What Is a Phase Sequence Relay? Types and Selection Criteria
A phase sequence relay is a monitoring relay that watches the order of the phases in a three-phase supply and opens its contact when the order is reversed, preventing reverse motor rotation.
A phase sequence relay (phase sequence protection relay) is a monitoring relay that watches the L1-L2-L3 order of the phases in a three-phase supply and, when the order is reversed, opens its output contact so that the motor contactor cannot pull in. It is used on pump, compressor, fan, conveyor, lift and crane motors where reverse rotation causes damage, and in construction site and generator panels whose connection changes often. The relay does not interrupt the motor current itself; its contact is wired into the contactor coil circuit.
Types
- Phase sequence only: Secures the direction of rotation (Entes FR-02, Siemens 3UG5511).
- Phase sequence and phase failure: Also trips when a phase is lost; the Siemens 3UG5512-1AR20 is additionally sensitive to 10% asymmetry.
- Analog-setting line monitoring relays: Monitor phase sequence, phase failure, asymmetry and undervoltage together (Siemens 3UG5514-1BR20).
- Digital-setting line monitoring relays: Settings are made digitally and automatic or manual reset is selectable (Siemens 3UG5616-1CR20, 3UG5618-1CR20).
Catalog models
| Model | Monitoring | Measuring voltage | Output |
|---|---|---|---|
| Entes FR-02 | Phase sequence | 220–230 V AC, three phases + neutral connected | Not specified |
| Siemens 3UG5511-1AR20 | Phase sequence | 3×160–690 V AC | 1 changeover contact |
| Siemens 3UG5511-1BR20 | Phase sequence | 3×160–690 V AC | 2 changeover contacts, automatic reset |
| Siemens 3UG5512-1AR20 | Phase sequence, phase failure, 10% asymmetry | 3×160–690 V AC | 1 changeover contact, automatic reset |
| Siemens 3UG5514-1BR20 | Phase sequence, phase failure, asymmetry, undervoltage | 3×160–690 V AC, 15–70 Hz | 2 changeover contacts |
| Siemens 3UG5616-1CR20 | Digital-setting line monitoring | Not specified | 2 changeover contacts, automatic or manual reset |
| Siemens 3UG5618-1CR20 | Digital-setting line monitoring | 3×90–690 V AC | 2 changeover contacts |
Values are taken from the product names and Siemens product data. The Siemens 3UG5501-1AW30 (0.5–500 kΩ sensitivity) and 3UG5642-1CW30 in this category are not phase sequence relays but relays with other monitoring functions.
Selection criteria
- Faults to monitor: If only the direction of rotation matters, a phase sequence relay is enough. To also stop the motor running on two phases, choose a model that monitors phase failure, and on an unbalanced supply asymmetry as well.
- Measuring voltage: The relay's voltage range must cover your supply. The Entes FR-02 is for a connection with neutral at 220–230 V AC; the Siemens 3UG55 models cover wide ranges such as 3×160–690 V AC.
- Neutral connection: If the panel has no neutral conductor, choose a model that does not need a neutral connection.
- Output contact: One changeover contact is enough to drive the contactor coil. If you also need a fault alarm or a signal to a PLC, choose a model with two changeover contacts.
- Reset: On machines where an unexpected restart is dangerous, prefer a model with selectable manual reset instead of automatic reset.
Common mistakes
- Using a phase sequence relay instead of a thermal overload relay: The relay does not monitor motor current; a thermal overload relay or motor protection circuit breaker is still needed against overload.
- Swapping only the relay's terminals when it trips: The relay goes quiet but the motor keeps turning the wrong way; correct the phase order on the supply side.
- Connecting the measuring terminals on the motor side of the contactor: The relay cannot see the supply before the contactor pulls in, so the motor never starts; the measuring terminals go on the supply side of the contactor.
Brands in the catalog
The Phase Sequence Protection Relays category has 9 active products: Siemens 8, Entes 1. Relays that monitor phase failure, asymmetry and voltage limits together are in Phase and Motor Protection Relays, and relays that monitor voltage only are in Voltage Protection Relays. When a generator or site connection is renewed, measure the order before the first start with Phase Sequence Testers.
Frequently asked questions
What does a phase sequence relay do?
It stops a three-phase motor from running in the wrong direction. If the phases are not in L1-L2-L3 order, the relay keeps its contact open and the contactor does not pull in; the motor does not start.
What is the difference between a phase sequence relay and a phase protection relay?
A phase sequence relay monitors only the order, and some models also phase failure. A phase protection relay also monitors phase failure, asymmetry and over- or undervoltage. Choose a phase sequence relay when only the direction of rotation matters, and a phase protection relay for motor protection.
How is a phase sequence fault corrected?
Swap any two phases on the supply side; this also reverses the motor's direction of rotation.
Does a phase sequence relay trip when a phase is lost?
It depends on the model. A relay that monitors only the phase sequence may not detect phase loss. The Siemens 3UG5512-1AR20 and 3UG5514-1BR20 also monitor phase failure.