Final answer:
The handheld instrument used to check the 3rd rail is called a rail tester or 3rd rail test sets. It is used to detect power or confirm the 3rd rail shut down in railway electrification.
Step-by-step explanation:
The handheld instrument used to check the 3rd rail in order to detect power or confirm the 3rd rail shut down is called a rail tester or 3rd rail test set.
This device is used in the field of railway electrification to test the status of the power supply on the third rail, which is the power source for electric trains.
By measuring the voltage or current on the 3rd rail, the rail tester can determine if power is present or if the rail has been shut down.