ANSI/IEEE device numbers are the compact language of power-system protection and control. They identify functions on single-line diagrams, schematics, trip matrices, relay settings and test documents without tying the design to one manufacturer.
This guide follows the active IEEE C37.2-2022 structure. The explanations below are original practical summaries—not copied standard definitions—and show both the operating principle and a typical application. Many numbers describe control, mechanical or auxiliary functions rather than protection relays, so the complete list is much wider than the familiar 21, 27, 50, 51, 67, 81 and 87 functions.
How to read a device code
- Base number: defines the general function, for example 50 for instantaneous overcurrent.
- Suffix: narrows the application, for example 50N for neutral/residual overcurrent or 87T for transformer differential.
- Combined numbers: show two important functions in one device, for example 50/51.
- Function 11: may identify a multifunction IED, but all enabled protection and control functions still need to be documented.
- Project legend: remains essential because suffix letters can have different meanings in different schemes.
Current-standard warning: Under IEEE C37.2-2022, numbers 95 through 99 are reserved for project-specific functions when no assigned number or official acronym is suitable. Therefore, 95 is not universally “Trip Circuit Healthy,” and 96 is not universally “Transmitter.” The official acronym for trip-circuit monitoring is TCM.
The most-used MV protection codes
| Code | Meaning | What the element evaluates |
|---|---|---|
| 21 | Distance protection | Apparent impedance/admittance and zone characteristic |
| 24 | Volts per hertz | V/f ratio for overfluxing |
| 27 / 59 | Under/overvoltage | Voltage below/above pickup |
| 32 | Directional power | Magnitude and direction of active/reactive power |
| 46 | Current unbalance | Negative-sequence or unbalanced current |
| 49 | Thermal protection | Measured temperature or thermal model |
| 50 / 51 | Instantaneous / inverse-time overcurrent | Current magnitude and operating time |
| 50BF | Current-monitored breaker failure | Trip initiation, breaker status and continued current |
| 67 / 67N | Directional phase / earth overcurrent | Overcurrent plus fault direction |
| 79 | Automatic reclosing | Trip cause, dead time, close permissives and reclose attempts |
| 81U / 81O / 81R | Under/overfrequency and ROCOF applications | Frequency or rate of change of frequency |
| 86 | Lockout | Latching master-trip logic requiring deliberate reset |
| 87 | Differential protection | Difference between quantities at the boundaries of a protected zone |
| TCM | Trip circuit monitor | Trip-path continuity and availability of trip voltage |
Complete ANSI/IEEE device-number list
Numbers 1–20
| No. | Function | How it works | Typical application |
|---|---|---|---|
| 1 | Master element | Initiates the overall command that places equipment in or out of service, directly or through permissive logic. | Master start/stop control in unit and plant schemes. |
| 2 | Time-delay starting or closing relay | Adds an intentional delay before or after a step in a starting, closing or protection sequence. | Delayed closing, staged starting and sequence timing. |
| 3 | Checking or interlocking relay | Evaluates equipment position or other conditions and permits, blocks or stops an operating sequence. | Breaker, disconnector and earthing-switch interlocks. |
| 4 | Master contactor | Makes or breaks the control circuits required to start equipment under permitted conditions or remove it under abnormal conditions. | Master control of machines and process units. |
| 5 | Stopping device | Shuts equipment down and holds it out of operation; unlike 86, it is not specifically the electrical lockout function for faults. | Normal stop and emergency-stop control circuits. |
| 6 | Starting circuit breaker | Connects a machine to its source of starting voltage. | Starting large motors, generators or converters. |
| 7 | Rate-of-change relay | Operates when the time rate of change of a measured quantity exceeds a set limit. | Rapid-change detection; the measured quantity must be defined in the project. |
| 8 | Control power disconnecting device | Connects or isolates the source feeding control buses and auxiliary circuits. | DC or AC control-supply isolation in switchgear and control panels. |
| 9 | Reversing device | Reverses a machine field, phase connection or another defined operating function. | Reversing motor starters and rotating-machine control. |
| 10 | Unit sequence switch | Changes the order in which multiple units are placed in or out of service. | Multi-pump, compressor or generating-unit sequencing. |
| 11 | Multifunction device | Performs three or more important functions; the enabled functions must be listed in drawings or settings records. | Modern numerical protection relays and bay-control IEDs. |
