Types of Protection Relays: Functions, Classification and Applications

Protection relays are the “decision makers” of an electrical power system. They watch current, voltage, frequency and other signals, spot a fault, and tell a circuit breaker to open. This guide explains every main type in plain English, with ANSI numbers, simple examples and clear tables.

What is a protection relay?

A protection relay is a device that continuously checks electrical quantities, compares them with set limits, and sends a trip signal when something abnormal happens. Abnormal conditions include short circuits, overloads, earth faults, overvoltage, undervoltage, abnormal frequency and phase imbalance.

One point causes a lot of confusion: the relay does not interrupt the fault current itself. It only makes the decision. The circuit breaker does the physical work of opening. If you want to see how breakers do this, read our guide to the different types of circuit breakers.

Device	Role	Interrupts fault current?
Protection relay	Detects the fault and decides to trip	No
Circuit breaker	Opens the circuit when told to	Yes
Fuse	Senses and breaks the fault in one device (it melts)	Yes
Control relay	Switching, interlocking and signalling (not fault detection)	No

How does a protection relay work?

A relay is one link in a chain called the protection system. Every link must work, or the fault stays on the network.

1. Current transformers (CTs) and voltage transformers (VTs) reduce high system values to safe levels, such as 1 A or 5 A and 110 V.
2. The relay reads these signals and compares them with its settings.
3. If the settings are exceeded, the relay closes a trip contact or sends a digital trip command.
4. A DC battery supply powers the trip coil, so tripping still works if the AC supply fails.
5. The circuit breaker opens and isolates the faulty section.

What makes a good protection system?

Requirement	Meaning in simple words
Sensitivity	Detects even the smallest fault it is meant to detect
Selectivity	Trips only the faulty section, so healthy parts stay live
Speed	Clears faults fast to limit damage and keep the grid stable
Dependability	Operates when it should
Security	Stays quiet when it should not operate

How are protection relays classified?

Each method answers a different question, so do not mix them in one flat list. A numerical relay (technology) can contain overcurrent (function) with an inverse-time curve (operating time) and protect a transformer (equipment) all at once.

Types of protection relays by technology

Relay technology has moved through four generations. Old relays are still working in many substations, so you will often meet all of them on one site.

Feature	Electromechanical	Static (solid-state)	Digital	Numerical / IED
How it works	Magnetic force moves a disc, armature or plunger	Analogue electronic circuits	Samples signals and processes them digitally	Microprocessor runs several protection programs
Functions per device	Usually one	Few	Several	Many, plus control and monitoring
CT burden	High	Lower	Low	Very low
Self-monitoring	None	Limited	Better	Continuous self-checks
Fault records	None	Very limited	Event records	Event, fault and disturbance records
Communication	Hardwired contacts	Limited	Some protocols	IEC 61850, DNP3, Modbus
Maintenance	Frequent mechanical checks	Moderate	Firmware-aware testing	Configuration and cyber checks
Status today	Legacy, still installed	Legacy	Common	Standard for new projects

Electromechanical relays

These work by magnetic attraction or induction. Attracted-armature relays use a plunger or hinged armature. Induction-disc and induction-cup relays use torque on a rotating disc or cup, which is why their operating time falls as current rises. They are rugged and simple, but they do one job each and need regular calibration.

Static relays

Static relays replace moving parts with transistors, diodes and amplifiers. They are faster, smaller and place less load on the CT than electromechanical designs. They are a stepping stone between the old and new generations.

Digital and numerical relays

Digital relays convert the measured signals into numbers and process them in a microprocessor. Numerical relays take this further, running many protection algorithms in one box with programmable logic, event recording and communication. In practice, manufacturers often use “digital” and “numerical” loosely, so treat the line between them as flexible rather than a strict rule.

Intelligent electronic devices (IEDs)

An IED is a numerical relay that also measures, controls and communicates. In a digital substation, IEDs share data over an Ethernet network using IEC 61850, which replaces large amounts of copper wiring. More on this in the numerical relay section below.

Types of protection relays by function

This is the classification that matters most when you choose or read about a relay. Each type below measures something different and guards against a different problem.

