What Is a 47 Relay? Function, Types & Applications Explained

What Is a 47 Relay?

A 47 relay is a phase sequence voltage relay — a protective device used in three-phase electrical systems to detect incorrect phase rotation or phase sequence conditions. In simple terms, it monitors the order in which voltage phases (typically labeled A, B, and C, or L1, L2, and L3) reach their peak values. If the sequence is wrong — for example, if phases arrive in the order A-C-B instead of the expected A-B-C — the relay trips or signals an alarm to prevent equipment damage.

The designation "47" comes from the ANSI/IEEE standard device numbering system, where each number corresponds to a specific type of protective relay function. Device number 47 is formally defined as the phase sequence voltage relay, and this numbering is universally used by electrical engineers, protection specialists, and equipment manufacturers worldwide.

This relay is especially critical in industrial facilities where three-phase motors, compressors, pumps, and HVAC systems depend on correct phase rotation to operate safely. A reversed phase sequence can cause a motor to spin in the wrong direction — a potentially catastrophic outcome in applications like conveyor belts, elevators, centrifugal pumps, and industrial fans.

The ANSI Device Numbering System and Why Number 47 Matters

The ANSI/IEEE C37.2 standard defines a set of device function numbers to standardize the identification of protective relay functions used in power systems. This system makes it easier for engineers across the globe to read single-line diagrams and protection schemes without ambiguity. Each number represents a distinct protection function:

• Device 27 — Undervoltage relay
• Device 46 — Reverse phase current relay or phase balance current relay
• Device 47 — Phase sequence voltage relay
• Device 51 — AC time overcurrent relay
• Device 59 — Overvoltage relay
• Device 67 — AC directional overcurrent relay

Among these, the 47 relay holds a unique role because it addresses a fault condition that is invisible to most other relay types: wrong phase sequence. An overvoltage relay won't detect a phase reversal. An overcurrent relay won't either. Only a dedicated phase sequence relay — the 47 relay — is designed specifically for this task.

Some modern multifunction protective relays incorporate the 47 function alongside others (such as 27, 59, and 46) in a single device, but the function itself remains defined by its ANSI number.

How a 47 Relay Works: The Technical Mechanics

Understanding the working principle of a 47 relay requires a basic understanding of three-phase power systems. In a standard three-phase system, voltages on phases A, B, and C are separated by 120° each and rotate in a fixed sequence. This rotation — called positive sequence — is what allows three-phase motors to generate torque in the correct direction.

Positive Sequence vs. Negative Sequence

Using symmetrical component analysis, any unbalanced three-phase system can be broken into three components:

• Positive sequence (V1): Balanced, normal rotation (A-B-C)
• Negative sequence (V2): Balanced, reversed rotation (A-C-B)
• Zero sequence (V0): In-phase components, associated with ground faults

A 47 relay monitors the ratio of negative sequence voltage to positive sequence voltage. When the negative sequence component rises above a set threshold — typically caused by a phase reversal, an open phase, or a severe phase imbalance — the relay detects this condition and issues a trip command or alarm.

Detection Methods Used in Modern 47 Relays

Older electromechanical 47 relays used rotating disc or induction cup mechanisms with auxiliary voltage transformers to detect phase sequence. Modern digital or numerical relays calculate the symmetrical components mathematically using sampled voltage data from potential transformers (PTs) connected to the bus or feeder being protected.

A typical digital 47 relay samples the three-phase voltages at rates of 1,000 to 10,000 samples per second and computes V1 and V2 continuously. If the ratio V2/V1 exceeds the pickup threshold — commonly set between 10% and 25% depending on the application — the relay initiates a trip with a programmable time delay.

Some 47 relays also incorporate a minimum positive sequence voltage threshold to prevent false trips during system startup or when all three voltages are low, such as during a blackout recovery.

What Faults Does a 47 Relay Detect?

The 47 relay is designed to respond to a specific class of voltage abnormalities in three-phase systems. Here are the primary fault conditions it detects:

Phase Reversal

This is the most common reason a 47 relay is installed. Phase reversal occurs when two supply phases are accidentally swapped — most often during maintenance, reconnection of cable terminations, or incorrect installation. For example, if an electrician swaps the connections for phases B and C, the sequence becomes A-C-B instead of A-B-C. The negative sequence component immediately jumps to nearly 100% of the positive sequence magnitude, and the relay trips within milliseconds to seconds depending on the time delay setting.

