Fuses and Thermal Relays in Electrical Circuits: A 2026 Engineering Note
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Industrial Automation · Protection Devices · Updated 2026-06-30
By KOEED Engineering Desk · 2026-06-30 · 7 min read · Engineering Notes
Fuses and thermal overload relays remain the first line of defense in any motor branch or power-distribution circuit. The 2024-era story — "fuse or bimetal strip, pick one" — no longer holds. Modern electronic motor-protection relays (EOLRs), semiconductor fuse classifications, and IEC 60947-4-1 trip classes 5 / 10 / 20 / 30 have reshaped the spec sheet. This note walks through the current logic, the latest product families stocked at KOEED, and what changed since 2024.
TL;DR
- Fuses follow IEC 60269 utilization categories:
gG(general),aM(motor),gR/gS(semiconductor DC-bus). - Thermal overload relays are classified by IEC 60947-4-1 trip class: 5 / 10 / 20 / 30. Class 5 is the new baseline for inverter-fed motors; Class 30 still dominates heavy-start applications.
- Electronic motor-protection relays (Siemens 3RB3, Schneider LR9D, ABB E200, Eaton C441) have largely replaced bimetal units above 32 A in 2026 panels.
- Selecting the right combination is a coordination problem (Type-1 / Type-2 per IEC 60947-4-1), not a component pick.
1. Why fuses and thermal relays still belong together
In any motor branch circuit, the fuse carries the short-circuit duty while the thermal relay (or modern electronic equivalent) handles sustained overload. They are not interchangeable. Skipping either side is the most common cause of nuisance tripping on one hand and welded contactors on the other. The original 2024 article framed it as a series element — it still is — but the way each element is selected has tightened under the latest IEC revisions.
2. Fuses under IEC 60269 (2026 view)
IEC 60269 is no longer a single document; it is a series. IEC 60269-1 covers general requirements, while the parts 60269-2 through 60269-6 cover specific fuse types and utilization categories. The two-letter codes below are the same in 2026 that they were in 2024 — they just show up in more panels:
| Utilization category | Typical duty | Branch-circuit role | Typical brand families stocked |
|---|---|---|---|
gG |
General-purpose cable & conductor protection | Distribution sub-circuit | Schneider TeSys DF, ABB OFAF, Eaton Bussmann NH gG |
aM |
Motor-branch short-circuit only (paired with contactor + overload) | Motor branch | Schneider DF2, ABB OFAA, Eaton Bussmann BS88 aM |
gR / gS
|
Semiconductor / DC-bus / drive input | VFD input, soft-starter, DC link | Eaton Bussmann 170M, ABB MS, Mersen Protistor |
gPV |
PV string / combiner-box (per 60269-6) | Solar string & array | Eaton Bussmann PV, ABB E90, Mersen HP15NH1 |
! Warning
A gG fuse is not a substitute for an aM fuse on a
motor branch. Using gG alone will let the contactor absorb the let-through energy that an
aM would have cleared — and Type-2 coordination per IEC 60947-4-1 will not hold.
For North American panels, UL 248 has been progressively harmonized with IEC 60269 since
the 2010s. In a 2026-dated panel you can quote a Class J or Class CC
fuse alongside the equivalent IEC gG; the time-current curves will line up to
within one frame size.
3. Thermal overload relays: trip class is the spec that matters
A thermal overload relay trips when the integrated heating of a bimetal strip (or its electronic model) crosses a threshold. The thing the datasheet calls out — and the thing most often misread on a 2024-vintage panel schedule — is the trip class defined in IEC 60947-4-1:
| Trip class | Trip time at 7.2× setting, from cold | Typical application |
|---|---|---|
| 5 | 0.5 – 5 s | VFD-fed motors, servo pumps, fans |
| 10 | 4 – 10 s | Standard DOL motors, conveyors, compressors |
| 20 | 6 – 20 s | Heavy-start pumps, mixers, crusher motors |
| 30 | 9 – 30 s | High-inertia fans, centrifuges, hoist motors |
The 2024 article lumped everything under "thermal relay." In 2026, the default for any inverter-fed motor is Class 5, because the VFD already limits the inrush; if you keep a Class 30 bimetal on a fan driven by a modern PowerFlex 525, it will nuisance-trip the first time the line sags.
