This time and the current rating in turn sets a maximum earth resistance permissible. 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.
If large enough, this current will trip an over-current device ( fuse or circuit breaker ) and disconnect the supply. Ī fault in the appliance which causes a live conductor to contact the casing will cause a current to flow in the earth conductor. It should also be the last to be broken when the plug is removed. Plugs are designed such that the connection to the protective earth conductor should be the first connection when plugged in. The earth connection is achieved with a three-conductor mains cable, typically ending with three-prong AC connector which plugs into a corresponding AC outlet. Hence, 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 India, USA, Canada and Japan). Any Class 1 equipment with a Schuko plug will act like a Class 0 equipment when connected to an ungrounded outlet.Īppliance 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 failure of such an equipment in a location where there are grounded equipment can cause fatal shock if one touches both. In many countries the plug of a class 0 equipment is such that it cannot be inserted to grounded outlet like Schuko. These appliances do not have their chassis connected to electrical earth. However, equipment of this class is common in some 120 V countries, and in much of the 230 V developing world, whether permitted officially or not.
A typical example of a Class 0 appliance is the old style of Christmas fairy lights. 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.
A single fault could cause an electric shock or other dangerous occurrence, without triggering the automatic operation of any fuse or circuit breaker. If permitted at all, Class 0 items are intended for use in dry areas only. These appliances have no protective-earth connection and feature only a single level of insulation between live parts and exposed metalwork.
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