The vast majority of household electrical appliances aren’t grounded. At first glance, it is not necessary: the chassis devices are securely isolated from the network, and work with them, usually in dry areas with non-conducting floors. But to rely entirely on technique impossible. If the breakdown or damage to insulation — faulty device becomes a source of serious danger. Fuses are useless: they will not burn out until there is a short circuit.
To avoid electric shock will help you an automatic device that will shut off the appliance from the network, only on the body voltage will be.
The machine is assembled according to the scheme of the trigger with one stable state (Fig. 1). In the case of the circuit “zero” or phases of a network on the body of the appliance rectified voltage through the chain V2, or V3, R1, R2 is supplied to the transistor V4, and it closes. The increased voltage on the collector of the transistor V5 opens relay K1, their contacts K1.1 is locked, and To 1.2 disables the appliance from the mains.