3.2 Temperature classification: the surface is the ignition source
Everything so far has been about sparks. This chapter is about the ignition source that never sparks at all: a surface that is simply hot enough. It needs no fault, no arc and no failure — a motor running normally on a warm afternoon will do — and it is the reason every piece of equipment approved for a classified location carries a temperature marking.
1 Ignition without a spark
Every flammable gas and vapor has a temperature at which it will ignite on contact with a hot surface, with no flame and no spark involved. Below that temperature the mixture sits there; above it, it lights. The value is a property of the substance, it is usually listed in Section 9 of the safety data sheet, and it varies enormously between materials — which is exactly why a single "don't let anything get hot" rule would be useless.
This matters more than most people expect because the surfaces in question are ordinary. A motor frame under load. A lighting fixture. A transformer or ballast. A drive heat sink. A heater. None of these is faulty, and none of them will set off a spark detector.
2 The rule, in OSHA's words
29 CFR 1910.307(c)(2)(ii): "Equipment shall be marked to show the class, group, and operating temperature or temperature range, based on operation in a 40-degree C ambient, for which it is approved. The temperature marking may not exceed the ignition temperature of the specific gas or vapor to be encountered."
Three separate ideas are packed into that. First, the marking exists — it is a required part of the nameplate, alongside class and group. Second, it is referenced to a 40 °C ambient (104 °F), so the number on the plate is not the temperature of the equipment in your particular room; it is the temperature it reaches under a defined reference condition. Third, and most importantly, the marking has to be below the ignition temperature of what is in the air.
That last sentence is the entire safety case in one line. The equipment cannot be capable of reaching a temperature that ignites the atmosphere. Not "should not". Not "under normal conditions". The approval itself is conditioned on it.
3 Temperature classes, and where the table lives
Rather than stamping a raw number on every device, the industry uses a set of standard temperature classes — the codes you see written as a T followed by a digit, sometimes with a letter, on equipment nameplates and in the marking example OSHA prints as Figure S-1. Each code corresponds to a maximum surface temperature the equipment will not exceed.
The table that assigns each code its maximum temperature is published in NFPA 70 and is not reproduced here, and this is one place where working from memory is genuinely dangerous. A T-code is not a rating you can rank by intuition: it stands for a specific temperature limit, and whether a particular code is acceptable in your area depends on the ignition temperature of the material in your area. Two documents settle it — the classification for the area, and the manufacturer's instructions for the equipment, which 1910.303(b)(2) requires listed or labeled equipment to be installed and used in accordance with.
Equipment can also be marked with an actual temperature or temperature range instead of a code — 1910.307(c)(2)(ii) permits either, saying "operating temperature or temperature range". Both mean the same thing: this is the hottest this device gets, and the atmosphere it is used in must ignite above that.
4 The 40 °C reference, and what happens in a hot plant
The marking is defined "based on operation in a 40-degree C ambient". Forty degrees Celsius is 104 °F — a hot day, and nowhere near the hottest place in an American plant. A boiler house, a dryer deck, a compressor room, a roof in Arizona and an unventilated enclosure in a Gulf Coast summer can all exceed it.
OSHA handles this directly in the last of the five exceptions to the marking rule. 1910.307(c)(2)(ii)(E): "Electric equipment suitable for ambient temperatures exceeding 40 °C (104 °F) shall be marked with both the maximum ambient temperature and the operating temperature or temperature range at that ambient temperature."
So the standard does not pretend that 40 °C covers everything. It requires equipment intended for hotter surroundings to carry two numbers: the maximum ambient it is suitable for, and what it runs at in that ambient. If you work somewhere that is routinely above 104 °F, that second marking is one you should learn to look for.
5 Dust changes the arithmetic completely
Everything above assumes the surface can lose its heat to the surrounding air. Put a layer of combustible dust on it and it cannot. That is not a side note — it is written into the definition of a Class II, Division 2 location in 29 CFR 1910.399, which describes accumulations "sufficient to interfere with the safe dissipation of heat from electric equipment."
The consequence is worth stating bluntly. In a dust area, the temperature marking on the nameplate is only valid for equipment that is clean. A motor certified to stay below a given surface temperature will exceed it under a blanket of flour, and the dust doing the insulating is also the fuel. Housekeeping in a Class II area is therefore not cosmetic and not merely about preventing a cloud: it is what keeps the equipment inside the approval it was installed under.
6 Putting group and temperature together
The two dimensions are independent and both have to be satisfied, which is why a nameplate carries both. A device can be right for the group and wrong for the temperature, or the reverse, and either mismatch is a real one.
| Group | Temperature marking | |
|---|---|---|
| What it describes | The ignitable properties of the atmosphere — how little energy ignites it, how easily flame escapes a gap | The maximum surface temperature the equipment reaches under a defined reference condition |
| Where it comes from | NFPA 70 group tables, applied to the material in your area by the classification study | The equipment's own approval, based on operation in a 40 °C ambient — 1910.307(c)(2)(ii) |
| The test to apply | Does the equipment's group cover the group of the area? | Is the marked temperature below the ignition temperature of what is in the air? |
| What breaks it in the field | A material change in the process; a substituted part with a different group | A hotter ambient than the equipment was marked for; a dust layer that blocks heat dissipation |
Neither of these is a judgment call you make with a tape measure. Both are read off documents — the classification drawing and the nameplate — and compared. Chapter 3 is about reading the second one correctly.
- A hot surface ignites a flammable atmosphere with no spark and no fault — ordinary running equipment is the source.
- 1910.307(c)(2)(ii) requires the class, group and operating temperature or temperature range on the nameplate, based on operation in a 40 °C (104 °F) ambient.
- The temperature marking may not exceed the ignition temperature of the specific gas or vapor to be encountered — the approval itself depends on it.
- Temperature classes are the T-codes on the nameplate; the table assigning each code its temperature limit is in NFPA 70, and the acceptable code for an area depends on the ignition temperature of the material there.
- Equipment suitable for ambients above 40 °C must be marked with both the maximum ambient and the operating temperature at that ambient — 1910.307(c)(2)(ii)(E).
- In a Class II area the temperature marking is only valid for clean equipment: a dust layer blocks heat dissipation, which is written into the Division 2 definition itself.
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