Phase Sequence Relay Symbol

The Phase Sequence Relay schematic symbol. Standard-specific details are listed below where available.

Definition: The Phase Sequence Relay symbol represents a three-phase protective device, designated K in schematics, that monitors the order (sequence) of phases L1, L2, and L3 in an AC supply and trips its output contacts when phase reversal, phase loss, or incorrect phase rotation is detected, per IEC 60617 and ANSI/IEEE 315.

Also known as: phase rotation relay, phase monitor relay, phase sequence monitor, reverse phase relay, phase failure relay.

What the Phase Sequence Relay symbol means

The Phase Sequence Relay symbol denotes a monitoring and protection relay connected to all three phases of an AC supply. It supervises the phase sequence (L1-L2-L3 or A-B-C) and the presence of all three phases, energising or de-energising its output contacts when conditions fall outside set thresholds.

In industrial control schematics this symbol appears in the incoming supply section of motor control centres (MCCs), protecting three-phase motors from running in reverse due to wrong phase order and from the mechanical and thermal damage caused by phase loss or unbalance.

How to identify the Phase Sequence Relay symbol

The Phase Sequence Relay symbol is typically drawn as a rectangle (representing the relay module body) with three input terminals labelled L1, L2, and L3 entering from the left or top, and two output contact terminals labelled NO (normally open) and NC (normally closed) on the right or bottom. Inside the rectangle the label 'PSR' or the phase-sequence designation may appear. Some representations use a circle with the relay coil symbol inside. The three-input, two-output configuration distinguishes it from a standard single-phase relay.

Function and usage boundary

A Phase Sequence Relay continuously monitors the three AC supply phases for correct rotation order, presence of all three phases, and sometimes phase voltage unbalance or phase loss. If the phases are reversed (e.g., L1-L3-L2 instead of L1-L2-L3) or if one phase is missing, the relay de-energises (or energises, depending on configuration) its output contacts to disconnect the load — typically a three-phase motor — preventing reverse rotation, overheating, or single-phasing damage.

Standards: IEC vs ANSI

IEC 60617	IEC 60617-07 and IEC 60255 series: the protective relay is drawn as a rectangle with function designation. Phase sequence monitoring relays carry the function code '47' in the ANSI/IEEE protective relay numbering scheme. IEC 60034-1 defines phase sequence requirements for rotating machines.
ANSI/IEEE 315	ANSI/IEEE C37.2-2008 assigns device number 47 specifically to the phase sequence voltage relay (also called reverse-phase or phase balance relay). The ANSI symbol follows IEEE 315 box-with-designation conventions: a rectangle with '47' inscribed.
Key difference	IEC uses a generic protective relay rectangle with function text; ANSI/IEEE C37.2 uses device number 47 inside the rectangle. Both use the same pin layout concept (three phase inputs, contact outputs), but ANSI labelling uses A/B/C for phases versus IEC L1/L2/L3.

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Logical terminals

Logical ID	Editor terminal name
l1	L1
l2	L2
l3	L3
no	NO
nc	NC

Where the Phase Sequence Relay symbol is used

Motor control centres (MCCs): protecting three-phase induction motors from reverse rotation caused by wrong phase order at site installation or after maintenance
HVAC systems: preventing compressor and fan motors from running backwards, which reduces efficiency and can cause mechanical damage
Pumping stations: ensuring pump rotation direction is correct before start-up, avoiding impeller damage
Conveyor and lift systems: safety interlock to halt drive if supply phase sequence is incorrect or a phase is lost
Industrial panels: incoming supply monitoring to detect phase loss or blown fuse on one phase before it can damage connected equipment
Generator changeover systems: verifying correct phase sequence when switching between utility and standby generator supplies

Key facts

The Phase Sequence Relay carries ANSI/IEEE C37.2 device number 47 and is designated K in IEC schematic notation for relay devices.
It monitors three-phase AC supplies on pins L1, L2, and L3, and provides output contacts NO (normally open) and NC (normally closed).
Phase reversal — swapping any two of the three supply phases — causes three-phase motors to run in the opposite direction, which the Phase Sequence Relay detects and prevents.
Single phasing (loss of one phase) causes three-phase motors to overheat and stall; the Phase Sequence Relay trips within its set delay (typically 0.1–5 s) to prevent this.
IEC 60255 governs the measurement and performance requirements for protective relays including phase sequence monitors.
Typical operating voltage range is 200–690 V AC three-phase, covering both low-voltage industrial (400 V) and medium-voltage applications via VTs.
Some Phase Sequence Relays also measure phase voltage unbalance, tripping if any phase deviates more than a set percentage (typically 5–10%) from the average.
In panel wiring diagrams the Phase Sequence Relay symbol is placed in the incoming supply section, upstream of all motor starters and contactors.

Frequently asked questions

What does the phase sequence relay symbol mean in a wiring diagram?

The Phase Sequence Relay symbol (device number 47 per ANSI/IEEE C37.2) in a wiring diagram means that three-phase supply monitoring is present at that point. The relay will trip its output contacts — disconnecting the load — if the phase order is reversed, if a phase is lost, or if voltage unbalance exceeds the set threshold.

What does the phase sequence relay symbol look like?

The Phase Sequence Relay symbol is drawn as a rectangle with three input terminals (L1, L2, L3) and two output contact terminals (NO and NC). In ANSI/IEEE C37.2 notation the numeral '47' appears inside the rectangle. In IEC schematic style it follows the standard protective relay box with a text label indicating its function.

What is the ANSI device number for a phase sequence relay?

The ANSI/IEEE C37.2 device number for a phase sequence relay (reverse-phase relay) is 47. This number is inscribed inside the relay rectangle symbol in ANSI-style electrical diagrams and uniquely identifies the phase sequence monitoring function.

What is the difference between IEC and ANSI symbols for a phase sequence relay?

In ANSI/IEEE C37.2 notation the phase sequence relay uses device number 47 inscribed in a rectangle, with phase inputs labelled A, B, C. In IEC 60617 / IEC 60255 notation the same device is drawn as a protective relay rectangle with a function description label, and phase inputs are labelled L1, L2, L3. Both represent the same physical device.

What are the pins on a phase sequence relay?

A Phase Sequence Relay has five key terminals: L1, L2, and L3 (three-phase supply inputs) plus NO (normally open output contact) and NC (normally closed output contact). Some models also include a common (COM) terminal for the output contacts and auxiliary supply terminals if the coil supply is separate from the monitored supply.

Why is a phase sequence relay used with three-phase motors?

Three-phase motors rotate in the direction determined by the phase sequence of their supply. If phases are reversed the motor runs backwards, which can damage the driven load (pump, compressor, conveyor). A Phase Sequence Relay detects incorrect phase order before start-up and prevents the motor contactor from closing until correct sequence is restored.

What standard defines phase sequence relay requirements?

Phase sequence relay performance requirements are defined in IEC 60255-47 (measuring relays and protection equipment — voltage phase sequence relays) and in ANSI/IEEE C37.2-2008 (device function numbers), where device 47 is assigned to the phase sequence voltage relay. IEC 60034-1 defines acceptable phase sequence for rotating machines.

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