Overload Relay Cross-Reference: LRD, TA, 3RU2 Replacement
What does "cross-reference" mean for a thermal overload relay? A cross-reference match compares two overload relays on the parameters that determine trip behavior — bimetal or electronic current range, IEC 60947-4-1 trip class, phase-loss sensitivity, and reset type — not just brand and catalog number. Get one of those parameters wrong and the replacement either nuisance-trips on every motor start or fails to protect the winding at all. This article covers how to cross-reference between Schneider TeSys LRD, ABB TA, and Siemens SIRIUS 3RU2 bimetallic relays: which parameters must match exactly, which frame and mounting differences break a "drop-in" claim, and when a cross-referenced relay changes the panel's declared coordination Type.
Why Part Number Alone Doesn't Cross-Reference: The Four Parameters
A catalog number encodes current range and frame, not trip curve shape. Two relays with an overlapping current window can still trip at different times for the same fault current if one is Class 10A and the other is Class 20. Swap a Schneider LRD3K (Class 10A) for a Siemens 3RU21 set to Class 10 and the motor sees roughly the same trip time band — close enough for most standard-duty pumps and fans. Swap either for an electronic relay left at its factory default Class 20 and the motor runs 6-20 s longer under a stalled-rotor condition before the relay opens.
What we see in the field: panel builders reuse an old cross-reference sheet from a prior obsolescence event without re-checking that the new relay's factory-default class matches the original. The current range lines up, the box gets closed, and the class mismatch surfaces months later as a burnt winding or a run of nuisance trips.
A usable cross-reference checks four parameters together, not one in isolation:
- Current setting range — the motor's FLC from the nameplate must sit inside the dial's marked window, ideally in the middle third rather than at either end.
- Trip class — Class 10A, 10, 20, or 30 per overload relay trip classes 10A, 10, 20 and 30 must match the load's start profile, not just the old relay's label.
- Phase-loss sensitivity — most modern bimetal relays from all three brands include a differential trip bar; older stock or bargain lines sometimes don't. Confirm before treating it as a given.
- Reset type and contact set — hand vs. auto reset, and whether the application wiring depends on both the 95-96 NC and 97-98 NO auxiliary contacts.
Schneider TeSys LRD, ABB TA, and Siemens 3RU2: Range, Frame, and Side-by-Side Comparison
Schneider's TeSys LRD covers roughly 0.10 A to 630 A across the LRD family and the larger-frame LR9F, clipping directly under LC1D contactors; LR2K/LR3K serves the smaller TeSys K motor starters. ABB's TA line — TA25DU, TA42DU, TA75DU, TA80DU, TA110DU, TA200DU — mounts under A/AF contactors with settings reaching about 200 A; TF42 is the compact version for AF09-AF38 frames. Siemens SIRIUS 3RU21 spans frame sizes S00, S0, S2, and S3 up to roughly 100 A, mounting directly onto 3RT2 contactors or standalone on a support rail.
None of the three ranges maps 1:1 onto another brand's frame breakpoints. A motor at 85 A sits inside ABB's TA110DU window and Schneider's LRD3365 range, but pushes past a Siemens 3RU21 S2 frame and into S3 — a different physical size, not just a different dial.
| Criteria | Schneider TeSys LRD | ABB TA | Siemens SIRIUS 3RU21 |
|---|---|---|---|
| Bimetal current range | ~0.10–630 A (LRD/LR9F) | up to ~200 A (TA25DU–TA200DU) | up to ~100 A (frames S00–S3) |
| Contactor frame tie-in | LC1D / LC1F (TeSys D/F) | A/AF contactors; TF42 for AF09–AF38 | 3RT2 (S00–S3) |
| Bimetal trip class | Class 10A | Class 10A | Class 10 |
| Electronic option | LR9 / TeSys T, selectable class | E-series (EF19–EF460), selectable Class 10/20/30 | 3RB30/3RB31, selectable Class 5/10/20/30 |
| Ground-fault detection | on select LR9 variants | on select E-series with thermistor input | on 3RB31 |
| Standard | IEC 60947-4-1 | IEC 60947-4-1 | IEC 60947-4-1 |
The table lines up bimetal-to-bimetal and electronic-to-electronic. Cross-referencing a bimetal Schneider LRD into a Siemens 3RB31 electronic relay is a real upgrade path, not a like-for-like swap, and should be priced and documented as one.
Verifying Trip Class Equivalence Before You Swap
The number on the relay body only means something against the test condition it was measured under. Before accepting a cross-reference, pull the trip class of both relays and check them against the same 7.2x-from-cold benchmark.
Formula: Trip Class Equivalence Check — Source: IEC 60947-4-1, trip class test condition
ttrip = f(7.2 × Iset) from cold: Class 10A = 2–10 s | Class 10 = 4–10 s | Class 20 = 6–20 s | Class 30 = 9–30 s
| Symbol | Description | Unit |
|---|---|---|
| Iset | Current dial setting on the overload relay, set to motor FLC | A |
| ttrip | Trip time at 7.2x Iset, tested from cold | s |
| Class | IEC 60947-4-1 trip class designation of the relay being cross-referenced | – |
If the replacement relay's class band is narrower or shifted from the original, re-check the motor's run-up time against it rather than assume the swap is transparent. See how to select and set an overload relay for a motor for the setting procedure itself.
