Protection Function Numbers
Fast lookup for common relay functions used in MV feeders, transformer panels and industrial protection systems.
| Code | Function | Typical use & interpretation |
|---|---|---|
| 21 | Distance protectionOvercurrent & earth fault | Line impedance / zone protection Uses apparent impedance and directional/zone logic to detect line faults. |
| 25 | Synchronism-check / synchronizingControl & system | Safe connection of AC sources Checks synchronism conditions such as voltage, frequency and phase angle before closing. |
| 27 | UndervoltageVoltage & frequency | Voltage supervision and tripping Acts when a selected voltage quantity falls below a defined setting with associated timing. |
| 32 | Directional powerControl & system | Reverse power and power-flow supervision Supervises the sign or direction of active or reactive power according to the relay function. |
| 46 | Negative-sequence current / phase unbalanceTransformer & machine | Motor and generator protection Detects unbalanced currents via negative-sequence or related criteria. |
| 47 | Phase-sequence / phase-balance voltageVoltage & frequency | Phase rotation and voltage balance Supervises phase sequence or voltage unbalance depending on relay implementation. |
| 49 | Thermal protectionTransformer & machine | Thermal overload model Models heating and/or monitors temperature to protect cables, motors or transformers. |
| 50 | Instantaneous overcurrentOvercurrent & earth fault | Fast phase overcurrent protection Operates for current above pickup without an intentional time delay; practical operation is not zero milliseconds. |
| 50N | Instantaneous neutral / residual overcurrentOvercurrent & earth fault | Earth-fault protection Common notation for instantaneous earth/neutral overcurrent sensing or residual-current calculation. N denotes a device-specific neutral/residual measurement convention; confirm CT connections and manufacturer terminology. |
| 51 | Time overcurrentOvercurrent & earth fault | Phase-fault and overload coordination Time-delayed overcurrent element, often inverse-time or definite-time. |
| 51N | Time neutral / residual overcurrentOvercurrent & earth fault | Delayed earth-fault protection Common notation for time-delayed neutral or residual-current protection. N/G suffix usage is not uniform among manufacturers; confirm measurement and setting logic. |
| 51G | Time ground overcurrentOvercurrent & earth fault | Measured ground-current protection Often denotes earth-fault protection using a dedicated ground CT input. G and N distinctions can vary between devices. |
| 59 | OvervoltageVoltage & frequency | Voltage supervision Acts above a voltage pickup with the relay-specific measuring and timing logic. |
| 59N | Neutral displacement / residual overvoltageVoltage & frequency | Earth-fault voltage detection Often supervises residual/zero-sequence voltage (for example calculated or measured 3U0). Meaning and voltage reference depend on measurement wiring and relay documentation. |
| 67 | Directional overcurrentOvercurrent & earth fault | Directional phase-fault protection Combines an overcurrent element with directional information from suitable polarizing quantities. |
| 67N | Directional earth-fault overcurrentOvercurrent & earth fault | Earth faults on interconnected MV feeders Uses residual/zero-sequence current with directional polarization appropriate to the network earthing method. Polarizing methods and signs are relay-specific; do not infer direction from the suffix alone. |
| 74 | Alarm / supervisory relayControl & system | Trip-circuit and status supervision General alarm/supervisory function; application-specific acronyms such as TCS are often used. |
| 79 | Automatic reclosingControl & system | Overhead-line auto-reclose Performs programmed reclose attempts after a protective trip where the system permits it. |
| 81U / 81O | Underfrequency / overfrequencyVoltage & frequency | Frequency protection Supervises low/high frequency using defined thresholds, delays and measurement filters. Suffix nomenclature may be shown as 81<, 81> or device-specific variants. |
| 86 | Lockout / master tripControl & system | Trip memory and interlocking Maintains a protective lockout condition until the specified manual/electrical reset sequence. |
| 87 | Differential protectionTransformer & machine | Internal-zone fault protection Compares zone-boundary currents after proper CT ratio, vector and saturation considerations. |
| 87T | Transformer differential protectionTransformer & machine | Transformer internal-fault protection Common application suffix for transformer differential, including vector-group/CT compensation. T is an application suffix; the underlying device-function number is 87. |