How does a relay receive a signal about a change in voltage and current on a high voltage circuit?

A relay receives a signal about a change in voltage and current through its control circuit, which is typically isolated from the high voltage circuit. When the voltage or current in the high voltage circuit exceeds a certain threshold, it activates an electromagnetic coil within the relay. This coil generates a magnetic field that moves an armature, closing or opening the relay contacts, thus allowing or interrupting the flow of electricity in the controlled circuit. The relay effectively acts as a switch that responds to the changes in the high voltage circuit while maintaining safe isolation.