When an appliance ceases to operate, there may be various causes for this, one of which is the blowing of the mains fuse. The monitor proposed here contains an LED that lights when that is the case. It is suitable for use with fuses rated from mili-amperes to amperes. As long as the fuse is intact, the full mains voltage exists across C2-R3-D3. Capacitor C2 and resistor R2 serve to limit the base current of T1. Diode D3 prevents C2 from being charged, which would cause the base current to drop to zero rapidly. Capacitor C1 and resistor R2 limit the current through D1, while D2 ensures that, the voltage across the LED does not exceed 2.7 V. At the same time, D2 prevents C1 from being charged.
As long as the mains voltage exists at Junction F1-C2, transistor T1 conducts and Short-circuits D1 and D2. When F1 blows, T1 is switched off, whereupon current flows through D1 and D2: the LED then lights. Resistor R1 must conform to relevant safety regulations. Furthermore, capacitors that still carry the mains voltage after the appliance is switched off, must become discharged (via R1) within a stipulated time. When the monitor is in use, remember at all times that certain of its parts are at potentially lethal, mains voltage.