This circuit can protect AC appliances. When the AC voltage in the mains increases above the set level , say 220 volts, relay triggers and breaks power from the appliance. Reconnects the appliance when the power returns normal.
Transistor Q1 and Zener diode acts as a switch to turn on / off the relay. When the voltage is normal ( as set by the preset ), Zener remains non conducting. Q1 off and the load gets power through the Common and NC contacts of Relay. When the input voltage in the transformer increases above the set level, corresponding voltage increase takes place in the preset and zener conducts. This switches on transistor and relay contacts break cutting power to the appliance.
Pin assignment of BD 139 is ECB from front side.