Relay type	ANSI no.	IEC symbol	Measures	Protects against	Typical use
Instantaneous overcurrent	50	I>>	Phase current	Severe phase faults	Feeders, transformers, motors
Time overcurrent	51	I>	Phase current	Overloads, lower-level faults	Feeders, transformers, motors
Earth fault	50N / 51N, 50G / 51G	I0>	Residual or neutral current	Faults to earth	Feeders, transformers
Directional overcurrent	67 / 67N	Directional I>	Current and direction	Faults in one direction	Ring mains, parallel feeders
Distance	21	Z<	Impedance (V ÷ I)	Line faults by distance	Transmission lines
Differential	87 (87T, 87G, 87B, 87L, 87M)	ΔI	Current in versus out	Internal faults	Transformers, generators, busbars
Undervoltage	27	U<	Voltage	Low voltage	Motors, buses
Overvoltage	59	U>	Voltage	High voltage	Generators, capacitor banks
Underfrequency	81U	f<	Frequency	Generation shortage	Load shedding
Overfrequency	81O	f>	Frequency	Excess generation	Generators, islands
Reverse power	32	P<	Direction of active power	Generator motoring	Generators
Negative sequence	46	I2>	Unbalanced current	Rotor heating	Motors, generators
Thermal overload	49	θ>	Heating model	Sustained overload	Motors, cables, transformers
Loss of excitation	40	—	Impedance or reactive power	Loss of generator field	Synchronous generators
Overexcitation (V/Hz)	24	V/Hz>	Volts per hertz	Core overheating	Transformers, generators
Buchholz (gas)	63 family	—	Gas and oil surge	Internal transformer faults	Oil-filled transformers
Breaker failure	50BF	—	Current after trip command	A breaker that fails to open	Substations
Auto-reclose	79	—	Breaker and line state	Temporary faults	Overhead lines
Synchro-check	25	—	Voltage, frequency, angle	Unsafe closing	Generators, bus couplers
Lockout	86	—	Trip logic	Unwanted reclosing	Transformer, generator schemes

Overcurrent relay (ANSI 50/51)

The simplest and most common type. It operates when current rises above a set pickup value. The 50 element trips with no intentional delay for large faults; the 51 element trips after a delay that shortens as current increases. It suits radial feeders, motors and transformers, and often acts as backup elsewhere.

Earth fault relay (ANSI 50N/51N)

A normal overcurrent relay is set above load current, so it can miss small earth faults. An earth fault relay measures only the residual (neutral) current, which is close to zero in a healthy system, so it can be set far more sensitively. The “N” suffix means the residual current is calculated from the three phases, while “G” means it comes from a separate core-balance CT.

Directional overcurrent relay (ANSI 67)

Where power can flow both ways, such as ring mains and parallel feeders, a standard overcurrent relay could trip for a fault behind it. A directional relay compares current with a voltage reference and operates only for faults in its set direction.

Distance relay (ANSI 21)

A distance relay divides voltage by current to calculate impedance. Because line impedance is roughly proportional to length, a lower value means a closer fault. Its settings are arranged in zones: Zone 1 trips instantly for faults on most of the protected line, while Zones 2 and 3 are time-delayed backup. Common characteristics are impedance, reactance and mho. It is the main protection for transmission lines.

Differential relay (ANSI 87)

This relay applies Kirchhoff’s current law: current entering a protected zone should equal current leaving it. Any difference means a fault inside. Because it responds only to internal faults, it is fast and very sensitive. Variants include 87T (transformer), 87G (generator), 87B (busbar), 87L (line) and 87M (motor).

Voltage and frequency relays (ANSI 27, 59, 81)

Undervoltage (27) and overvoltage (59) relays watch voltage. Underfrequency (81U) relays support load shedding when generation falls short, and overfrequency (81O) relays protect against too much generation or islanding.

Reverse power, negative sequence and thermal relays (ANSI 32, 46, 49)

A reverse power relay detects a generator drawing power from the grid and acting as a motor, which can damage the prime mover. A negative sequence relay detects unbalanced currents that overheat rotors. A thermal overload relay uses a heating model of the motor, cable or transformer to trip before damage occurs. For motor circuits, it also helps to understand the breakers beside them; see MCB vs MCCB vs ACB.

Loss of excitation and overexcitation relays (ANSI 40, 24)

Loss of excitation (40) protects a synchronous generator if its field is lost. Overexcitation (24) measures volts per hertz and trips when a transformer or generator core risks overheating from over-fluxing.

Buchholz relay (ANSI 63)

A Buchholz relay is a mechanical device between the transformer tank and its conservator. Slow gas build-up from a minor fault triggers an alarm; a sudden oil surge from a serious fault trips the transformer. It detects problems that electrical relays may miss.

