Batteries

Batteries can store large amounts of energy and as such present a problem for intrinsic safety.

It is fairly easy to understand how a battery of cells could be connected together such that when a switch is opened or closed a spark is generated, with sufficient energy to trigger an explosion.

Less obviously an individual cell may fail internally causing a short circuit and dissipating its energy as heat across its internal resistance. In this event the surface temperature of the cell could trigger an explosion. Zinc carbon and zinc chloride cells were fairly benign in this respect but some modern alkaline cells are reaching the point where they are no longer suitable for T4 rating.