| 12 | Overspeed device | Operates when machine speed rises above its permitted value. | Turbines, generators and large motors. |
| 13 | Synchronous-speed device | Detects operation at or near synchronous speed using speed, slip frequency, voltage or another suitable quantity. | Synchronous-machine starting and field-application schemes. |
| 14 | Underspeed device | Operates when machine speed falls below a preset value. | Motors, turbines, conveyors and process drives. |
| 15 | Speed or frequency matching device | Matches and holds the speed or frequency of one machine or system close to another. | Synchronizing controls and parallel operation. |
| 16 | Data communications device | Supports serial or Ethernet communications used for protection, automation and control; suffixes identify functions such as switch, gateway or firewall. | Substation networks, protocol gateways and managed Ethernet switches. |
| 17 | Shunting or discharge switch | Opens or closes a bypass or discharge path around apparatus such as a field, capacitor or reactor, excluding resistor switching assigned to 73. | Field discharge and capacitor/reactor discharge circuits. |
| 18 | Accelerating or decelerating device | Closes circuits that increase or decrease machine speed. | Stepped motor control and legacy machine acceleration schemes. |
| 19 | Starting-to-running transition contactor | Transfers a machine automatically from its starting connection to its normal running connection. | Reduced-voltage, star-delta and other motor-starting arrangements. |
| 20 | Electrically-operated valve | Electrically operates or controls a valve in a liquid, gas, air or vacuum line. | Hydraulic breaker mechanisms, cooling, gas and vacuum systems. |
Numbers 21–40
| No. | Function | How it works | Typical application |
|---|---|---|---|
| 21 | Distance (impedance) relay | Calculates apparent impedance or admittance from electrical quantities and operates when the result crosses a defined characteristic. | Transmission and sub-transmission line protection; selected MV line applications. |
| 22 | Equalizer circuit breaker | Makes or breaks equalizing or current-balancing connections between units or regulating equipment. | Older multi-unit machine-field and regulator installations. |
| 23 | Temperature control device | Switches control equipment when a measured temperature crosses a set level. | Switchgear space heaters, ventilation and cooling control. |
| 24 | Volts-per-hertz relay | Monitors the voltage-to-frequency ratio and operates above or below assigned limits. | Transformer and generator overfluxing protection. |
| 25 | Synchronizing or synchronism-check relay | Checks voltage magnitude, phase angle and frequency difference before allowing two energized systems to be connected. | Generator, incomer, bus-coupler and transfer closing circuits. |
| 26 | Apparatus thermal device | Responds to excessive or low temperature of apparatus or a medium, excluding the load-carrying windings specifically covered by 49. | Oil, enclosure, cooling-medium and equipment temperature supervision. |
| 27 | Undervoltage relay | Operates when measured voltage falls below a setting, with any required delay and phase logic. | Bus supervision, motor protection, transfer schemes and generator protection. |
| 28 | Flame detector | Detects presence or loss of pilot or main flame. | Boilers, gas turbines and combustion systems. |
| 29 | Isolating contactor or switch | Disconnects one circuit from another specifically for emergency operation, maintenance or testing. | Control, test and maintenance isolation. |
| 30 | Annunciator relay | Provides multiple non-self-resetting visual indications following operation of protective or control devices. | Substation alarm panels and first-out indication. |
| 31 | Separate excitation device | Connects a machine field circuit to a separate excitation source during starting. | Synchronous converters and legacy rotating-machine systems. |
| 32 | Directional power relay | Measures power magnitude and direction and operates when flow in the selected direction crosses a setting. | Generator reverse power, import/export limits and anti-motoring protection. |
| 33 | Position switch | Changes contact state when equipment reaches a specified position. | Racking, mechanism, valve and switch position indication. |
| 34 | Master sequence device | Establishes the operating order of major equipment using multi-contact or programmable logic. | Automatic plant start/stop and sequential switching. |
| 35 | Brush-operating or slip-ring short-circuiting device | Moves machine brushes, shorts slip rings or operates mechanical rectifier contacts. | Wound-rotor motors and older rotating machines. |
| 36 | Polarity or polarizing-voltage device | Permits operation only for a defined polarity or confirms that a polarizing voltage is available. | DC schemes and supervision of directional-relay polarizing quantities. |
