Undervoltage Relay Symbol
Definition: The Undervoltage Relay symbol represents a protective relay device that monitors supply voltage and trips its output contacts when the voltage falls below a preset threshold, standardised in IEC 60617 protective relay conventions and ANSI/IEEE C37.2 device numbering (ANSI Device Number 27 — undervoltage relay), with terminals Coil A, Coil B (voltage sensing inputs), VCC (auxiliary supply), and Trip (output contact).
Also known as: UV relay, undervoltage protection relay, ANSI Device 27, voltage monitoring relay, low voltage relay, underpotential relay.
What the Undervoltage Relay symbol means
The Undervoltage Relay symbol represents a protection device that continuously monitors the voltage on one or more phases of an electrical supply and sends a trip signal to a circuit breaker, contactor, or alarm when that voltage drops below a set threshold for a defined time delay. This protects motors, sensitive electronic equipment, and distribution systems from damage caused by low voltage, which can cause motor overheating, control circuit failure, and process disruption.
In electrical single-line diagrams and protection coordination drawings, the undervoltage relay symbol appears connected across the protected bus or phase voltage with its trip output wired to the shunt-trip coil of a circuit breaker or the coil of a contactor. The relay is identified by ANSI Device Number 27 in power system protection, a universal numbering scheme defined in IEEE C37.2 and used worldwide on protection diagrams regardless of relay manufacturer or model.
How to identify the Undervoltage Relay symbol
The Undervoltage Relay symbol is drawn as a rectangle labelled with 'UV', '27', or 'Undervoltage Relay', with terminal stubs for: Coil A and Coil B (the two voltage-sensing input terminals connected across the monitored phase or line voltage), VCC (auxiliary DC or AC supply for the relay's internal electronics), and Trip (the normally-open or change-over output contact that actuates when the voltage threshold is exceeded). On IEC-style protection diagrams the relay coil is sometimes shown as a rectangle with a voltage-type qualifying symbol inside.
Function in a circuit
An undervoltage relay continuously compares the sensed voltage on its Coil A–Coil B input terminals against its internal set-point. When the measured voltage falls below the threshold (typically adjustable 50–90% of nominal) for longer than the relay's pick-up time delay (typically 0.1–5 seconds), the Trip output contacts change state — opening normally-closed contacts or closing normally-open contacts — which can de-energise a contactor, trigger a circuit-breaker shunt trip, or activate an alarm. Once voltage recovers above the reset level (usually a few percent above the trip level), the relay can be set to auto-reclose or require manual reset depending on the protection scheme.
Standards: IEC vs ANSI
| IEC 60617 | IEC 60617 protective relay symbols: the undervoltage relay uses the standard relay rectangle symbol with the voltage-type functional designator. IEC 60255 series (measuring relays and protection equipment) defines the technical performance requirements for undervoltage relays. |
|---|---|
| ANSI/IEEE 315 | ANSI/IEEE C37.2-2008 (standard for electrical power system device function numbers and contact designations): Device Number 27 is the official designation for an undervoltage relay. This number appears inside or beside the relay symbol on ANSI-style protection single-line diagrams. |
| Key difference | IEC protection diagrams use the functional rectangle with an internal qualifier; ANSI diagrams write the device number '27' inside or beside a circle or rectangle. Both conventions are widely used internationally, with ANSI Device 27 being universally recognised in power engineering regardless of local standards preference. |
Terminals / pins
| Pin | Name |
|---|---|
| coil_a | Coil A |
| coil_b | Coil B |
| vcc | VCC |
| trip | Trip |
Typical values
Typical settings: trip threshold 50–90% of nominal voltage (adjustable); time delay 0.05–30 s; reset/hysteresis 2–5% above trip threshold; auxiliary supply 24 V DC, 48 V DC, 110 V DC, or 230 V AC depending on relay type; contact rating 5–10 A, 250 V AC / 30 V DC; operating temperature -20°C to +70°C.
