Phase Failure Relay vs Single Phasing Preventer vs Phase Sequence Relay
Three-phase motors are badly hurt by supply problems: a blown fuse, a loose cable lug, a reversed connection after maintenance. Three common devices protect against these problems: the phase failure relay, the single phasing preventer (SPP) and the phase sequence relay. Their names overlap and so do their jobs. Here is how they differ.
What each device does
Phase failure relay
Monitors the three supply voltages and trips if a phase is missing or, on many models, if voltage is unbalanced or out of limits. It looks at the supply.
Single phasing preventer (SPP)
A traditional Indian term for a device that stops a motor running on two phases. Many SPPs sense voltage, so they work like a basic phase failure relay. Some also detect reverse phase.
Phase sequence relay
Checks that the phases are in the correct order (R-Y-B) so the motor turns the right way. This matters for pumps, compressors, conveyors and hoists, where reverse rotation causes damage or danger. See our article on reverse phase protection.
Voltage sensing vs current sensing
This is the key point many buyers miss. A voltage-sensing relay at the panel incoming may not see a fault that happens after it, such as a blown fuse in the motor feeder or a broken cable to the motor. In those cases the running motor can also back-feed a voltage onto the dead phase, so a voltage relay may not trip.
A current-sensing motor protection relay measures the current actually flowing to the motor. If one phase stops carrying current, wherever the break is, it trips. That is why current-based phase-loss protection is more dependable for the motor itself.
| Function | Phase failure relay / SPP (voltage) | Phase sequence relay | Current-sensing motor relay |
|---|---|---|---|
| Missing phase at supply | Yes | Some models | Yes |
| Phase lost after the relay (feeder fuse, cable) | May miss it | No | Yes |
| Wrong phase sequence | Some models | Yes | Yes (EOCR DS, 3DE / 3EZ) |
| Voltage unbalance / under-over voltage | Yes (many models) | No | Sees the resulting current unbalance |
| Overload, locked rotor | No | No | Yes |
| Prevents starting on a bad supply | Yes | Yes | Trips shortly after start |
Which do you need?
- To protect each motor: use a current-sensing motor protection relay. The EOCR SS covers phase loss with overload and locked rotor; the EOCR DS adds reverse phase; the EOCR 3DE / 3EZ adds unbalance, undercurrent and earth fault.
- To stop a whole panel from starting on a bad supply: add a voltage-sensing phase failure or monitoring relay at the incoming.
- Where reverse rotation is dangerous: make sure phase sequence protection is included, in the motor relay or a separate relay.
For more detail, see single phasing protection and phase unbalance protection.
Frequently Asked Questions
Is a single phasing preventer the same as a phase failure relay?
Mostly. Both stop a motor running on two phases; many SPPs are simple voltage-sensing phase failure relays. Check whether the model also detects reverse phase and unbalance.
Why did my motor burn even though an SPP was installed?
A voltage-sensing SPP can miss a phase lost downstream of it, because the running motor back-feeds voltage onto the dead phase. A current-sensing motor protection relay detects the missing current directly.
Do I need a separate phase sequence relay?
Not if your motor protection relay already includes reverse phase protection, as the EOCR DS and 3DE / 3EZ do.
Talk to Concord
Concord Automation & Controls has supplied motor protection relays and industrial relays to Indian industry since 1996 and is the exclusive ComatReleco (Switzerland) distributor for India, Bangladesh and Mauritius. Share your motor, pump or panel details and we will recommend the right model: call or WhatsApp +91 96670 40511, email contact@concordrelay.com, or use our contact page.