Electromechanical device containing a coil and set of contacts.
Relay contacts can be in one of two states:
- normally open (NO)
- normally closed (NC).
Relays may be operated electricly, using pneumatics, or a combination of both.
The load impedance imposed by a relay on its source, expressed in ohms and phase angle at specified conditions.
Modern solid state protective relays have very small burdens, while some older electromechanical relays had significant burdens, requiring the use of high output instrument transformers to drive them.