Ambient Temperature Compensation in Thermal Overload Relays

What is ambient temperature compensation in a thermal overload relay? It is a second, current-independent bimetal element built into the relay that cancels the deflection caused by the surrounding air, so a relay compensated per IEC 60947-4-1 holds its calibrated trip current across roughly -5 to +55/60 C ambient instead of drifting with the enclosure. Skip it and the same relay trips a healthy motor early in a hot cabinet, or lets a real overload run longer than the dial says in a cold one. This article covers why a bare bimetal drifts with ambient in the first place, how the compensating strip cancels that drift, the IEC ambient window a compensated relay is rated for, mounting practice inside the panel, why electronic relays behave differently, and how thermal overload relays from Schneider, ABB, and Siemens implement compensation.

Why a Bare Bimetal Strip Drifts with Panel Temperature

A thermal overload relay's primary bimetal strip bends because of heat, full stop — it cannot tell whether that heat came from motor current flowing through it or from the air around it. Set the dial to the motor's full-load current (FLC) and the strip is calibrated to reach its trip deflection at a specific combination of current-heating plus whatever ambient heat it started with. Raise the starting temperature and the strip needs less current-heating to finish the bend; drop it and the strip needs more. On an uncompensated element the trip current effectively slides down as the cabinet gets hotter.

This matters because control panels are rarely at outside-air temperature. A VFD heatsink, a bank of contactor coils, direct sun on a steel enclosure door, or simply a poorly ventilated cabinet in a hot process area all push the internal air above ambient. A relay that reads correctly on the test bench at 20 C can trip early — or late — once it is living inside that enclosure.

How the Compensating Bimetal Cancels the Drift

Manufacturers solve this with a second bimetal strip mounted next to the primary one. The compensating strip is not heated by the motor current; it only sees the ambient air, the same air the primary strip sits in. It is oriented to bend in the direction that offsets the primary strip's ambient-driven deflection. Warm the enclosure and both strips bend together — the compensating strip's bend subtracts from the primary's reading, leaving only the current-driven component to trip the relay. Cool the enclosure and the same cancellation runs the other way.

The result is a relay whose trip point tracks the dial setting and the motor current, not the temperature of the box it lives in. This is standard on the mainstream bimetallic lines from thermal overload relays across the major brands — it is not an optional upgrade on a modern relay, it is baked into the mechanism.

The IEC Ambient Window and What "Compensated" Actually Promises

IEC 60947-4-1 defines the ambient band a compensated relay is designed to hold its calibration across, roughly -5 to +55/60 C depending on the model. Inside that band the trip current stays close to the dial setting. Compensation is not a guarantee of zero drift at any temperature — it is a bounded promise over a stated range, and manufacturers publish that range on the datasheet rather than a single blanket number. Push a relay past its rated ambient window, hot or cold, and the compensation mechanism runs out of margin the same way any calibrated instrument does outside its rated operating conditions.

What Happens on a Non-Compensated or Poorly Mounted Relay

A handful of low-cost bimetallic relays skip the compensating strip entirely. Their trip point genuinely walks with the cabinet temperature, which shows up as nuisance tripping on hot days or as unprotected overload on cold ones. More common in practice: a compensated relay mounted somewhere the compensation cannot help, because the compensating strip is reading a different air pocket than the motor's actual thermal environment.

What we see in the field is less often a defective relay and more often a mounting decision — a relay bolted directly above a VFD heatsink, or squeezed between two contactors whose coils run warm all day, trips a perfectly healthy motor on a summer afternoon. The compensation is doing its job on the air it can measure; the problem is that air is not representative of the panel as a whole.

Mounting Practice: Put the Relay in the Motor's Thermal Environment

The compensating bimetal assumes the relay sits in the same ambient as the rest of the motor's control gear, which is the assumption the standard's test conditions are built on. Two practical rules follow from that:

Keep it away from local heat sources

Avoid mounting the overload relay directly against a VFD heatsink, a transformer, or a cluster of coils that run continuously warm. Even a compensated relay is being asked to read an ambient that no longer matches the rest of the enclosure.

Respect the enclosure's own ventilation

A tightly packed cabinet with no airflow runs hotter in the top third than near the bottom vents. Where the relay lands in that gradient changes the ambient it actually sees, independent of anything printed on its label.

Electronic Overload Relays: Less Sensitive to Their Own Ambient

Electronic (solid-state) overload relays replace the bimetal strip with current transformers or shunts feeding a microcontroller that runs a thermal model of the motor in software. Because the trip decision comes from a calculation rather than a physical strip bending in the panel air, an electronic relay is less sensitive to its own ambient temperature than a bimetallic one — there is no compensating strip to speak of because there is no primary strip drifting in the first place.

That does not remove the case for careful mounting — the microcontroller and its supply electronics have their own temperature ratings — but the specific failure mode of a bimetal drifting with cabinet heat does not apply the same way. Where a panel runs consistently hot, an electronic unit from the thermal vs electronic overload relay comparison is worth weighing against a bimetal purely on this point, separate from its other extras like ground-fault and stall protection.

Compensation Across Schneider, ABB, and Siemens Ranges

All three of the major European brands build ambient compensation into their mainstream bimetallic lines as standard, not as an option to specify separately. Schneider's TeSys LRD clips under LC1D (TeSys D) contactors and is temperature compensated and phase-loss sensitive across its range. ABB's TA line — TA25DU through TA200DU and larger, mounted under A/AF contactors — carries the same compensation as standard. Siemens' SIRIUS bimetallic overload relays follow the same cross-brand convention: set to motor FLC, Class 10 or 10A, phase-loss sensitive, and ambient compensated, mounting directly onto 3RT2 contactors or on a standalone support.

The differences between the three show up in frame breakpoints, the top current a bimetal element reaches before you move to a CT-based electronic unit, and terminal/mounting ecosystem — not in whether ambient compensation exists at all. It is close to a baseline expectation on any current-production bimetallic motor protection overload relay from a tier-one manufacturer.

Frequently Asked Questions

Conclusion

Ambient compensation is the reason a thermal overload relay can be calibrated once on a bench and trusted to hold that calibration in a panel that runs warmer in July than in January. The mechanism is simple — a second bimetal strip cancelling the ambient component of the primary strip's deflection — but it only works within its rated band and only if the relay sees the same air the motor's control gear does. For the broader mechanism this article builds on, see the thermal overload relay engineering guide, and for the setting procedure itself, the overload relay selection and setting article covers matching the dial to motor FLC.