570
IEC PROTECTION CLASS
The electrical insulation classes are the homogeneous grouping defined by the IEC (International Electrotechnical Commission) which refers
to the technical specifications that are applicable to an electric device to limit the risk of electrocution resulting from a failure of the system.
CLASS
DESCRIPTION
0
These appliances have no protective-earth connection and feature only a single level of insulation between live parts and exposed metalwork. If permitted at all, Class 0
items are intended for use in dry areas only. A single fault could cause an electric shock or other dangerous occurrence, without triggering the automatic operation of any
fuse or circuit breaker. Sales of such items have been prohibited in much of the world for safety reasons, for example in the UK by Section 8 of The Low Voltage Electrical
Equipment (Safety) Regulations 1989 and New Zealand by the Electricity Act. However, equipment of this class is common in 110V countries, and in much of the 220V
developing world, whether permitted ofcially or not. These appliances must have their chassis connected to electrical earth.
I
These appliances must have their chassis connected to electrical earth (US: ground) by a separate earth conductor (coloured green/yellow in most countries, green in the
US, Canada and Japan). The earth connection is achieved with a 3-conductor mains cable, typically ending with 3-prong AC connector which plugs into a corresponding
AC outlet. 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 if a fault
occurs the supply will be removed automatically (this is sometimes referred to as ADS = Automatic Disconnection of Supply).
A fault in the appliance which causes a live conductor to contact the casing will cause a current to flow in the earth conductor. If large enough, this current will trip an
over-current device (fuse or circuit breaker (CB)) and disconnect the supply . The disconnection time has to be fast enough not to allow fibrillation to start if a person is
in contact with the casing at the time. This time and the current rating in turn sets a maximum earth resistance permissible. To provide supplementary protection against
high-impedance faults it is common to recommend a residual-current device (RCD) also known as a residual current circuit breaker (RCCB), ground fault circuit interrupter
(GFCI), or residual current operated circuit-breaker with integral over-current protection (RCBO), which will cut of the supply of electricity to the appliance if the currents
in the two poles of the supply are not equal and opposite.
II
A Class II or double insulated electrical appliance is one which has been designed in such a way that it does not require a safety connection to electrical earth
(ground). he 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. This is usually achieved at least in part by having at least two layers of insulating material between live parts
and the user, or by using reinforced insulation.
In Europe, a double insulated appliance must be labelled Class II or double insulated or bear the double insulation symbol (a square inside another square).
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.
The designation "Class II" should not be confused with the designation "Class 2", as the latter is unrelated to insulation (it originates from standard UL 1310, setting
limits on maximum output voltage/current/power).
III
A Class III appliance is designed to be supplied from a separated/safety extra-low voltage (SELV) power source. The voltage from a SELV supply is low enough
that under normal conditions a person can safely come into contact with it without risk of electrical shock. The extra safety features built into Class I and Class II
appliances are therefore not required. For medical devices, compliance with Class III is not considered sufcient protection, and further more-stringent regulations
apply to such equipment.