Without the 47 relay, a three-phase induction motor connected to a reversed sequence will start spinning backward. In a pump application, this can cause water hammering, seal damage, or flooding. In an elevator system, it could mean the car moves in the opposite direction from intended — a serious safety risk.

Phase Unbalance

A 47 relay also responds to severe voltage unbalance — a condition where the three phase voltages are not equal. According to NEMA standards, a voltage unbalance exceeding 5% can reduce motor efficiency and lifespan significantly. A 3.5% voltage unbalance can cause a 25% increase in motor heating. At unbalance levels above 5–10%, the negative sequence voltage rises enough to trigger the 47 relay, providing protection before thermal damage occurs.

Single Phasing (Loss of One Phase)

When one of the three supply phases is lost — due to a blown fuse, a tripped breaker on one phase, or a broken conductor — the system operates on only two phases. This condition generates a very large negative sequence component and causes motors to experience extreme thermal stress. A 47 relay picks up on this severe unbalance and trips the equipment before it burns out. Some studies have shown that single phasing is responsible for over 30% of three-phase motor failures in industrial environments where protection is inadequate.

Incorrect Phase Sequence at Energization

In generator protection and bus transfer applications, a 47 relay prevents energizing a load bus or closing a breaker when the incoming supply has an incorrect phase sequence relative to the reference system. This is especially important in synchronizing applications where two sources must match not only in voltage and frequency but also in phase rotation before being paralleled.

Types of 47 Relays

47 relays are available in several technology generations, each with different characteristics suited to different applications:

Standalone vs. Integrated 47 Relay Elements

Standalone 47 relay units are still widely used in motor control panels and smaller industrial installations. They typically have a voltage input, a set of output contacts (normally open and normally closed), and a simple LED indicator for status. Examples include devices from manufacturers like Schneider Electric, ABB, and Siemens that are DIN-rail mounted and cost between $50 and $300 depending on features.

In larger power systems, the 47 function is usually embedded within a multifunction Intelligent Electronic Device (IED) such as the GE Multilin 469, SEL-300G, or Schweitzer SEL-547. These devices combine 10 to 30 relay functions in a single chassis, communicate over SCADA networks via IEC 61850 or DNP3, and provide event logging, oscillography, and remote setting capabilities — features impossible in standalone electromechanical units.

Where Is a 47 Relay Used? Real-World Applications

The 47 relay appears across a wide range of industries and system types wherever three-phase electrical equipment is installed and phase rotation integrity is critical.

Industrial Motor Protection

This is by far the most common application. Three-phase induction motors are the workhorses of manufacturing facilities, water treatment plants, mining operations, and oil refineries. A phase sequence relay — the 47 relay — is frequently installed as a standard component in motor control centers (MCCs) to block motor starting if the supply phase sequence is incorrect or if one phase is missing. Some codes and standards, including NFPA 70 (NEC) in specific contexts and IEC 60034 for rotating machinery protection, recommend or require phase sequence protection for motors above certain horsepower ratings.

HVAC and Building Systems

Large commercial and industrial HVAC systems rely on chillers, cooling towers, and air handling units — all driven by three-phase motors. Phase reversal in a chiller compressor motor can cause the compressor to run backward, destroying the lubrication system and resulting in catastrophic mechanical failure within minutes. Chiller compressors can cost $50,000 to $500,000 to replace, making a $100–$200 phase sequence relay an obvious investment. Most chiller manufacturers specify phase sequence protection as a standard requirement in their installation guidelines.

Elevator and Lifting Equipment

Phase reversal in elevator drive systems is a serious safety concern. If a hoist motor runs in reverse, the elevator car moves in the opposite direction from the control command — going down when it should go up, or worse, moving when the brakes are not fully released. EN 81-20 and other elevator safety standards in Europe and Asia explicitly require phase failure and phase reversal protection. The 47 relay — or a combined phase sequence and phase loss relay — is a mandatory component in most elevator electrical control panels in these markets.

Generator Protection and Paralleling

When a generator is brought online and paralleled with a utility grid or another generator, the incoming machine must match the existing bus in voltage, frequency, and phase rotation. A 47 relay element within the generator protection IED verifies that the generator's phase sequence matches the bus before the synchronizing breaker is permitted to close. Closing across an incorrect phase sequence would create a massive fault current — potentially damaging the generator, the transformer, and connected equipment instantaneously.

