Loop checking means inspection of full loop of instruments- junction box- control room wiring after the installation to ensure proper working.
Before the start of the plant loop checking ensures that the signal from the transmitters/control passes on line and receives at the receiver end and carries out the commands/instructions.
We can do loop checking in two ways. In first method we give the actual input in to the instrument, in the other method we give corresponding signals depends upon the output of the instrument. During the loop check we give some known input and check the corresponding output at other side (control room). If the output is corresponding to input, the loop is passed OK. If not some fault exists in the loop.
For example, if you want to check ON/OFF valve during the loop check, you give 24V DC (command) from control room, then valve must open fully, and when you give 0V DC (command withdrawn) the valve must close.
There are two types of loops, closed loop and open loop. Open loop is used for indications only whereas closed loop is for control.