The most common form of hazardous area electrical equipment is the flameproof enclosure type (Ex d). The internal apparatus may include components that arc or have surfaces that become hot. Gas may also be present inside the enclosure, so it must fulfill the following three conditions:

• The enclosure must be strong enough to withstand an internal explosion without suffering damage.
• The enclosure must prevent the flame and hot gases from being transmitted to the external flammable atmosphere.
• The external surface temperature of the enclosure must remain below the ignition temperature of the surrounding gas under all operating conditions.

Flame and hot gas transmission from a flameproof enclosure is prevented because all joints—such as flanges, spigots, shafts, and bores—are closely machined to achieve a small gap, which is less than a defined maximum. When an internal explosion occurs, the pressure is released through this narrow gap between machined faces, which cools the gas sufficiently to prevent ignition of the external flammable atmosphere.

The maximum allowable gap depends on three factors:

• The type of gas for which the apparatus is rated.
• The width of the joint.
• The volume of the enclosure.

• Light fittings
• Motors
• Starters
• Push buttons and alarms within the hazardous zones

• The flameproof enclosure surface must be kept clean, corrosion-free, and properly secured.
• On lighting fittings, the cement bonding the lamp glass to its frame must be closely inspected for cracks or indentations.
• All bolts must be in place, of the correct type, and evenly torqued.
• The edges of flame path flanged joints must not be painted or otherwise altered.
• Exposed flameproof equipment on deck must be made weatherproof using the correct gaskets or "O" rings.
• All Ex d fittings may be opened for inspection of flame path surfaces for signs of corrosion, pitting, or scratches.
• No modification of Ex d equipment is permitted without certification authority approval.
• This includes the lamp size and its rating for a particular light fitting; only the correct lamp must be used.