| 37 | Undercurrent or underpower relay | Operates when current or power falls below a threshold. | Motor underload, pump dry-running and generator low-forward-power protection. |
| 38 | Bearing protective device | Responds to excessive bearing temperature, wear or another abnormal bearing condition. | Motors, generators, turbines and large rotating equipment. |
| 39 | Mechanical condition monitor | Detects abnormal mechanical conditions other than bearing-specific conditions, such as vibration, eccentricity, expansion or seal failure. | Turbines, generators, motors and compressors. |
| 40 | Field (over/under-excitation) relay | Detects an abnormal machine excitation condition, commonly from voltage, current, reactive power or impedance relationships. | Synchronous generator loss-of-excitation and overexcitation schemes. |
Numbers 41–60
| No. | Function | How it works | Typical application |
|---|---|---|---|
| 41 | Field circuit breaker | Opens or closes the field circuit of a machine. | Generator and synchronous-machine excitation systems. |
| 42 | Running circuit breaker | Connects a machine to its normal running supply; the designation can also cover a frequently operated running contactor. | Large motor and machine running circuits. |
| 43 | Manual transfer or selector device | Manually transfers control or voltage circuits to change the operating plan. | Local/remote, auto/manual and source-selection switches. |
| 44 | Unit sequence starting relay | Starts the next available unit when the normally preceding unit is unavailable. | Standby pumps, compressors and generating units. |
| 45 | Abnormal atmospheric condition monitor | Responds to smoke, fire, damaging fumes, explosive mixtures or another defined atmospheric hazard. | Substations, battery rooms and enclosed electrical rooms. |
| 46 | Reverse-phase or phase-balance current relay | Detects reverse sequence, phase-current unbalance or excessive negative-sequence current. | Motor and generator protection against unbalance and phase loss. |
| 47 | Phase-sequence or phase-balance voltage relay | Detects wrong phase sequence, voltage unbalance or excessive negative-sequence voltage. | Motor supplies, source verification and phase-loss/reversal supervision. |
| 48 | Incomplete sequence relay | Trips, resets or locks out equipment when a starting, operating or stopping sequence is not completed in time. | Motor start failure, locked rotor and process-sequence supervision. |
| 49 | Machine or transformer thermal relay | Uses measured temperature or a thermal model to protect current-carrying windings and elements from overheating. | Motors, transformers, generators and thermally limited cables. |
| 50 | Instantaneous overcurrent relay | Operates when current exceeds a pickup value with no intentional delay; 50TD denotes intentional definite time and 50BF current-monitored breaker failure. | Fast short-circuit protection and breaker-failure schemes. |
| 51 | AC inverse-time overcurrent relay | Operates above current pickup with an operating time that generally decreases as current increases. | Time-graded feeder, incomer and backup protection. |
| 52 | AC circuit breaker | Makes and interrupts an AC power circuit under normal, fault or emergency conditions. | The primary interrupting device in MV switchgear. |
| 53 | Field excitation relay | Forces DC machine excitation to build during starting or responds when machine voltage reaches a defined value. | DC generators and excitation systems. |
| 54 | Turning-gear engaging device | Controls or supervises engagement and disengagement of a turning gear with a machine shaft. | Large turbine-generator maintenance and cooldown systems. |
| 55 | Power factor relay | Operates when AC power factor rises above or falls below a set boundary. | Generator, synchronous motor and capacitor-control applications. |
| 56 | Field application relay | Applies field excitation to a synchronous motor at the correct point in its slip cycle. | Synchronous-motor starting. |
| 57 | Short-circuiting or grounding device | Automatically or manually shorts or grounds a circuit. | Earthing switches, crowbar circuits and field-discharge arrangements. |
| 58 | Rectification failure relay | Detects failure of a power rectifier or diode to conduct or block correctly. | Excitation systems, chargers and rectifier banks. |
| 59 | Overvoltage relay | Operates when measured voltage exceeds a setting. | Bus, generator, transformer, capacitor-bank and neutral-displacement protection. |
| 60 | Voltage or current balance relay | Compares two circuit inputs or outputs and operates when their voltage or current difference exceeds a criterion. | CT/VT circuit balance, dual-source comparison and unbalance schemes. |
Numbers 61–80
| No. | Function | How it works | Typical application |
|---|---|---|---|
| 61 | Density switch or sensor | Responds to a density threshold or rate of change of density. | Temperature-compensated insulating-gas monitoring in switchgear. |
