Appliance classes

A single fault could cause an electric shock or other dangerous occurrence, without triggering the automatic operation of any fuse or circuit breaker.

Appliance class I is not only based on the basic insulation, but the casing and other conductive parts are also connected with a low-resistant earth conductor.

The basic requirement is that no single failure can result in dangerous voltage becoming exposed so that it might cause an electric shock and that this is achieved without relying on an earthed metal casing.

As such, the appliance should not be connected to an earth conductor because the high-impedance casing will cause only low-fault currents that are unable to trigger the fusible cut-out.

[1] Insulated AC/DC power supplies (such as cell-phone chargers) are typically designated as Class II, meaning that the DC output wires are isolated from the AC input.

A Class III appliance is designed to be supplied from a separated extra-low voltage (SELV) power source.

The voltage from a SELV supply is low enough such that under normal conditions a person can safely come into contact with an energized conductor without risk of electrical shock.

For medical devices, compliance with Class III is not considered sufficient protection, and furthermore, stringent regulations apply to such equipment.

For instance, a laptop or mobile phone might qualify as a Class III appliance if it is charged via an external SELV adapter, even though the onboard battery could pose a fire risk.