Powder-filling (also called sand-filling) superficially resembles oil-filled and encapsulated equipment. However, it operates differently, and has several unique requirements.

Contrary to popular opinion, powder-filling does not keep an explosive atmosphere away from the electrical parts. Instead, it is designed such that if an ignition does occur, the explosion will be quenched by the powder before it reaches the outside explosive atmosphere. This occurs because the sand particles provide thousands of tiny flame paths, which the internal flame is unable to penentrate.

Obviously, the fill must be performed so that there is no possibility that openings or gaps in the powder could lead to an ignition hazard, and there are particular tests for enclosures. Paradoxically, sealed equipment must be equipped with a vent, but the vent can't allow the powder to escape.

Powder filling is a relatively uncommon technique. This is due, in part, to the fact that it does not suit a lot of equipment, including anything with accessible terminals, some keypads, any type of open-air sensor and/or any kind of visual indicator or display. It is also unsuited to equipment requiring service or periodic maintenance. It is, arguably, best suited to non-rotating equipment operating at high temperatures, but such equipment is by definition rare, as it tends to self-destruct in short order.

Powder-fill is, in fact, so rare that some agencies have never issued a type 'q' certificate. Rumor has it that the responsible IEC committee was ready to drop the technique entirely, but kept it around for the benefit of the one (or maybe two) OEM(s) worldwide that actually use(d) it.

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