Protection against direct voltage
Protection against direct contact electric shock is based on normal common sense measures such as insulation of live parts, use of barriers or enclosures, protection by obstacles or protection by placing live parts out of reach. As a result, under normal conditions it is not possible to touch the live parts of the installation or equipment inadvertently.
Protection against indirect contact electric shock is slightly more complicated hence a number of options are given in BS 7671 for the installation designer to consider.
Amplifiers that have no capacitor at their output may, in the case of a defect apply a direct voltage to the loudspeakers and this can destroy the drive units. The circuit shown can prevent such a catastrophe.
It is best to give the circuit a separate power supply: this minimizes any work on the amplifier(s). This supply must. however. be switched synchronously with that to the amplifier(s). since on power-on T1 ensures that the relay (which switches the loudspeaker inputs ) is energized after some delay. The delay is determined by the time constant R3C3.
Assuming that the amplifier operates correctly. its output signal at point b is linked to point a. and thence to the loudspeaker(s). via the contact. Owing to the time constant R3-C3. bipolar Capacitor C1-C2 cannot be charged by the a.c. the signal at point b. if, however. a defect causes a direct voltage at that point. the Capacitor will be charged via R2. Depending on the polarity of the direct voltage, either T2 or T3 will be switched on, which removes the base current from T4 and this results in the de-energizing of the relay: the amplifier output is then removed from the loudspeaker(s).
The supply to the protection circuit must be neither regulate nor smoothed. True, C4 provides some smoothing. but the important thing is that after the amplifier has been switched off. this Capacitor is discharged more rapidly then than the smoothing Capacitors in the amplifier power supply. This ensures that the relay is de-energized before the amplifier can produce a click in the loudspeaker(s). Depending on the relay. the current drawn by the circuit is about 50 mA.