What Is a Phase Protection Relay? Selection Guide
A phase protection relay monitors a three-phase network for phase loss, phase sequence and voltage imbalance; here are the selection criteria.
What a phase protection relay is and where it is used
A phase protection relay is an electronic relay that monitors a three-phase network for faults such as phase loss, phase sequence errors and voltage imbalance (asymmetry), and reports to the control circuit through its output contact. It is also searched for as a motor phase protection relay, motor protection relay or phase failure relay.
It is used on three-phase motor loads such as pumps, compressors, elevators, fans and conveyors, and at mains incomers. A phase failure can damage a motor, while a reversed phase sequence can make loads such as pumps or compressors rotate in the wrong direction. According to manufacturer catalogs, these relays are used to prevent motors from being damaged by a phase failure or voltage imbalance.
Types of protection relays
The category is divided into subgroups by the quantity being protected:
- Phase and Motor Protection Relays: motor phase protection relays that monitor phase loss, phase sequence and asymmetry.
- Voltage Protection Relays: single-phase and two-phase relays that monitor overvoltage or undervoltage.
- Phase Sequence Protection Relays: devices that check only the phase sequence.
- Current Protection Relays: products that monitor current, such as undercurrent control relays.
- Thermistor relays: relays that monitor temperature through a thermistor (PTC) sensor input.
- Overcurrent secondary protection relays: a single product is listed in this group, which is a separate subcategory.
What to look at when choosing a phase protection relay
- Neutral connection: Some models also detect neutral loss and require a neutral connection, while others work without a neutral. If the enclosure has no neutral conductor, look for a type that works without a neutral.
- Fault types protected against: Is phase loss alone enough, or are phase sequence and asymmetry needed as well? For pumps and compressors, where reverse rotation causes damage, look for phase sequence monitoring.
- Asymmetry threshold: On some relays the threshold is fixed, on others it is adjustable. If you want to avoid false tripping on an unbalanced network, consider an adjustable type.
- Delay: The pick-up and drop-out delay can be fixed or adjustable; the delay prevents unnecessary tripping during short voltage fluctuations.
- Supply voltage: The relay can be 230 V, 400 V or wide-range. Confirm the model that suits your mains voltage from the manufacturer data.
- Output contact: Check in the catalog the current capacity of the contact to be used in the control circuit and whether it is changeover (CO) or normally open (NO); the wiring diagram follows the manufacturer data.
- Thermistor input: If the motor has a PTC sensor, you need a model that also monitors temperature.
Difference between a phase protection relay and a thermal overload relay
A thermal overload relay monitors motor current overload. A phase protection relay, on the other hand, checks for phase loss, sequence and imbalance on the voltage side. A thermal overload relay does not cover every phase protection function on its own; whether both devices are needed together is determined by the project and the manufacturer data. For the overload side, see the pages for thermal overload relays and motor protection circuit breakers.
Brands in this category
The category lists ABB digital protection units, distribution automation relays and arc protection equipment; Tense undercurrent control, digital thermal and motor phase protection relays; Siemens fault and arc protection devices with auxiliary devices; Entes motor phase and voltage protection relays; and Schneider Electric phase loss, phase sequence and voltage monitoring relays.
Order notes
Products are sold per piece. Some products are listed as "made to order"; for these, the lead time is confirmed separately. If you know the manufacturer model code, you can search by stock code or model code. If the relay current is being considered together with the cable and fuse calculation, the current and fuse calculation from active power tool can help.
Comparison and selection table
| Selection Criterion | Phase Sequence & Phase Failure Relay | Over/Under-Voltage & Asymmetry Relay | PTC Thermistor / Liquid Level Protection Relay |
|---|---|---|---|
| Monitored Faults | L1-L2-L3 phase presence and correct phase rotation | Adjustable high/low voltage limits & phase unbalance (%) | Motor winding PTC temperature or tank electrode level |
| Adjustment Controls | Fixed threshold, compact plug-and-play DIN module | Adjustable trip thresholds (%Un) and trip/reset delay (s) | PTC short/open detection or adjustable fluid resistance (kΩ) |
| Neutral Requirement | 3-wire (no neutral, 3×400 V) or 4-wire (3P+N) | 4-wire (monitors phase-to-neutral) or 3-wire (phase-to-phase) | Auxiliary supply (24 V AC/DC or 230 V AC) + sensor input |
| Typical Application | Compressors, elevators, conveyors & directional pumps | Main switchboards, generators & voltage-sensitive loads | Heavy-duty motor thermal protection & submersible pumps |
Frequently asked questions
What faults does a phase protection relay protect against?
Manufacturer catalogs state that a phase protection relay protects against phase loss, phase sequence errors and voltage imbalance. Some models also monitor neutral loss or the motor PTC sensor. Because the faults covered vary by model, you need to check the product's selection table.
Can a phase protection relay be used in enclosures without a neutral?
Yes, there are models that work without a neutral. In the Entes selection table, some models detect neutral loss, while others are listed as neutral-free. If the enclosure has no neutral conductor, look for a relay whose selection table specifies operation without a neutral; suitability is confirmed from the product's manufacturer data. This distinction is decisive when selecting.
How does a phase protection relay disconnect the motor?
When the relay detects a fault, its output contact changes position and this contact signals the control circuit; for example, on Entes models the output is 1 CO or 1 NO contact. How the contact is wired into the contactor control varies by model. Always apply the wiring diagram according to the product's manufacturer data.
Is a phase protection relay the same as a voltage protection relay?
No. A phase protection relay monitors missing phases, sequence and imbalance between the three phases. A voltage protection relay, on the other hand, checks the voltage level, such as overvoltage and undervoltage; it comes in single-phase or two-phase types. In some applications both functions are required together, and suitable models are listed in separate subgroups.
Is a phase protection relay needed when there is a thermal overload relay?
A thermal overload relay monitors motor current overload; a phase protection relay checks for phase loss, sequence and imbalance on the voltage side. A thermal overload relay does not cover every phase protection function on its own. Whether both are needed together depends on the application; the decision is made according to the project and the manufacturer data.