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Protective Relay Terms and Definitions
Acceptance test
A calibration and functional test of a new or replacement relay to ensure it is in proper working order prior to installation.
Air gap
Sometimes used for contact separation or for magnetic air gap.
Ampere-turns
The product of the number of turns in a magnetic coil and the root means square (rms) current in amperes passing through the coil.
Armature
Hinged or pivoted moving part of the magnetic circuit of an electromagnetic relay. It is used in a general sense to mean any moving part that actuates contacts in response to a change in coil current.
Armature contact
Sometimes used for a movable contact.
Armature relay
A relay operated by an electromagnet that, when energized, causes an armature to be attracted to a fixed pole (or poles).
Auxiliary relay
A relay that operates in response to opening and closing of its operating circuit to assist another relay or device in performing a function.
Back contacts
Sometimes used for the stationary contact of single-pole, normally closed contacts. Same as normally closed contacts.
Backstop
The part of a relay that limits movement of the armature away from the pole piece or core.
Backup relaying
Supplementary relaying designed to operate if a primary relay should malfunction or a circuit breaker fails to operate. Backup relaying usually disconnects more of the power system than just the part with the faulty element, since this is necessary to remove the abnormal condition and to minimize the effect on the remainder of the system.