What is a mains failure relay?

A mains failure relay is a type of relay that activates when the mains supply fails. There are many different types of mains failure relay that monitor a range of parameters and can be adjustable, both for the time before they activate after failure and the level at which they consider the mains to have failed.

Where is a mains failure relay used?

In power generation, a mains failure relay is used inside an Automatic transfer switch ATS panel to detect that the mains has failed.

How does the mains failure relay work?

Again, there are many different types, but the relay is normally open or closed. When the mains is available it will be in one state and when the mains fails it will switch to the opposite state. This open and closed state allows other electronics,  like the ATS panel, to read this signal and perform other actions.