1.4 Ignition sources: what the equipment brings, what people bring

1 Two families of ignition source

It helps to separate them from the start, because they are controlled by completely different systems. Permanent ignition sources are the installed equipment: switches, starters, motors, lights, instruments, heaters. These are controlled by the hazardous location classification and by the equipment approvals that Part 3 of this course covers. Transient ignition sources arrive with an activity: welding, grinding, a vehicle, a phone, a spark from a dropped tool, a static discharge off a person. These are controlled by permits, procedures and housekeeping.

A plant can get the first family completely right — every fitting approved, every seal poured, every nameplate correct — and lose the building to the second. The classified installation is the foundation, not the whole house.

2 What the equipment brings: arcs, sparks and heat

Electrical ignition comes in two forms and both are addressed explicitly in 1910.307. The first is the arc or spark: a switch opening under load, a contactor closing, a brush motor commutating, a loose terminal, an insulation failure. The second is the hot surface, which needs no fault at all — a motor frame, a lamp, a ballast, a heater, a drive at full load on a hot day.

The hot surface is the one people forget, and OSHA's marking rule exists because of it. 1910.307(c)(2)(ii) requires equipment to 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", and then adds the rule that ties the marking back to the chemistry: "The temperature marking may not exceed the ignition temperature of the specific gas or vapor to be encountered."

That single sentence is the entire logic of temperature classification, and Part 3, Chapter 2 unpacks it. For now, hold on to the idea: the equipment must not be able to reach a temperature at which the atmosphere ignites on its own, with no spark at all.

3 Hot work: the transient source with its own standard

Welding, cutting and brazing are the ignition source that arrives on a work order, and 29 CFR 1910.252(a) is built around a simple hierarchy. First, "if the object to be welded or cut cannot readily be moved, all movable fire hazards in the vicinity shall be taken to a safe place." Second, if it cannot be moved and the fire hazards cannot be removed, "then guards shall be used to confine the heat, sparks, and slag." Third — and this is the paragraph people skip — "if the requirements stated in paragraphs (a)(1)(i) and (a)(1)(ii) of this section cannot be followed then welding and cutting shall not be performed."

The special precautions that follow give the numbers everybody quotes. Fire watchers are required whenever appreciable combustible material is closer than 35 feet (10.7 m) to the point of operation, or is farther but easily ignited by sparks, or where wall and floor openings within a 35-foot radius expose combustible material in adjacent areas including concealed spaces, or where combustibles on the far side of a metal partition, wall, ceiling or roof are likely to be ignited by conduction or radiation. Where combustible materials are on the floor, the floor must be swept clean for a radius of 35 feet.

Two more provisions matter in a classified area. The fire watch must be maintained "for at least a half hour after completion of welding or cutting operations to detect and extinguish possible smoldering fires" — the delay that catches out crews who leave when the arc stops. And 1910.252(a)(2)(iv) puts authorization in writing: before cutting or welding is permitted, the area shall be inspected by the individual responsible for authorizing the work, who designates the precautions to be followed "preferably in the form of a written permit."

4 Static: the ignition source nobody is holding

Moving a nonconductive liquid through a pipe, splash-filling a container, blowing dust through a duct or pulling a plastic liner out of a drum all separate charge. When the accumulated charge finds a path to ground, it does so as a spark — and the energy needed to ignite a vapor is far less than the energy you can feel in your hand.

This is why bonding and grounding requirements appear all through the flammable liquids standard rather than in the electrical standard. At a bulk plant, 29 CFR 1910.106(f)(3)(vi) says Category 1 or 2 flammable liquids, and Category 3 with a flashpoint below 100 °F (37.8 °C), "shall not be dispensed into containers unless the nozzle and container are electrically interconnected"; and 1910.106(f)(3)(v) requires tank car loading through open domes to be protected against stray currents by bonding the pipe to at least one rail and to the rack structure, with insulating sections where excessive stray currents are known to exist. The standard even names the case where it is not required: liquids that are "nonaccumulators of static charges", and the higher-flashpoint categories.

The practical version for anyone transferring flammable liquid is short. Bond the container you are pouring from to the container you are pouring into, ground the receiving container, keep the fill pipe down near the bottom rather than splashing from the top, and do not break the bond until the transfer is finished.

5 Friction, impact and process heat

Mechanical ignition sources do not announce themselves. A bearing running dry gets hot enough to ignite dust long before anybody notices the noise. A belt slipping on a jammed pulley does the same. Tramp metal in a mill throws sparks. A grinder makes them by design. An oven, a dryer or a furnace is a permanent open heat source in a plant that also handles fuel.

The incident record in Chapter 3 is largely a record of exactly this. The Massachusetts foundry investigation listed maintenance of ovens and equipment safety devices among the causal factors. The Kentucky plant's likely ignition scenario was a small fire extending from an unattended oven. The Indiana plant's primary explosion was near a chip melting furnace. In none of those cases was the ignition source an electrical fault — and in all of them, the classified electrical installation, however good, was irrelevant to how the fire started.

6 The source-by-source summary

Look at the right-hand column as a whole. Exactly one row is controlled by the classification system. The other six are controlled by what people do. That balance is the reason this course has a Part 4.