Siemens SIRIUS 3RU2 and 3RB3 Overload Relays: Full Range Review
What is the Siemens SIRIUS overload relay range? SIRIUS covers motor overload protection in two families: the 3RU21 bimetallic line in frame sizes S00 to S3, and the 3RB3 electronic (solid-state) line split into 3RB30 (standard) and 3RB31 (with ground-fault detection), both built to IEC 60947-4-1. Picking wrong between them means either overspending on electronics a simple pump doesn't need, or under-protecting a load that needs stall and ground-fault coverage. This review walks the frame breakpoints, setting ranges, mounting rules onto 3RT2 contactors and 3RV2 MPCBs, and where 3RB30 stops being enough.
SIRIUS 3RU21: Bimetallic Frame Sizes and Ranges
3RU21 is Siemens' mainstream bimetallic overload relay, built in four frame sizes — S00, S0, S2, S3 — each covering a fixed current band. S00 and S0 start around fractional amps and top out in the low double digits; S2 and S3 extend the family up to roughly 100 A. Trip class is fixed at Class 10, the default for standard-duty motors with a normal run-up. Reset is selectable hand or auto, the relay is temperature compensated, and it carries a differential trip mechanism sensitive to phase loss.
Mounting is direct: 3RU21 clips onto the base of a matching-frame 3RT2 contactor with no wiring between them, or it bolts to a standalone base for remote installation next to a manual motor starter. Frame size has to match the contactor frame — an S0 relay does not clip onto an S2 contactor. What we see in the field: builders sometimes order the relay current range without checking frame size against the already-selected contactor, and the two arrive incompatible for direct mounting.
SIRIUS 3RB30: Electronic Overload for Standard-Duty Motors
3RB30 replaces the bimetal element with current transformers feeding a solid-state trip circuit. The setting ratio widens to roughly 1:4 against about 1:1.5 on 3RU21, so one 3RB30 unit covers a range that would need two or three 3RU21 frame sizes. Trip class is selectable — 10, 20, or 30 — from a rotary switch on the front, letting one relay serve a standard pump on Class 10 and, reset to Class 30, a centrifuge on the same hardware.
3RB30 adds true phase-loss detection (not just the differential bimetal effect), thermal memory that survives a power cycle so a hot motor doesn't get a full cold-start allowance right after tripping, and stall/locked-rotor protection that a bimetal element cannot resolve on its own. It does not include ground-fault detection — that's the line that separates it from 3RB31.
SIRIUS 3RB31: Adding Ground-Fault Detection
3RB31 is mechanically and functionally the same base as 3RB30 plus a ground-fault (earth-fault) detection channel, reading current imbalance to ground through an internal zero-sequence measurement or an external core-balance CT depending on the variant. This matters on long motor feeders, wet or corrosive environments, and any load where an insulation breakdown to frame is a real risk before it becomes a full short circuit.
If the application has no meaningful ground-fault exposure — a dry indoor pump on a short, well-terminated cable — 3RB31's extra channel adds cost without a corresponding risk to cover. Reserve it for feeders with real earth-fault exposure, not as a default upgrade.
Setting and Trip Class Across the Range
All three relays set the same way: dial to the motor's nameplate full-load current (FLC), not to the breaker rating or a rounded-up number. On 3RU21 the narrow 1:1.5 band means the relay model has to be chosen close to the actual FLC from the start. On 3RB30/3RB31 the 1:4 band gives more room to size once and adjust later if the application changes.
Formula: Trip Class Test Condition — Source: IEC 60947-4-1, trip class definition
ttrip measured at I / Iset = 7.2, relay cold before test
| Symbol | Description | Unit |
|---|---|---|
| ttrip | Time to trip at the test multiple | s |
| I | Applied test current | A |
| Iset | Dial or programmed setting (= motor FLC) | A |
Class 10 trips in 4-10 s at 7.2x setting from cold — the default on 3RU21 and the low end of 3RB30/31's selectable range. Class 20 (6-20 s) and Class 30 (9-30 s) exist only on the 3RB3 line and are reserved for loads whose run-up outlasts what a Class 10 element tolerates: large fans, centrifuges, some conveyor drives with high starting inertia. Selecting the class wrong in either direction either nuisance-trips on every start or lets an actual overload run too long before the relay reacts.