Restricted earth fault relay (REF)

REF is a high-sensitivity differential scheme that covers only the winding to earth. It detects low-level earth faults near the neutral of a transformer or generator that a full differential relay might not see.

Arc flash relay

Arc flash relays use light sensors, often with a current check, to detect an arc inside switchgear and trip within a few milliseconds. Their speed greatly reduces damage and risk to people. They are used inside switchgear panels.

Breaker failure, auto-reclose, synchro-check and lockout relays

• Breaker failure (50BF): if current still flows after a trip command, it trips the neighbouring breakers.
• Auto-reclose (79): recloses a line after a trip. Most overhead-line faults, such as lightning, are temporary, so supply is restored quickly. A permanent fault trips it again and locks out.
• Synchro-check (25): allows a breaker to close only when voltage, frequency and phase angle on both sides match.
• Lockout (86): latches the trip and blocks closing until someone resets it by hand.

Pilot and communication-based protection

Pilot schemes compare information from both ends of a line over a communication link. Line differential and phase comparison protection are examples. They clear faults along the whole line quickly, which distance relays alone cannot always do.

Types of protection relays by operating time

Type	How it behaves	Best for
Instantaneous	Trips with no intentional delay once pickup is exceeded	Severe, close-in faults
Definite time	Trips after a fixed delay, whatever the current	Simple grading where fault current barely changes
Inverse time (IDMT)	Higher current means faster trip	Feeders and transformers needing coordination

IDMT stands for inverse definite minimum time. Two settings shape the curve: the pickup current (Is), and the time multiplier setting (TMS), which moves the whole curve up or down. IEC 60255-151 and IEEE C37.112 define the standard shapes. The IEC formula is:

t = TMS × k ÷ ((I ÷ Is)^α − 1)

IEC curve	k	α	At 2 × Is	At 5 × Is	At 10 × Is	At 20 × Is
Standard inverse (SI)	0.14	0.02	10.03 s	4.28 s	2.97 s	2.27 s
Very inverse (VI)	13.5	1	13.50 s	3.38 s	1.50 s	0.71 s
Extremely inverse (EI)	80	2	26.67 s	3.33 s	0.81 s	0.20 s
Long-time inverse (LTI)	120	1	120.00 s	30.00 s	13.33 s	6.32 s

Notice how the extremely inverse curve is slowest at low current but fastest at high current. That steep shape suits fuse coordination and motor feeders, while long-time inverse suits protection that must ride through long starting currents.

Protection relays by protected equipment

Different equipment fails in different ways, so each gets its own mix of relays. The table shows typical functions. It is a guide, not a design recipe.

Equipment	Typical relay functions
Generator	87G differential, 40 loss of excitation, 32 reverse power, 46 negative sequence, 49 thermal, 24 V/Hz, 27/59 voltage, 81 frequency, 50BF
Transformer	87T differential, REF, 50/51 and 50N/51N overcurrent, 63 Buchholz, 24 over-excitation, 49 thermal, 86 lockout
Motor	49 thermal overload, 50/51 overcurrent, 46 unbalance, 27 undervoltage, 50N/51N earth fault, 87M for large motors
Feeder	50/51 and 50N/51N, 67/67N where power flows both ways, 79 auto-reclose, 50BF
Transmission line	21 distance, 87L line differential, 67/67N backup, 79 auto-reclose, pilot schemes, 50BF
Busbar	87B busbar differential (high or low impedance), 50BF, 86 lockout
Capacitor bank	59 overvoltage, unbalance protection, 50/51 overcurrent, 50N/51N earth fault

Most of these relays sit in switchgear panels at substations and plants, which is where you will usually see them in the field.

Protection zones, primary protection and backup

A protection zone is the part of the system a given relay is responsible for. Zones overlap at circuit breakers so that no gap exists between them.

• Primary protection acts first for faults in its own zone.
• Backup protection acts if the primary relay or its breaker fails. Local backup sits at the same location; remote backup sits at the next substation.
• Coordination means setting the time and current of each relay so the nearest one to the fault operates first.

Numerical relays, IEC 61850 and cybersecurity

Modern substations are moving to digital designs. Numerical relays are the building blocks, and IEC 61850 is the language they speak.

