Phase-loss detection on three-phase motors — why a thermal overload relay alone can be too slow

The guide on motor overload relays and trip class covers how a thermal overload relay protects a motor against a sustained, excessive current. This article covers a specific fault scenario where that thermal protection doesn't always respond fast enough: the loss of one supply phase to a three-phase motor that is already running.

The phenomenon: running on two phases

If one of the three supply phases to a running three-phase motor is lost — for example due to a loose connection terminal, a blown fuse, or an interrupted conductor — the motor, thanks to its own mechanical inertia and the remaining rotating field from the two surviving phases, often simply keeps running. The motor then effectively operates as a severely unbalanced machine: the current in the remaining windings rises significantly compared with the normal, balanced operating current. In a delta-connected motor, the current in the winding still fed via both remaining phases can rise to on the order of double the normal phase current, with a correspondingly elevated line current in the remaining supply conductors.

Why this is dangerous for the motor

This elevated, unbalanced current causes uneven, locally increased heating of the motor windings — with a real risk of insulation damage (winding-to-winding short circuit) if the condition persists too long. Because the motor often still appears to function mechanically (sometimes with an audible hum and reduced torque, but not necessarily an obviously visible fault), a phase-loss condition can go unnoticed for a considerable time without separate detection.

Why a thermal overload relay isn't always fast enough here

A thermal overload relay (see the trip-class guide) protects against excessive current by simulating the heating of a bimetallic element (or its electronic equivalent), and only trips once that simulated temperature exceeds a set limit. This works well for a gradual, symmetrical overload, but for a phase-loss condition it only responds after the time needed to build up enough thermal energy — a period during which the uneven, concentrated heating of the most-heavily-loaded winding can already cause damage before the average, simulated temperature reaches the relay's trip threshold.

Phase-failure and voltage-monitoring relays: direct detection of unbalance

A separate phase-failure or voltage-monitoring relay, typically built to IEC 60947-5-1 (control-circuit devices) — not to be confused with IEC 60947-4-1, which governs the phase-failure sensitivity of a thermal overload relay that is integrated into a motor starter — continuously monitors the three supply phases (voltage and/or current) for mutual unbalance and for the complete loss of a phase, and trips the motor directly once a set unbalance or phase-loss threshold is exceeded — often within a fraction of the time a thermal overload relay would need to detect the same fault. Many of these relays also monitor phase-sequence reversal at the same time (see the guide on phase-rotation recognition) and under-/over-voltage, as additional protection functions built into the same device.

Practical relevance

For motors that are critical to a process (for example in industry or horticulture, see also the guide on motor starting methods) or in installations where a loose terminal or blown phase fuse isn't immediately visible in practice, a separate phase-failure relay is a valuable addition to the standard thermal overload protection — not a replacement for it, but a faster detection method specifically aimed at this fault scenario.

Common mistakes

Relying solely on the thermal overload relay for protection against a phase-loss condition, even though this relay only responds after significant thermal build-up.
Not immediately recognising a motor running on two phases — the absence of an obviously visible fault (the motor is still turning) can give the impression that nothing is wrong, while the windings are meanwhile overheating unevenly.
Letting a phase-failure relay monitor voltage only in a situation better detected via current unbalance (or vice versa), without considering the specific failure mode of the installation — the correct monitored quantity depends on where in the supply chain a phase can be lost.
Not accounting for the winding configuration (star versus delta) when assessing the risk — in a delta-connected motor, the current unbalance in the most heavily loaded winding during a phase-loss condition can be considerably larger than the rise in average line current alone would suggest.