This circuit can protect your breadboard or prototype circuit instantly when it detects a short circuit in the assembled circuit. This can save the valuable components from burning. It allows only maximum 150 mA current to pass through the circuit under test. When it senses more than 150 mA load current due to short circuit, it cut off the power supply to the load.
R2 forms the current sensor. As long as a current of less than 150 mA passes through the circuit under test, T1 will be out of bias since its base voltage is less than 0.7 volts. When the current passing through R2 increases above 150 mA due to a short circuit in the circuit under test, T1 conducts. T1 then drains the relay current and the relay de-energize. Relay contacts open and breaks the power. Power supply can be established only after removing the short circuit and pressing S1 again.