Schneider LRD vs ABB TA vs Siemens 3RU2: Overload Relay Comparison

What is the difference between Schneider TeSys LRD, ABB TA, and Siemens SIRIUS 3RU2 overload relays? All three are bimetallic thermal overload relays built to IEC 60947-4-1, set to the motor's full-load current (FLC) and rated Class 10A or Class 10, and each snaps directly under its own manufacturer's contactor line rather than a competitor's. The consequence is that the choice is rarely about protection quality — bimetal is bimetal — and almost always about the frame you already committed to, the amperage the bimetal element tops out at before you need an electronic unit, and how deep the panel builder's stock of that brand's accessories already runs. This article compares current range, trip class, reset options, phase-loss sensitivity, and the electronic step-up line (LR9/TeSys T, E-series, 3RB30/3RB31) across Schneider, ABB, and Siemens so a panel builder can size a relay without guessing at cross-compatibility.

What the Three Families Have in Common

Strip away the part numbers and the three lines converge on the same design. Three bimetal strips, one per phase, bend as motor current heats them; a differential trip bar releases the 95-96 NC contact once the bend crosses the dial's set point. The dial is calibrated in amps and set to nameplate FLC, not to the contactor's rated current. Trip time follows an inverse-thermal curve — a small overload takes minutes to trip, a large one takes seconds — so a normal 6-8x FLC starting inrush passes without nuisance tripping while a sustained 1.2x overload still clears before the windings cook.

Phase-loss sensitivity is standard on all three: a lost phase pushes roughly 1.7x current onto the remaining two windings, and the differential trip mechanism reacts to the imbalance between phases faster than the plain thermal element would. Ambient compensation is also standard — a second bimetal element offsets drift in the trip point across roughly -5 to +55/60 C, provided the relay sits in the same ambient as the motor's control gear rather than bolted to a hot panel wall on its own. Hand (manual) reset is the default mounting on all three; auto reset is selectable where an unattended restart is acceptable.

Schneider TeSys LRD and LR9: Range and Reset

TeSys LRD is Schneider's mainstream bimetal line, clipping straight under the LC1D (TeSys D) contactor with no separate mounting kit. The LRD/LR9F family spans roughly 0.10 A to 630 A, with LR9F carrying the larger frames once the amperage outgrows LRD's block size. Below that, LR2K and LR3K (TeSys K) cover small motors on compact contactors. Class is 10A across the bimetal range, hand or auto reset is a switch on the front face, and temperature compensation and phase-loss sensitivity are built in — nothing to specify separately.

Where a motor needs a wider setting ratio or selectable trip class, Schneider steps up to LR9 under the TeSys T electronic platform: current transformers replace the bimetal strips, the setting ratio widens from about 1:1.5 to roughly 1:4, and Class becomes a dial or software selection instead of a fixed part number. What we see in the field: builders standardized on TeSys D for years tend to stay on LRD through the whole current range they carry, only reaching for LR9/TeSys T when a specific motor calls for ground-fault or stall detection the bimetal line doesn't offer.

ABB TA Series and E-Series: Range and Reset

ABB's bimetal line is the TA series — TA25DU, TA42DU, TA75DU, TA80DU, TA110DU, TA200DU and larger — mounting under the A and AF contactor frames, with setting ranges reaching roughly 200 A before the line runs out of bimetal frame. TF42 is the compact variant sized for the AF09-AF38 contactors. Class 10A, temperature compensation, phase-loss sensitivity, and hand/auto reset match the pattern set by the other two brands; ABB does not differentiate here.

Above the bimetal ceiling, or where selectable class matters, ABB's E-series (EF19, EF45, EF65, EF96, EF146, EF205, EF370, EF460) is solid-state: a 1:3 to 1:4 setting ratio, Class 10/20/30 selectable on the unit, and on some models a thermistor input and phase-loss detection layered on top. The step from TA to E-series is a bigger jump in both capability and part count than Schneider's LRD-to-LR9 step, since E-series covers eight distinct frame sizes rather than one electronic family spanning the full range.