Mounting and Coordination with 3RT2 and 3RV2
3RU21 and 3RB30/31 both mount directly onto the matching-frame 3RT2 contactor as part of the SIRIUS load feeder system, or onto a standalone base wired to a manual motor starter. The overload relay's auxiliary contact (95-96 NC) wires into the 3RT2 coil circuit so a trip drops the contactor out regardless of what's controlling it. Upstream, the short-circuit protective device is either a fuse or a 3RV2 MPCB — the overload relay does not clear short circuits, it only protects against sustained current above FLC.
Siemens publishes coordination tables pairing specific 3RV2 MPCB ratings with specific 3RT2 contactor and 3RU21/3RB3 combinations for both Type 1 and Type 2. Deviating from a published pair — swapping in an off-table breaker rating to save cost — voids the declared coordination and the short-circuit rating that comes with it.
3RU21 vs 3RB30 vs 3RB31: Side-by-Side
| Criteria | 3RU21 | 3RB30 | 3RB31 |
|---|---|---|---|
| Sensing element | Bimetal strips | Current transformer, solid-state | Current transformer, solid-state |
| Setting ratio | ~1:1.5 | ~1:4 | ~1:4 |
| Trip class | Class 10 fixed | 10/20/30 selectable | 10/20/30 selectable |
| Phase-loss protection | Differential bimetal | True phase-loss | True phase-loss |
| Stall/locked-rotor | No | Yes | Yes |
| Ground-fault detection | No | No | Yes |
| Frame range | S00-S3, up to ~100 A | Wide, above S3 into larger frames | Wide, above S3 into larger frames |
Where Each Line Fits in a Motor Control Center
In a motor control center with dozens of feeders, mixing 3RU21 on the small, standard-duty motors and reserving 3RB3 for the larger or higher-inertia loads keeps bill-of-materials cost down without under-protecting the loads that actually need the wider setting ratio or the extra fault channels. This depends on how much the panel builder already standardizes on one relay family for spares and training — some shops run 3RB30 across the board specifically to cut the number of distinct spare part numbers, accepting the higher unit cost per feeder.
All three relays fall under the same thermal overload relay engineering guide logic for the basic protection role: they sit between the contactors and the motor, they don't replace the motor protection circuit breakers or fuse upstream, and setting always starts from nameplate FLC, covered in more depth in how to select and set an overload relay for a motor.
Frequently Asked Questions
What's the practical difference between 3RU21 and 3RB30?
3RU21 is bimetallic, fixed at Class 10, with a narrow ~1:1.5 setting ratio. 3RB30 is electronic, selectable across Class 10/20/30, with a wider ~1:4 ratio plus stall and true phase-loss protection that the bimetal element doesn't provide.
Does 3RU21 mount directly on a 3RT2 contactor without an adapter?
Yes, as long as the relay frame size (S00, S0, S2, or S3) matches the contactor frame size. Mismatched frames require a standalone base and separate wiring instead of direct clip mounting.
Should every feeder use 3RB31 instead of the cheaper 3RB30?
No. 3RB31 adds ground-fault detection, which is worth the extra cost on long feeders or wet/corrosive environments with real earth-fault exposure. A dry, short, well-terminated feeder rarely needs it, and 3RB30 covers the same overload and stall protection for less.
Can a 3RU21 be reset automatically after a trip?
Yes, 3RU21 offers a hand/auto reset selector. Manual (hand) reset is the default choice for most motors so a person confirms the fault is cleared before restart; auto reset is reserved for loads where an unattended restart is safe.
What trip class should I start with if I don't know the motor's run-up time?
Start at Class 10 — it covers the vast majority of standard-duty pumps, fans, and general machinery. Step up to Class 20 or 30 on the 3RB3 line only if the motor is nuisance-tripping on start because its run-up genuinely exceeds the Class 10 curve.
Conclusion
3RU21 handles the bulk of standard-duty motor protection at lower cost and a simpler bimetal mechanism. 3RB30 earns its place on higher-inertia loads or wherever stall protection and a wider setting ratio matter more than unit price. 3RB31 is the narrow case: real ground-fault exposure on the feeder, not a default upgrade. Match the relay to the load's actual run-up and fault exposure, set it to nameplate FLC, and confirm the frame or range against the paired 3RT2 contactor and, where a short-circuit rating is declared, the coordinated 3RV2 MPCB or fuse ahead of it. For trip-class mechanics in more depth, see overload relay trip classes 10A, 10, 20 and 30, and for the coordination rules behind the Type 1/Type 2 tables, see IEC 60947-4-1 contactors and motor starters standards.
