ten techniques of the division system
29 CFR 1910.307(f) lists ten acceptable protection techniques for electric and electronic equipment in hazardous (classified) locations, each with the locations it is permitted in. The permissions below are the standard's; the plain-language column is this course's description, because the formal definitions of these techniques live in NFPA 70 and are not reproduced here.
| Technique | Permitted for equipment in | The idea behind it |
|---|---|---|
| Explosionproof apparatus | Class I, Division 1 and 2, for which it is approved | Contain. The enclosure withstands an internal explosion and lets the burnt gases out only through long, tight paths that cool them below ignition temperature. |
| Dust ignitionproof | Class II, Division 1 and 2 | Exclude and contain. Dust is kept out of the enclosure, and the surface stays cool enough not to ignite dust lying on it. |
| Dust-tight | Class II, Division 2 and Class III | Exclude. Dust cannot enter in a quantity that would interfere with operation or be ignited inside. |
| Purged and pressurized | Any hazardous (classified) location for which it is approved | Exclude. Clean air or inert gas is supplied to hold the inside of the enclosure above the surrounding pressure so the atmosphere cannot get in. |
| Nonincendive circuit | Class I, Division 2; Class II, Division 2; Class III, Division 1 or 2 | Limit. The circuit cannot release enough energy to ignite the atmosphere under normal operating conditions. |
| Nonincendive equipment | Class I, Division 2; Class II, Division 2; Class III, Division 1 or 2 | Limit, applied to the whole device rather than a circuit. |
| Nonincendive component | Class I, Division 2; Class II, Division 2; Class III, Division 1 or 2 | Limit, applied to a part with contacts that would otherwise be a source. |
| Oil immersion | Class I, Division 2, for current-interrupting contacts | Exclude. The contacts open under oil, so the arc never meets the atmosphere. |
| Hermetically sealed | Class I, Division 2; Class II, Division 2; Class III, Division 1 or 2 | Exclude. The contacts are sealed inside a chamber the atmosphere cannot enter. |
| Other protection techniques | Any hazardous (classified) location, if it meets paragraph (c) | The open door for anything that satisfies the three routes of (c) — most often intrinsic safety. |
Scan the middle column. Only two entries cover Class I, Division 1: explosionproof, and purged and pressurized — plus intrinsic safety through (c)(1). Everything else on the list is a Division 2 technique. That is why Division 1 areas cost what they cost, and why moving a boundary line on a drawing is an expensive decision.
3 The eight techniques of the zone system
The zone system uses the international letter designations, and 1910.307(g)(3) requires one or more of the following for equipment in zone-classified locations:
| Technique | Permitted for equipment in |
|---|---|
| Flameproof "d" | Class I, Zone 1 |
| Purged and pressurized | Class I, Zone 1 or Zone 2 |
| Intrinsic safety | Class I, Zone 0 or Zone 1 |
| Type of protection "n" (subdivided into nA, nC and nR) | Class I, Zone 2 |
| Oil immersion "o" | Class I, Zone 1 |
| Increased safety "e" | Class I, Zone 1 |
| Encapsulation "m" | Class I, Zone 1 |
| Powder filling "q" | Class I, Zone 1 |
Notice which technique is the only one on that list permitted in Zone 0: intrinsic safety. In the zone system, the place where the atmosphere is present continuously admits exactly one approach — make the energy too small to ignite it. That single fact is the best argument for understanding intrinsic safety properly, which is the next zone.
One footnote from the same paragraph carries into Part 4: flameproof "d" equipment "may not be suitable for use at temperatures lower than −20 °C (−4 °F) unless they are approved for use at lower temperatures" — the low ambient note to (g)(4).
4 Intrinsic safety is a property of the system, not the box
1910.307(c)(1) gives intrinsically safe equipment a privilege nothing else has: "Equipment and associated wiring approved as intrinsically safe is permitted in any hazardous (classified) location for which it is approved."
Read the phrase "and associated wiring". This is the point technicians most often miss. An intrinsically safe instrument is not safe because the instrument is special; it is safe because the whole loop — the field device, the cable, and the barrier or isolator back in the safe area that limits what can be delivered down that cable — has been designed as one energy-limited system. Change any element and the case is no longer made.
Practical consequences on the floor: do not substitute the barrier, do not extend or re-route the cable outside its specified parameters, do not add a device to the loop, and do not connect a general-purpose test instrument to an intrinsically safe circuit unless it is approved for that use. The system's documentation — the control drawing referenced in its listing — is what defines the permitted combinations, and 1910.303(b)(2) requires listed equipment to be installed and used in accordance with it.
5 "Explosionproof" is the most misleading word in the trade
It sounds like a promise that nothing will explode inside. It is the opposite. An explosionproof enclosure is designed on the assumption that the atmosphere will get inside and will be ignited by whatever the enclosure contains. Its job is to survive that event and to stop the flame from propagating out into the room.
Two things do that work, and both are physical. The first is strength: the housing has to take the internal pressure without rupturing, which is why the castings are thick and heavy. The second is the flame path: the long, precisely machined joint between the body and the cover, and between the housing and each threaded entry. Burnt gases leave through that path, and the path is long enough and tight enough that they are cooled below the ignition temperature of the surrounding atmosphere before they emerge.
This is the reason the requirements in 1910.307(d) and (g)(2)(iii) are about threads. "All conduits shall be threaded and shall be made wrench-tight", and in a zone installation the conduit must be threaded with an NPT standard conduit cutting die providing a 3/4-inch taper per foot and 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."
Every bolt, every thread engagement and every machined face is part of a pressure vessel and a flame arrester. Part 4 opens with what happens when one of them is missing.
6 Choosing between them, in practice
You will rarely choose a technique — a designer does — but knowing why a plant looks the way it does makes the rest of the job easier.
- Motors, switchgear, panels, lighting in Division 1: explosionproof, because the power involved rules out energy limitation and a purge system on every fitting would be unmanageable.
- Analyzer houses and large control enclosures: purged and pressurized, because it is cheaper to protect one large volume with clean air than to build it as a casting — and because the purge can be monitored and alarmed.
- Transmitters, thermocouples, switches, communications: intrinsic safety, because the power is tiny and the maintenance advantage is enormous — energy-limited circuits are the only ones that can be worked on live.
- Dust areas: dust ignitionproof in Division 1, dust-tight in Division 2 — plus the housekeeping that keeps surface temperatures where the marking says they are.
- Anything in Zone 0: intrinsic safety, and only intrinsic safety, under 1910.307(g)(3)(iii).
- Every protection technique is one of three ideas: contain the explosion, exclude the atmosphere, or limit the energy below what can ignite.
- 1910.307(f) lists ten division techniques with the locations each is permitted in; only explosionproof, purged and pressurized, and intrinsic safety reach Class I, Division 1.
- 1910.307(g)(3) lists eight zone techniques, and intrinsic safety is the only one permitted in Zone 0.
- Intrinsic safety covers "equipment and associated wiring" — it is a property of the whole loop, including the barrier and the cable, not of the field device alone.
- Explosionproof does not mean nothing explodes inside; it means the enclosure contains the explosion and cools the escaping gases through a machined flame path.
- That is why 1910.307(d) requires all conduits threaded and made wrench-tight, and (g)(2)(iii) specifies the NPT die and the 3/4-inch taper per foot.
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