Siemens SIRIUS 3RU2 and 3RB3: Range and Reset

Siemens' bimetal line is SIRIUS 3RU21, built in frame sizes S00, S0, S2, and S3, mounting directly onto 3RT2 contactors or standing alone on a support rail bracket where the contactor and relay aren't co-mounted. Class 10 (not 10A) covers the bimetal range up to roughly 100 A, which is the tightest bimetal ceiling of the three brands — Siemens pushes motors above that amperage onto the electronic line sooner than Schneider or ABB do.

3RB30 and 3RB31 are the solid-state step-up: selectable Class 5/10/20/30 (the only one of the three offering Class 5, a faster curve for light-duty, low-inertia loads), a wide 1:4 setting ratio, phase-loss protection, and — on 3RB31 specifically — ground-fault detection built into the relay rather than requiring a separate zero-sequence CT. Both 3RU21 and 3RB30/31 are part of the SIRIUS load-feeder system alongside 3RT2 contactors and 3RV2 MPCBs, and Siemens' coordination tables are published against that specific trio, not against mixed-brand combinations.

Trip Class Across the Three Brands

Trip class is the one spec that decides motor compatibility independent of brand. A standard pump or fan with a short run-up sits comfortably on Class 10A or Class 10 — Schneider and ABB both default their bimetal lines here, Siemens 3RU21 as well, just under a different class label (10 rather than 10A, a narrower trip window but the same practical range). A high-inertia load — a large fan, a centrifuge, a crusher — needs Class 20 or 30 so the run-up current doesn't cross the trip curve before the motor reaches speed; that pushes the choice toward the electronic lines on all three brands, since none of the bimetal ranges offer Class 20/30 as standard.

See our full breakdown of overload relay trip classes 10A, 10, 20 and 30 for how to match a class to a specific load's start time, and the selection walkthrough in how to select and set an overload relay for a motor.

Comparison Table: Frame, Range, Class, Mounting

Which Brand Fits Your Panel

Frame lock-in decides most of this before the spec sheet does. An LRD relay does not clip under a 3RT2 contactor, and a 3RU21 does not clip under an LC1D — each bimetal relay is built to its own manufacturer's contactor block, so the practical starting point is whatever contactor line the panel already standardizes on. Mixing brands means abandoning the direct-mount and wiring the overload relay as a standalone unit with jumper leads, which works electrically but adds panel space and a wiring step most builders skip unless a specific spare-parts or pricing reason forces it.

Past that, three things tip the decision: how high the bimetal range needs to reach before jumping to electronic (Schneider's 630 A ceiling on LRD/LR9F is the widest of the three), whether Class 5 matters for a light, fast-cycling load (only Siemens 3RB3 offers it), and whether ground-fault detection needs to live inside the relay itself rather than as a separate zero-sequence CT (3RB31 is the only bimetal-family sibling with that built in — ABB and Schneider both handle ground fault at the electronic tier through separate accessories or thermistor inputs). Service factor and duty cycle still set the actual dial value regardless of brand; that part of the calculation does not change.

All three brands publish coordination tables pairing their overload relay, contactor, and upstream short-circuit protective device (SCPD) for a declared Type 1 or Type 2 rating — see overload relay coordination: Type 1 vs Type 2 for what that rating actually guarantees after a fault. Cross-brand combinations fall outside those published tables, which is the practical reason panel builders stay within one manufacturer's overload relay, contactor, and motor protection circuit breaker or fuse for a given starter, even when individual components would fit physically.

Conclusion

Schneider TeSys LRD, ABB TA, and Siemens SIRIUS 3RU2 protect a motor the same way — bimetal strips heated by motor current, set to FLC, Class 10/10A, phase-loss sensitive, temperature compensated. The differences that matter sit in the amperage ceiling before you need an electronic unit, the selectable class options on that electronic line, and which brand's contactor frame the relay physically mounts to. Pick the family that matches the contactor already specified in the starter, confirm the bimetal range covers the motor's FLC, and only step up to an electronic line when the load's inertia, ground-fault requirement, or setting ratio demands it. For the full mechanics behind how these relays trip and how to size one from a motor nameplate, see the thermal overload relay engineering guide and browse the current range of thermal overload relays stocked across all three brands.