ANSI Device Numbers

Adds a set delay before an operation in a sequence.

Trips when the impedance seen from the relay falls inside a set reach — used on transmission lines.

Protects transformers and generators from overexcitation.

Permits closing only when voltage, frequency and phase angle on both sides match closely enough.

Operates when voltage falls below a setting; used for load shedding, transfer and motor protection.

Detects power flowing the wrong way, such as a generator motoring or export past a utility interconnection.

Responds to excessive bearing temperature.

Detects loss of generator excitation.

Protects rotating machines from heating caused by unbalanced or single-phased supply.

Blocks or trips on reversed phase rotation or unbalanced voltage.

Models or measures equipment temperature, typically motor or transformer overload.

Trips without intentional delay above a current setting.

Trips backup breakers if the primary breaker does not clear a fault in time.

Trips after a delay that shortens as current rises (an inverse curve).

Overcurrent elements applied to residual or ground current. The suffix identifies how the current is measured.

The breaker itself; 52a and 52b are its auxiliary contacts that follow and oppose its position.

Operates when voltage rises above a setting.

Detects ground faults on ungrounded or high-resistance-grounded systems from neutral displacement.

Compares two circuits, often to detect a blown VT fuse.

Adds a delay to an opening or stopping sequence.

Responds to gas or liquid pressure; on transformers, the sudden-pressure relay.

Detects insulation failure to ground, for example 64G on generator stator windings.

Overcurrent protection that operates only for current flowing in a chosen direction.

Blocks tripping or reclosing under set conditions, such as power swings.

Drives an alarm rather than a trip.

Detects loss of synchronism.

Recloses a breaker automatically after a trip, usually on overhead lines.

Operates on under- or over-frequency (81U / 81O), for load shedding and generator protection.

Selects between sources or conditions automatically.

Trips and holds breakers open until manually reset, so equipment is not re-energized into a fault.

Compares current entering and leaving a zone (transformer 87T, bus 87B, generator 87G, line 87L) and trips for faults inside it.

A disconnect or load-break switch.

Trips a breaker or prevents it from reclosing.