Mounting and Accessory Differences That Break a Drop-In Swap
Current range and trip class are electrical questions. Mounting is mechanical, and it fails just as often. ABB's TF42 clips onto the AF09-AF38 contactor body; it does not fit a Schneider LC1D or a Siemens 3RT2 without an adapter, and no adapter is standard across all three brands. Siemens 3RU21 has its own base-mount and direct-mount variants tied to 3RT2 frame sizes S00 through S3. Schneider's LRD/LR9 range ties to LC1D and LC1F frame breakpoints in the same way.
Cross-referencing an overload relay from one brand to another almost always means cross-referencing the contactor underneath it too, unless the relay is being converted to standalone/base mounting with separate wiring. Budget for that when a customer asks for a "like-for-like" swap on an obsolete panel.
Bimetallic vs. Electronic Cross-References
An electronic overload relay can usually stand in for a bimetallic one from another brand, but not the reverse without losing features. Electronic relays — ABB's E-series (EF19 through EF460), Siemens 3RB30/3RB31, Schneider's LR9/TeSys T — carry a wider 1:3 to 1:4 setting ratio versus about 1:1.5 on bimetal, plus selectable class, ground-fault detection on some models (3RB31), and thermal memory that survives a power cycle. Crossing a bimetal original into an electronic replacement is close to a strict upgrade on protection features, at a higher unit cost and with a current transformer or shunt to install.
Going the other way — replacing an electronic relay with a bimetal one because it's in stock — drops phase-imbalance detection, stall protection, and the wide setting ratio. That trade only makes sense as a temporary fix, and it should be flagged, not treated as equivalent. See thermal overload relay engineering guide for the full bimetal-vs-electronic breakdown.
When Cross-Reference Isn't Enough: Coordination Type
A panel's declared coordination Type (IEC 60947-4-1) covers the specific combination of SCPD, contactor, and overload relay tested together by the manufacturer. Type 1 allows the starter to be damaged after a short circuit as long as it doesn't endanger persons; Type 2 requires the starter be serviceable afterward, with at most light, easily separated contact welding. Swapping any one of the three components — including "just the overload relay" — voids the manufacturer's tested Type unless the new combination appears in a published coordination table for that exact SCPD, contactor, and relay set. Full background: overload relay Type 1 vs Type 2 coordination.
This is the step that gets skipped most often. A procurement team matches current range and trip class, closes the cross-reference, and never re-pulls the coordination table for the new brand combination. On a Type 2 panel, that's a documentation gap that shows up at the next audit, not a nuisance trip that shows up on day one.
Buying Cross-Referenced Overload Relays
For a straight replacement — same brand family, same frame, obsolete part number — stock the direct successor rather than hunting for an exact discontinued match; Schneider, ABB, and Siemens all publish successor part numbers for retired LRD, TA, and 3RU2 codes. For a cross-brand swap driven by lead time or a multi-vendor panel, work from the four-parameter check above before ordering, and confirm the paired contactor is available from the same brand if the mounting is frame-integrated. Stoklink stocks current-production thermal overload relays from all three brands alongside matching contactors and motor protection circuit breakers, which keeps the cross-reference and the order on one line item instead of three separate vendor lead times.
Frequently Asked Questions
Can I replace a Schneider TeSys LRD with an ABB TA relay directly?
Only if the current setting range, trip class, and contactor frame all match. The relay itself won't mount on a Schneider LC1D contactor — ABB's TA and TF relays are built for A/AF frames — so a direct cross-brand swap usually means changing the contactor too.
Do cross-referenced overload relays need the same trip class?
Yes, or a class matched to the load's actual start time. A Class 10A relay and a Class 20 relay both sit within IEC 60947-4-1, but they trip at different points on the same fault current, which changes protection margin and nuisance-trip risk on high-inertia loads.
What happens if I use a Class 20 relay where a Class 10 was specified?
The motor gets a longer allowed run-up time before the relay trips, which can be correct for a high-inertia load or unnecessary heat exposure for a standard pump or fan. Check the motor's actual start time against both classes before accepting the substitution as equivalent.
Can an electronic overload relay replace a bimetallic one from another brand?
Generally yes, and it adds features — wider setting ratio, selectable class, phase-imbalance and stall protection on most electronic lines. It costs more and needs a current transformer or shunt, so treat it as an upgrade decision, not a plain substitution.
Does cross-referencing an overload relay affect Type 2 coordination?
It can. Type 1 and Type 2 ratings under IEC 60947-4-1 apply to the tested combination of SCPD, contactor, and overload relay. Changing the relay brand without checking the manufacturer's coordination table for the new combination can quietly downgrade a Type 2 panel to untested status.
Where can I check contactor-relay mounting compatibility across brands?
Each manufacturer's coordination and accessory tables list which overload relay frames mount on which contactor frames. Cross-check the relay's current range and frame against the specific contactor line before ordering, rather than relying on a generic cross-reference chart.
Conclusion
A reliable cross-reference between Schneider TeSys LRD, ABB TA, and Siemens SIRIUS 3RU2 overload relays checks four things every time: current setting range against the motor's nameplate FLC, trip class against the load's start profile, phase-loss sensitivity, and reset/contact configuration. Mounting footprint and coordination Type sit on top of that and decide whether the swap is a drop-in or a partial panel rebuild. Treat catalog cross-reference sheets as a starting point, not a final answer, and re-verify against the current datasheet before an order goes in.
