How to select phase sequence and voltage monitoring relays: phase failure, unbalance and under/over-voltage protection for 3-phase motors.
A phase sequence relay (also called a phase rotation relay, phase failure relay or 3-phase monitor) guards a three-phase supply against the faults that destroy motors and trip whole panels. It continuously watches the three line voltages and the order in which they appear. If the sequence reverses, a phase drops out, the voltage strays outside limits, or the phases become badly unbalanced, the relay opens its output contact and drops the contactor feeding the load — before the motor cooks or the drive faults.
Think of it as cheap insurance in front of an expensive machine. A 22 kW compressor, a 75 kW pump or a conveyor with a 200 A soft starter costs far more than the relay that protects it, and a reversed phase at commissioning can spin a pump backwards or wreck a gearbox in seconds.
Three-phase motors are built to run in one direction. If two supply lines are swapped during a reconnection, the magnetic field rotates the wrong way and the motor attempts to start against its own load — drawing locked-rotor current and overheating fast. A wrong-sequence relay detects the reversed rotation order (L1-L2-L3 versus L1-L3-L2) and blocks starting.
A single phase loss (phase failure) is equally destructive. With one phase open, a running motor becomes a single-phase machine that keeps drawing current through the remaining windings, overheating until the insulation fails. Phase-failure protection must drop the contactor the instant a phase voltage disappears, not after a thermal delay.
A voltage monitoring relay extends the basic phase guard with limits on each phase. You set a nominal voltage (for example 400 V AC or 230 V) and an allowable band, and the device trips when any phase falls below the under-voltage threshold or rises above the over-voltage threshold. Unbalance protection trips when the spread between the highest and lowest phase exceeds roughly 8–10 %.
For sites on unstable grids — remote plants, farms, ports, developing-market utilities — this is the layer that saves variable-frequency drives and PLCs from repeated brown-outs. Many units combine sequence, loss, unbalance and voltage limits in one 22.5 mm or 35 mm DIN-rail module, which is why specifiers often order a single "3-phase monitor" rather than several separate devices.
Pick the relay to the supply it watches, not to the load it protects. The critical parameters are:
| Parameter | What to specify | Why it matters |
|---|---|---|
| Supply voltage | 208/230/400/415/480 V AC, 50/60 Hz | Must match the measured bus; some units auto-range |
| Measuring range | ±10–20 % of nominal | Sets the under/over-voltage trip band |
| Unbalance trip | 5–15 % selectable | Protects motors from negative-sequence heating |
| Reset mode | auto / manual | Manual reset prevents unexpected auto-restart of machinery |
| Output | 1–2 changeover relays, 5–8 A | Drives the contactor coil or a PLC input |
| Mounting | 35 mm DIN rail | Standard panel layout; confirm width (17.5/22.5/35 mm) |
| Delay / hysteresis | 0.1–10 s adjust | Avoids nuisance trips on transient dips |
For export, look for IEC 60255 (measuring relays) and the relevant CE / CCC marks depending on destination. Units with screw terminals and clear LED phase indication speed up field commissioning. For generator-to-mains changeover boards a phase-sequence check is mandatory before paralleling, which is exactly where these relays earn their keep.
A phase failure (loss) relay only drops the load when one phase disappears. A phase sequence relay also checks the rotation order and is usually combined with voltage and unbalance monitoring in a single 3-phase monitor. For new panels, specify the combined unit.
The relay watches the supply feeding the VFD input. The VFD has internal protection, but the external monitor guards the incoming mains (sequence, loss, sag) that the drive's own electronics may not fully catch during start-up.
With the panel de-energised, wire a known-good 3-phase source, energise, then lift one phase wire to confirm the output contact opens and the contactor drops. The LED phase indication should show which phase is missing.
Not usually. Most modern 3-phase monitors combine under-voltage, over-voltage, unbalance, sequence and loss in one module. Specify separate units only if you need different trip bands for different loads on the same board.
Send the ratings, dimensions and quantities you are working to and we will confirm the closest match from the ELEHPD catalogue, the compliance documents available for it and the lead time. OEM branding, private model numbering and buyer-specific cartons are available on qualified volumes.