Where the Undervoltage Relay symbol is used
- Motor protection panels where low voltage causes motor overheating and reduced torque
- Critical process control panels where underpotential can cause PLC or drive faults
- Automatic transfer switch (ATS) circuits that detect mains undervoltage to initiate generator changeover
- Generator protection panels monitoring bus voltage to prevent paralleling at wrong voltage
- Substation feeder protection schemes using ANSI Device 27 to detect loss of supply
- Data centre UPS coordination circuits monitoring upstream supply voltage quality
Example
In a motor control centre (MCC) for a pump motor, an undervoltage relay (ANSI 27) symbol is connected across phases A and B of the 415 V AC bus with its Trip contact wired in series with the motor contactor coil circuit. If the bus voltage drops below 360 V (87% of nominal) for more than 1.5 seconds, the relay trips, de-energises the contactor, and stops the motor — preventing the motor from restarting automatically on low voltage and causing damage.
Key facts
- The Undervoltage Relay symbol represents ANSI Device Number 27, a protective relay that trips when monitored voltage falls below a preset threshold, per IEEE C37.2-2008.
- Terminals: Coil A and Coil B (voltage sensing inputs connected across the monitored phase), VCC (auxiliary supply), and Trip (protection output contact).
- IEC 60255 series defines the performance requirements for undervoltage relays; IEC 60617 defines the schematic symbol; ANSI C37.2 Device 27 is the equivalent designation.
- Typical trip threshold range: 50–90% of nominal voltage, with an adjustable time delay of 0.05–30 seconds to prevent nuisance tripping on brief voltage dips.
- The relay's VCC auxiliary supply is often independent of the monitored voltage so the relay remains powered and can trip even when the monitored supply is severely depressed.
- ANSI Device 27 is one of the standard function numbers appearing on power system protection single-line diagrams alongside Device 51 (overcurrent) and Device 59 (overvoltage).
- Auto-reclose or manual-reset behaviour after voltage recovery is configurable; critical motor loads typically use manual reset to prevent automatic restart.
- Undervoltage protection is mandatory for motors under IEC 60947-4-1 (low-voltage switchgear and controlgear) and NFPA 70 NEC Article 430 (motors).
Frequently asked questions
What does the undervoltage relay symbol look like?
The undervoltage relay symbol is drawn as a rectangle labelled 'UV', '27', or 'Undervoltage Relay' with four terminal stubs: Coil A and Coil B (voltage sensing inputs), VCC (auxiliary supply), and Trip (output contact). On ANSI protection single-line diagrams the device number 27 appears inside a circle or rectangle; on IEC diagrams a functional qualifier may appear inside the relay rectangle.
What does the undervoltage relay symbol mean in a circuit?
The undervoltage relay symbol means there is a voltage-monitoring protection device at that point. It continuously measures the voltage across its sensing inputs and triggers its Trip contact when that voltage drops below a set threshold, protecting motors, equipment, and control systems from the damaging effects of low supply voltage.
What is ANSI Device 27?
ANSI Device 27 is the official device function number for an undervoltage relay as defined in IEEE C37.2-2008 (Standard Electrical Power System Device Function Numbers and Contact Designations). The number 27 appears on protection single-line diagrams worldwide to identify any relay that monitors voltage and trips on undervoltage conditions.
What is the difference between an undervoltage relay and an overvoltage relay?
An undervoltage relay (ANSI 27) trips when measured voltage falls BELOW its set threshold, protecting against low-voltage conditions. An overvoltage relay (ANSI 59) trips when measured voltage rises ABOVE its threshold, protecting against overvoltage. Both are often used together on the same bus in critical protection schemes, with separate set-points and time delays.
What are the terminals of an undervoltage relay?
The four functional terminals are: Coil A and Coil B (the voltage sensing inputs, connected across the phase or line voltage being monitored), VCC (the auxiliary DC or AC supply powering the relay's internal circuitry independently of the monitored voltage), and Trip (the output contact that changes state to actuate a breaker, contactor, or alarm when undervoltage is detected).
What standard defines the undervoltage relay?
ANSI/IEEE C37.2-2008 assigns Device Number 27 to undervoltage relays. IEC 60255 series (measuring relays and protection equipment) defines performance requirements, and IEC 60617 provides the schematic symbol conventions. Both ANSI and IEC are used internationally in power system protection drawings.
Where is an undervoltage relay used?
Undervoltage relays are used in motor control centres to stop motors during low-voltage conditions, in automatic transfer switches (ATS) to trigger generator start when mains voltage is lost, in substation feeders for supply loss detection, and in critical process panels to prevent control equipment from operating on inadequate supply voltage.
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