• GOOSE messages let IEDs send fast trip and blocking signals to each other over Ethernet instead of copper wires.
• Sampled values carry digitised CT and VT data across the network.
• Recorders store fault and disturbance data, helping engineers find the cause after a trip.
• Self-supervision raises an alarm if the relay itself develops a fault.

Connected relays also create cyber risk. IEC 62351 covers security for power-system communications, and NERC CIP applies to critical assets in North America. Good practice includes controlled access, network separation, secure configuration, firmware control and logging.

ANSI device numbers, IEC symbols and key standards

ANSI numbers (from IEEE C37.2) name the function of a device, such as 51 for time overcurrent. IEC symbols such as I>, U< and Z< describe the measured quantity. They work side by side but are different systems, so never treat them as exact matches. In India you will see both on drawings, along with CEA regulations, CBIP guidance and utility specifications.

Standard	What it covers
IEEE C37.2	Device function numbers, acronyms and contact designations
IEC 60255-151	Functional requirements for over- and under-current protection (Edition 1.0, 2009)
IEEE C37.112	Inverse-time characteristic equations for overcurrent relays (2018)
IEC 61850	Communication networks and systems for power utility automation
IEC 62351	Cybersecurity for power-system communications
IEEE C37.90	Relay performance and environmental requirements

Why protection relays misoperate

A misoperation means the protection failed to operate, operated too slowly, or tripped when it should not. The ReliabilityFirst 2025 assessment recorded 133 misoperations in its region in 2024, down from a high of 277 in 2016. Its 2020–2024 analysis shows where the causes lie:

Human performance and equipment problems account for most cases. The practical lessons are simple:

• Check relay settings and logic against the approved settings file after every change.
• Test CT and VT wiring, polarity and ratios during commissioning.
• Test the trip circuit and DC supply, not only the relay.
• Review event and fault records after each operation.
• Follow the manufacturer’s and utility’s test routines. Secondary injection checks the relay; primary injection checks the whole CT-to-relay path.

How big is the protection relay market?

Published market sizes vary because each research firm uses a different scope. The estimates for 2025 range from about USD 2.27 billion to USD 3.8 billion, so treat them as a range, not a fixed figure. Reports agree that Asia-Pacific is the largest region and that digital and numerical relays are growing fastest.

Frequently asked questions

What are the main types of protection relays?

The main types by function are overcurrent, earth fault, directional, distance, differential, voltage, frequency, reverse power and thermal relays. By technology they are electromechanical, static, digital and numerical.

What is the most commonly used protection relay?

The overcurrent relay (ANSI 50/51) is the most widely used because it is simple and suits feeders, motors and transformers.

Does a protection relay interrupt fault current?

No. The relay detects the fault and sends a trip signal. The circuit breaker interrupts the current.

What is the difference between a numerical and a digital relay?

Both process signals digitally. Numerical relays are the more advanced multifunction type with programmable logic, recording and networking. Manufacturers often use the terms loosely.

What does ANSI device number 21 mean?

It is a distance relay, which measures impedance to find how far along a line a fault lies.

What is an ANSI 50/51 relay?

ANSI 50 is an instantaneous overcurrent function and ANSI 51 is a time-delayed overcurrent function.

What is the function of a differential relay?

It compares current entering and leaving a protected zone and trips if they differ, which indicates an internal fault.

Which relays protect a transformer?

Typically differential (87T), Buchholz (63), restricted earth fault, overcurrent (50/51) and earth fault (50N/51N), plus thermal and over-excitation protection.

What is an IDMT relay?

An inverse definite minimum time relay trips faster as fault current rises. The TMS setting moves its curve up or down.

What is the difference between primary and backup protection?

Primary protection acts first in its own zone. Backup protection acts only if the primary protection or its breaker fails.

Conclusion

Protection relays can be sorted by technology, function, operating time and protected equipment. For day-to-day work, function is the most useful lens: overcurrent and earth fault for feeders, distance for lines, differential for transformers, generators and busbars, and voltage, frequency and thermal relays for supporting roles. Pair that with the right zones, backup and coordination, and numerical relays give a fast, well-recorded and reliable protection system.

Sources

• IEEE C37.2 – Device function numbers
• IEEE C37.112-2018 – Inverse-time characteristics
• IEC 60255-151:2009 – Over/under-current relays
• NERC MIDAS data reporting instructions (Composite Protection System)
• ReliabilityFirst 2025 Misoperation Assessment
• Schneider Electric – Protection relays