Double Insulated Appliances and Safety
Date: Fall 2013
How does a double insulated appliance or power tool prevent electrical shock?
It is first necessary to understand how a shock can most likely occur. Basically for appliances that plug into the wall: metal=bad and plastic=good. The principal danger of any item is that if it has a metal casing, the metal casing will become energized as a result of a malfunction. If that happens then electricity will easily flow through someone touching it who is standing on a good ground conductor. That person could be shocked, perhaps fatally.
If there is no metal then there is little possibility of a shock even if things become damaged inside or wires become frayed or worn. So if a manufacturer builds an appliance with a plastic shell, or with two layers of an insulating material around live parts, then the item will be very safe. Such a safe item does not need a three wire grounding plug. It can use two wires because there is nothing really for the grounding pin to attach to. Such items are called double insulated.
It is still possible to suffer harm with a DI appliance because there is still live voltage inside of it. A plastic toaster, for example, is pretty safe, but if you insert a knife into it and hold onto the kitchen sink with the other hand, a shock could occur. Or a plastic drill also has 120 volts inside of it, but you would need to probe around with a wire in order to come in contact with 120 volts. Or if you drop the entire appliance into the water it could be bad.
As you have guessed, a metal body on something can be quite dangerous. For example, an old metal fan is fine when it is in good condition, but what if one of the 120 volt wires becomes frayed and touches the metal of the fan itself? The metal fan will be at a 120 volt potential and could give a shock to someone holding or touching it. (That is assuming that the circuit breaker does not open up. Depending on the fan, a frayed cord could cause a short circuit that the circuit breaker or fuse would interrupt, but more likely a frayed cord would cause the fan body to become energized.) That is why most things are made of plastic nowadays. Or if there are external metal parts then the manufacturer must design carefully to make sure that these parts do not become energized even if the item is damaged.
Power cords carry three wires.
One cord carries voltage and line current from the energy source.
The 2nd cord carries a return current to complete the circuit.
The 3rd cord establishes ground between the case of the power tool and earth ground to assure that the tool user does not get electrocuted.
Sometimes a connection between the tool case and the first two wires gets established and this 3rd wire assures that nobody will get hurt in this case.
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Update: November 2011