Loop Check vs Function Test: Troubleshooting Causes and Fixes
Feature image: a loop check proves the signal path; a function test proves what the system does with it. Conceptual illustration. A loop check and a function test are two commissioning tests that are constantly mixed up, and the mix-up has a price. A loop check proves that a signal gets from a field device to the operator display (or from the controller out to a final element) correctly, on the right tag, in the right engineering units. A function test proves that the control and safety logic then does the right thing with those signals: that interlocks, permissives, voting, alarms and trips behave as specified. When either one fails, or when one is skipped and the other is asked to do its job, people lose days chasing a wiring fault that looks like a logic fault or a logic fault that looks like a wiring fault. This guide shows how to troubleshoot loop check versus function test problems: how to tell which test a failure belongs to, the causes that show up most often in each, t...