Design of electrical appliances
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If appliances that use mains electricity should develop a fault the consequences to the user can be lethal. In the design of electrical appliances steps are taken to prevent this. It is always possible for appliances to become faulty. However, the design precautions taken are such that a single fault will not result in any danger to the user.
On appliances that have large areas of exposed metal, say a PC or an electric fire, this metal is connected to the Earth pin of the mains plug. The idea is that if high voltages should develop within the PC due to a single fault, this cannot reach the user, as the whole unit is enclosed in a "safe" earthed case. This type of protections is known as Class I.
The other way of providing protection is by the use of two separate layers of Insulating Oil Testers. If a single fault resulted in the first layer of insulation being breached, then the second layer of protection is still available. This method is used in handheld appliances such as drills and hair dryers and is generally know as Class II.
Class II appliances are inherently safe and require less frequent testing. They are always indicated by the "double box" symbol.