Water and Wastewater Treatment

Pumping stations at water utilities rely on large centrifugal pumps driven by three-phase motors. If a pump starts in reverse, it not only fails to move water — it can also unscrew impellers, damage shaft seals, and cause waterlogging in the motor windings. Phase sequence relays are standard protection in these facilities. The cost of a single pump repair can exceed $20,000, not counting the operational downtime or the risk of sewage overflow or water supply disruption.

Data Centers and Critical Power Systems

Modern data centers use large uninterruptible power supply (UPS) systems and precision cooling units, all powered by three-phase feeds. Phase sequence monitoring — performed by 47 relay elements embedded in transfer switches and power distribution units — ensures that standby and alternate power sources have the correct sequence before being connected to critical loads. An incorrect phase connection discovered only after equipment failure in a data center can result in losses far exceeding the hardware cost alone.

How to Set and Configure a 47 Relay

Correct settings are essential for any protective relay to perform its function without nuisance tripping or failing to operate when needed. The 47 relay has a relatively small number of settings compared to more complex protection functions, but each one matters.

Negative Sequence Voltage Pickup (V2 Pickup)

This is the threshold at which the relay starts timing toward a trip. It is usually expressed as a percentage of the nominal positive sequence voltage or as a percentage ratio V2/V1. Typical settings range from 10% to 30% of nominal voltage. A lower pickup makes the relay more sensitive but increases the risk of false trips during normal system switching events. A higher pickup provides more security but may miss moderate unbalance conditions.

For motor protection applications, a V2 pickup of 15–20% is common. For generator paralleling applications, the pickup may be set as low as 5% to ensure phase sequence integrity before breaker closure.

Time Delay (Trip Delay)

Most 47 relays include a programmable time delay before the trip output is issued. This delay prevents unnecessary tripping during brief, transient unbalance events caused by switching operations on adjacent feeders. Time delays typically range from 0 to 60 seconds, with 1–5 seconds being a common choice for motor protection applications. For phase reversal detection specifically, some engineers set the delay to near zero (instantaneous) because a fully reversed phase sequence is unambiguous and immediate tripping is appropriate.

Minimum Positive Sequence Voltage (Voltage Supervision)

Some relays include a minimum voltage supervision function that blocks the 47 element from operating when the positive sequence voltage (V1) falls below a threshold — typically 50–70% of nominal. This prevents the relay from tripping during a complete loss of voltage (blackout), which would otherwise produce a large V2/V1 ratio due to very low V1, even if the underlying system condition is simply a total power outage rather than a phase sequence problem.

Output Configuration

The 47 relay output contacts can be configured to:

• Trip a circuit breaker or contactor directly
• Initiate a lockout relay (Device 86) that prevents automatic restart
• Trigger an alarm at the control room or SCADA system
• Block closing commands to a synchronizing breaker

In motor protection schemes, the 47 relay output is typically wired into the motor starter control circuit, preventing the contactor from energizing if phase sequence conditions are not met. This means the motor simply will not start rather than starting in the wrong direction.

47 Relay vs. Related Protective Relay Functions

Engineers often compare or combine the 47 relay with adjacent relay functions that also address phase-related problems. Understanding the distinctions helps in designing effective protection schemes.

47 Relay vs. 46 Relay (Phase Balance Current Relay)

The Device 46 relay measures current unbalance rather than voltage unbalance. It detects negative sequence current (I2) rather than negative sequence voltage (V2). While both relays respond to phase imbalance conditions, a 46 relay is typically used when current transformers (CTs) are available and when the goal is to protect a specific machine or feeder based on actual current flowing through it. A 47 relay, measuring voltage, provides system-level protection that is independent of what load is connected — making it suitable for protecting the supply bus itself or detecting problems before any load is even connected.

In practice, both 46 and 47 relay elements are often included together in a motor protection IED to provide comprehensive phase protection: the 47 element catches supply-side phase sequence problems, while the 46 element detects load-side current imbalance that could indicate a failing winding or mechanical fault in the motor.

47 Relay vs. 27 Relay (Undervoltage)

A 27 relay trips when the magnitude of the voltage drops below a threshold. It does not respond to phase sequence changes. A phase reversal with all three voltages at rated magnitude would not cause a 27 relay to operate at all. The 47 relay fills this gap by monitoring sequence rather than magnitude, making them complementary rather than redundant functions.

