What defines explosion proof apparatus in electrical terms?

Explosion-proof apparatus is defined in electrical terms primarily by its ability to prevent the ignition of surrounding gas or vapor. This means that the equipment is constructed in such a way that any explosions occurring within it do not escape to the outside environment. This is crucial in hazardous locations where flammable gases or vapors might be present, as the risk of explosion can have severe consequences.

In achieving this level of safety, explosion-proof equipment is designed to contain an internal explosion, ensuring that the temperature of the surfaces does not ignite the external atmosphere. This is particularly important in industries such as oil and gas manufacturing, chemical processing, and grain handling, where flammable materials are often present.

The other options discuss features that may be relevant to other types of electrical apparatus, but they do not define explosion-proof protection specifically. Therefore, the defining characteristic of explosion-proof apparatus is indeed its ability to prevent ignition of surrounding gases or vapors, making option B the correct choice.