| 62 | Time-delay stopping or opening relay | Adds an intentional delay to a shutdown, stopping or opening command; 62BF is commonly a breaker-failure scheme without current monitoring. | Delayed shutdown, opening logic and non-current-monitored breaker failure. |
| 63 | Pressure switch | Responds to a pressure threshold or a defined rate of pressure change. | Transformer sudden-pressure devices, gas systems and hydraulic mechanisms. |
| 64 | Ground detector relay | Detects insulation failure from normally insulated machine or apparatus circuits to ground; it is not the normal CT residual-overcurrent designation. | Generator field/stator ground and ungrounded-system insulation supervision. |
| 65 | Governor | Regulates the fluid, steam or other medium supplied to a prime mover to control starting, speed, load or stopping. | Hydro and turbine generator control. |
| 66 | Notching or jogging device | Limits operations within a time period or periodically energizes a circuit for incremental movement. | Motor starts-per-hour protection, breaker operation limits and machine jogging. |
| 67 | AC directional overcurrent relay | Combines overcurrent pickup with a directional decision using a polarizing quantity. | Ring networks, parallel sources, distributed generation and directional earth-fault protection. |
| 68 | Blocking relay | Initiates or applies blocking to prevent tripping for external faults, power swings or out-of-step conditions. | Communications-assisted line protection and power-swing blocking. |
| 69 | Permissive control device | Allows a control action only when specified conditions are satisfied. | Close permissives, process interlocks and transfer schemes. |
| 70 | Rheostat | Varies resistance in an electrical circuit and includes electrically operated or position-supervised arrangements. | Field, speed and legacy regulator control. |
| 71 | Liquid level switch | Operates at a liquid-level threshold or a specified rate of level change. | Transformer oil, sump, coolant and hydraulic-reservoir supervision. |
| 72 | DC circuit breaker | Makes and interrupts a DC power circuit under normal or fault conditions. | DC traction, battery, excitation and industrial DC systems. |
| 73 | Load-resistor contactor | Switches a load-limiting, starting or braking resistor. | Motor starting, dynamic braking and field-discharge systems. |
| 74 | Alarm relay | Operates a visual, audible or transmitted alarm; unlike 30, it is not specifically a multi-point non-resetting annunciator. | Protection alarms, trip-circuit alarms and equipment warning circuits. |
| 75 | Position changing mechanism | Moves a main device from one defined position to another. | Circuit-breaker truck racking and movable equipment positioning. |
| 76 | DC overcurrent relay | Operates when current in a DC circuit exceeds a setting. | Battery feeders, excitation systems and DC distribution. |
| 77 | Telemetering device | Transmits a signal representing a measured quantity or receives and reconstructs that quantity at a remote location. | Legacy remote metering and analog telemetry. |
| 78 | Phase-angle measuring relay | Operates for a specified phase angle between two voltages, two currents, or a voltage and current. | Out-of-step detection, pole-slip protection and phase-angle supervision. |
| 79 | AC reclosing relay | Controls automatic reclosing and lockout of an AC interrupter after a trip. | Overhead feeders and line circuits where faults may be temporary. |
| 80 | Flow switch | Operates at a fluid or gas flow threshold or rate of flow change. | Transformer cooling, pumps, gas and hydraulic systems. |
Numbers 81–99
| No. | Function | How it works | Typical application |
|---|---|---|---|
| 81 | Frequency relay | Responds to frequency or rate of change of frequency above or below assigned limits. | Under/overfrequency, load shedding, generation and loss-of-mains schemes. |
| 82 | DC load-measuring reclosing relay | Controls automatic closing or reclosing of a DC interrupter, generally using load-circuit conditions. | DC traction and industrial DC networks. |
| 83 | Automatic selective control or transfer relay | Automatically selects between sources or conditions and performs a transfer operation. | Automatic transfer schemes and redundant auxiliary supplies. |
| 84 | Operating mechanism | Represents the complete electrical mechanism or servomechanism of equipment not otherwise assigned a function number. | Tap changers, induction regulators and special mechanisms. |
| 85 | Pilot communications, carrier or pilot-wire relay | Has its operation, restraint or logic modified by a protection signal sent or received over communications. | Line differential, transfer-trip and permissive/blocking schemes. |
| 86 | Lockout relay | Trips and latches associated equipment out of service until deliberately reset locally or remotely. | Transformer, busbar, generator and serious equipment-fault master trip. |