47 Relay vs. Phase Loss Relay

A phase loss relay (sometimes marketed as an open-phase relay) is a simpler, usually lower-cost device that only detects when one of the three phases disappears completely. It does not measure sequence or unbalance — just presence or absence of voltage on each phase. A 47 relay, by contrast, can detect phase reversal and significant unbalance even when all three phases are present. For comprehensive protection, a 47 relay provides a superset of what a phase loss relay offers, though dedicated phase loss relays may be used in cost-sensitive applications where only open-phase detection is needed.

Testing and Commissioning a 47 Relay

Like all protective relays, the 47 relay must be tested during commissioning and at regular maintenance intervals to confirm it will operate correctly when needed. Relay testing for a 47 element typically involves the following steps:

1. Inject balanced three-phase voltages at the relay's voltage input terminals in the correct sequence (A-B-C) and verify the relay remains in the non-operated state.
2. Swap two phases (e.g., inject A-C-B sequence) and verify the relay picks up and trips within the specified time delay.
3. Inject an unbalanced set of voltages where one phase voltage is reduced to simulate a phase imbalance condition, then verify the relay responds at the expected V2/V1 threshold.
4. Remove one phase voltage entirely and confirm the relay trips to verify open-phase detection.
5. Verify the time delay by measuring the actual elapsed time between pickup and trip output using a timer or relay test software.
6. Check output contacts for proper operation using a continuity tester or secondary injection relay test set.

Modern digital 47 relays can often perform self-diagnostics and generate test records automatically. IEC 61850-compliant devices can be tested remotely via GOOSE messaging simulation using software tools. Commissioning test records should be retained as part of the facility's protection documentation and referenced during future maintenance activities.

Maintenance intervals for relay testing vary by industry and standard. IEEE C37.118 and NERC PRC standards govern utility transmission protection. Many utilities test their protective relays on a 6-year cycle for numerical relays and a 4-year cycle for electromechanical types. Industrial facilities typically follow manufacturer recommendations or internal maintenance programs.

Common Mistakes When Applying a 47 Relay

Despite its relatively simple function, the 47 relay is sometimes misapplied or misconfigured in the field. These mistakes can result in either nuisance tripping or, worse, failure to protect equipment during a genuine fault.

• Incorrect PT connections: If the potential transformer secondaries feeding the 47 relay are wired in the wrong sequence, the relay may interpret a correct system sequence as reversed and trip unnecessarily — or vice versa. Always verify PT wiring with a phase rotation meter before enabling the 47 element.
• Setting the pickup too low: A V2 pickup set below 10% of nominal can cause the relay to trip during normal but slightly unbalanced supply conditions, particularly in areas served by long distribution feeders where some voltage unbalance is inherent. This leads to repeated nuisance trips and loss of confidence in the protection system.
• Omitting voltage supervision: Without a minimum voltage inhibit, the relay may trip during a complete power outage when the voltages are near zero, generating a false "phase sequence fault" alarm that complicates post-event analysis.
• Bypassing the relay during maintenance: Temporarily bypassing the 47 relay during maintenance and forgetting to restore it leaves the equipment unprotected. All relay bypasses should be logged and subject to a formal restoration procedure.
• Assuming the relay replaces all phase protection: The 47 relay detects sequence and voltage unbalance, but it does not replace thermal overload protection, overcurrent protection, or ground fault protection. It should always be part of a coordinated protection scheme, not a standalone safeguard.

The Cost-Benefit Case for Installing a 47 Relay

The economic argument for the 47 relay is straightforward. The cost of the relay itself — whether a simple standalone unit at $100–$300 or a multifunction IED with the 47 element included among many others at $1,000–$10,000 — is trivially small compared to the potential cost of the equipment it protects.

Consider a real-world scenario: a 500 hp three-phase motor driving a critical process pump in a chemical plant. The motor alone may cost $30,000–$80,000 to replace, not counting:

• Crane and rigging costs for removal and reinstallation: $5,000–$20,000
• Rewinding or repair lead time: 2–6 weeks
• Production downtime at a rate of $10,000–$100,000 per day depending on the process
• Secondary damage to driven equipment (pump impellers, seals, couplings)
• Potential environmental or safety incidents if the process is hazardous

A $200 phase sequence relay that prevents a single such event pays for itself many thousands of times over. This is why many insurance carriers and engineering procurement contractors (EPCs) specify phase sequence protection as mandatory for motors above 10 hp in facilities where process continuity is critical.

Beyond equipment protection, reliable phase sequence monitoring improves overall system reliability and reduces the frequency of root-cause investigations following unexplained motor failures or direction-of-rotation incidents. It also simplifies regulatory compliance in industries where electrical safety standards require documented protection against specific fault modes.