What is the purpose of an oil flow indicator in a transformer?

An oil flow indicator confirms that cooling oil is circulating where it should, especially in forced oil systems (OFAF or ODAF). It helps detect blocked coolers, pump failures, closed valves, or air locking that can cause overheating. By giving local or alarm contacts, it supports protective actions before winding hot spots rise into damaging ranges, preserving insulation life and reducing the risk of gas generation and thermal faults.