| 87 | Differential protective relay | Compares two or more currents or other quantities and operates on their percentage, phase-angle or quantitative difference. | Busbar, transformer, motor, generator and line unit protection. |
| 88 | Auxiliary motor or motor generator | Drives auxiliary equipment such as pumps, fans, exciters or rotating amplifiers. | Station auxiliaries and machine support systems. |
| 89 | Line switch | Disconnects, isolates or load-switches an AC or DC power circuit, especially when electrically operated or fitted with electrical accessories. | Disconnectors, load-break switches and circuit switchers. |
| 90 | Regulating device | Maintains one or more quantities near a target or within a control band. | Automatic voltage control, tap regulation, speed and load control. |
| 91 | Voltage directional relay | Responds when voltage across an open switching device exceeds a value in a defined direction. | Special source-transfer and legacy directional-voltage schemes. |
| 92 | Voltage and power directional relay | Permits connection based on directional voltage difference and disconnects for excessive reverse power. | Legacy tie and source-paralleling control. |
| 93 | Field-changing contactor | Raises or lowers machine field excitation by one step. | Generator and synchronous-machine excitation control. |
| 94 | Tripping or trip-free relay | A self-resetting relay that trips equipment, permits immediate tripping, or prevents immediate reclose while a closing command persists. | Master trip output and trip-free breaker control. |
| 95 | Project-specific function | Available only when no standard number 1–94 or official function acronym is suitable; its meaning must be defined. | Unique, documented project applications only. |
| 96 | Project-specific function | Available only when no standard number 1–94 or official function acronym is suitable; it has no universal IEEE meaning. | Unique, documented project applications only. |
| 97 | Project-specific function | Available only when no standard number 1–94 or official function acronym is suitable. | Unique, documented project applications only. |
| 98 | Project-specific function | Available only when no standard number 1–94 or official function acronym is suitable. | Unique, documented project applications only. |
| 99 | Project-specific function | Available only when no standard number 1–94 or official function acronym is suitable. | Unique, documented project applications only. |
The 22 official IEEE C37.2-2022 function acronyms
The 2022 standard also defines function acronyms that are used in the same manner as device numbers. These prevent the creation of unofficial numbers above 99 and cover modern digital-substation, recording and monitoring functions.
| Acronym | Official function | Practical meaning |
|---|---|---|
| AFD | Arc flash detector | Detects an unintended electrical arc; used in fast switchgear arc-flash schemes. |
| CLK | Clock or timing source | Receives and distributes an accurate time signal such as GNSS-derived IRIG-B or PTP. |
| DDR | Dynamic disturbance recorder | Records low-frequency oscillations and abnormal frequency or voltage excursions for system analysis. |
| DFR | Digital fault recorder | Captures triggered voltage/current waveforms and discrete states around power-system faults. |
| ENV | Environmental sensor | Measures environmental variables such as weather, ice, geomagnetic or seismic conditions. |
| FDC | Fault data concentrator | Retrieves and stores fault records from multiple substation IEDs for engineering access. |
| FLOC | Fault locator | Calculates or reports the estimated location of a fault. |
| HIZ | High-impedance fault detector | Detects faults whose current may be too low or irregular for conventional overcurrent elements. |
| HMI | Human-machine interface | Displays system information and accepts authorised operator control. |
| HST | Historian | Continuously gathers states and values so trends and past behaviour can be analysed. |
| IID | Instantaneous impulse detection | Triggers action when measured impulses exceed time, amplitude or count criteria. |
| LGC | Scheme logic | Implements programmed multi-device protection, interlocking or remedial-action logic. |
| MET | Substation metering | Calculates and stores electrical quantities such as volts, amps, power, demand and energy. |
| MU | Merging unit | Converts sensor inputs into time-synchronised digital measurements and may interface binary I/O. |
| PDC | Phasor data concentrator | Collects and forwards synchrophasor and discrete data from PMUs or other PDCs. |
| PMU | Phasor measurement unit | Produces accurately time-referenced phasors, frequency and related measurements. |
| PQM | Power quality monitor | Monitors power-quality quantities such as voltage variations, unbalance, harmonics and transients. |
| RAS | Remedial action scheme | Monitors system conditions and automatically applies predetermined corrective action; also called SPS in some projects. |
