Signalling relays contain a mechanical flag to indicate the operation of the relay. They are used for operation indication of protection functions in a protective assembly, for DC supervision, or with transformer mechanical protections as indication and contact. y of quality products and innovation remains the same. Trip circuit. In order to quickly and accurately determine which relay is faulty, we connect an indicator light module in parallel at both ends of the relay coil to display the power gain and loss status of the relay, which is often encountered during use. Ensure proper operation and fault management. Ready LED (green) blinks slowly when the standard protection functions of the electronic trip unit are operational.