| RIO | Remote input/output device | Interfaces process signals with substation automation without acting as the main SCADA control-centre gateway. |
| RTU | Remote terminal unit / data concentrator | Provides the primary data and command interface between the substation and the SCADA control centre. |
| SER | Sequence of events recorder | Time-tags and records changes of state for post-event reconstruction. |
| TCM | Trip circuit monitor | Supervises breaker trip-circuit continuity and trip voltage, alarming when either is lost. |
Common suffixes and combined designations
| Designation | Practical interpretation | Typical application |
|---|---|---|
| 21P / 21G | Phase / ground distance | Line protection zones |
| 27P / 27N | Phase / neutral or residual undervoltage, as defined by the project | Bus and generator schemes |
| 32R / 32F | Reverse / forward power | Generator anti-motoring and power-flow limits |
| 46 | Often implemented as negative-sequence current I2 or an I2²t model | Motor and generator rotor-heating protection |
| 49M / 49T | Motor / transformer thermal application | Thermal image or winding-temperature protection |
| 50P / 51P | Phase overcurrent | Feeder phase faults |
| 50N / 51N | Neutral or residual overcurrent | Earth-fault protection using residual connection or calculated residual |
| 50G / 51G | Ground overcurrent; verify whether G means ground or generator in the specific drawing | Core-balance CT or ground-current input |
| 50BF | Current-monitored breaker failure | Retrip and upstream/adjacent breaker tripping |
| 50TD | Definite-time overcurrent | Fixed-delay overcurrent stages |
| 51V | Voltage-restrained or voltage-controlled time overcurrent, depending on the scheme definition | Generator backup protection |
| 52a | Auxiliary contact open when the breaker is open; closes when the breaker closes | Closed-position indication and trip-circuit logic |
| 52b | Auxiliary contact closed when the breaker is open; opens when the breaker closes | Open-position indication and close-circuit logic |
| 59N | Neutral/residual overvoltage | Earth-fault detection in impedance-earthed or unearthed systems |
| 64REF / 87N | Common project/manufacturer designations for restricted earth-fault protection | Transformer or generator winding zones |
| 67P / 67N / 67G | Directional phase / neutral / ground overcurrent | Parallel and ring networks |
| 81U / 81O / 81R | Underfrequency / overfrequency / rate-of-change-of-frequency application | Load shedding, generation and loss-of-mains schemes |
| 87B / 87T / 87M / 87G / 87L | Busbar / transformer / motor / generator / line differential | Fast unit protection with defined CT boundaries |
Suffix caution: C37.2 allows prefixes and suffixes to clarify a function, but their exact meaning should be defined in the drawing legend, device-function list or relay-setting record. For example, G may mean ground in 51G but generator in 87G.
Examples by protected object
| Protected object | Common functions | Engineering purpose |
|---|---|---|
| MV feeder | 50/51, 50N/51N, 67/67N, 46, 50BF, 79, 52 | Phase/earth faults, directionality, breaker backup and optional reclosing |
| Power transformer | 87T, 64REF or 87N, 49, 24, 50/51, 63, 86 | Internal winding faults, earth faults, thermal/overfluxing conditions and mechanical protection |
| MV motor | 49, 48 or 51LR, 46, 50/51, 37, 27, 66 | Thermal overload, stalled start, unbalance, short circuit, underload and start-frequency limits |
| Busbar | 87B, 50BF, 86 | Fast bus-zone protection, failed-breaker backup and latching trip |
| Generator | 87G, 40, 32R, 46, 49, 24, 27/59, 81 | Internal faults, excitation, reverse power, unbalance, thermal, voltage and frequency conditions |
Why the code alone is not enough
An ANSI/IEEE number identifies the function, not the full protection design. 51 does not state the CT ratio, pickup, curve family, time multiplier, reset behaviour, directional supervision, inrush blocking or trip destination. Likewise, 87T does not define transformer vector-group compensation, differential slope, inrush restraint, zero-sequence elimination or CT requirements.
Every project should therefore provide, at minimum:
- a protection philosophy and single-line diagram;
- a function list with every suffix defined;
- CT/VT ratios, classes, polarity and circuit arrangement;
- settings and coordination calculations;
- logic diagrams, trip matrix and breaker-failure scope;
- test procedures and final as-left records.
Related lesson
Before applying the codes, read Electrical Protection in MV Switchgear: A Practical Guide for protection zones, selectivity, primary and backup protection, the trip chain and common MV fault examples.
Primary reference
Engineering note: This guide is an educational interpretation. Where a contractual or safety decision depends on the exact designation, consult the licensed current standard, the project legend and the protection-relay manufacturer’s documentation.