4. Bimetal vs. electronic: where the market has moved
Bimetal thermal relays are still in production and still widely stocked. They are cheap, rugged, and need no auxiliary power. Above 32 A — and increasingly above 16 A in new panels — electronic motor-protection relays have displaced them because they:
- Offer selectable trip class (5 / 10 / 20 / 30) on the device itself.
- Provide phase-loss, phase-sequence and ground-fault detection as standard.
- Communicate over IO-Link, Modbus RTU, or PROFIBUS into the PLC scan.
- Hold ±5 % accuracy across the full −25 °C to +70 °C band; bimetal drifts with ambient.
| Brand | Bimetal series (current 2026) | Electronic series (current 2026) | Typical current range |
|---|---|---|---|
| Schneider | TeSys LRD (01…35) | TeSys LR9D / LR9F | 0.1 – 630 A |
| Siemens | SIRIUS 3RU2 | SIRIUS 3RB3 | 0.1 – 630 A |
| ABB | TF / EF series | E200 / E210 electronic | 0.1 – 800 A |
| Eaton | C440 / C441 bimetal | C441 electronic / PowerXL DG1 | 0.3 – 1500 A |
| Allen-Bradley | 193-E / 193-ED (Eutectic series) | 193-EC (E3 Plus electronic) | 0.1 – 800 A |
5. Coordination — the missing piece in the 2024 article
Picking a fuse and a thermal relay individually is not enough. The two have to be coordinated per IEC 60947-4-1 Type-1 or Type-2:
- Type-1: under short circuit, the contactor may weld but the installation is safe. Replaced after the fault.
- Type-2: no welding, no damage. The contactor is reusable after a typical fault. This is what 2026 EPC specs call for.
Type-2 coordination is published by every major brand in tables that map their aM fuse rating to their contactor / overload pair. If you do not check the table, you will overspec the fuse and blow it on every motor start.
6. Reset behavior and maintenance implications
The original article highlighted auto-reset after the fault clears. In practice:
- Bimetal: ambient-temperature sensitive. A 10 °C swing shifts the trip curve by ~5 %. Cabinets above 40 °C ambient derate the setting.
- Electronic: ambient-independent in spec, but needs 24 V DC or a current-transformer supply. Loss of aux power = no protection.
- Self-resetting is now rare on new panels. Most 2026 panels mandate manual reset for personnel safety, with auto-reset only on unattended pump or fan loads.
7. What changed since 2024
> Note — what shifted in the spec book
- IEC 60269-6 (photovoltaic fuses) is now mainstream in combiner-box designs — expect it on any 2026 solar retrofit.
- Class 5 trip class has become the de-facto default for VFD-driven motors, replacing Class 10 in most datasheet selections.
- gR / gS semiconductor fuses are now standard on PowerFlex 525, 755 and Altivar 320/600 DC-bus inputs.
- Electronic EOLRs with IO-Link have moved below 16 A (Schneider LR9D and Siemens 3RB3 now ship from 0.1 A upward with the same electronic option).
- UL 248 / IEC 60269 cross-reference is now listed in major brand catalogs directly — one frame size, one curve.
8. Sourcing at KOEED
We stock active and EOL bimetal / electronic motor-protection relays across Schneider TeSys, Siemens SIRIUS, Allen-Bradley 193-EC and ABB EF / E200. Common orders we ship: LRD01…LRD35, 3RU2116, 193-EC2ADB, EF19-18.5, plus BS88 / NH aM fuses from Eaton Bussmann and Schneider DF2BN.
Send the motor nameplate (kW, FLA, duty, ambient, drive type) and we will return a fuse + contactor + overload coordination proposal in one quote.
Need a fuse + thermal relay coordination quote?
Send the motor nameplate, FLA, ambient, and drive type to Moritta@KOEED.COM. Active stock, EOL stock, and cross-references — one quote within 24 hours.
Send My BOM →Related on KOEED Blog
- PLC Error Code Database — fault code lookup for Siemens / Mitsubishi / Omron.
- Modbus CRC Calculator — when the protection relay reports over Modbus RTU.
- Schneider TeSys collection — LR9D electronic relays and DF2BN aM fuses, in stock.
- AI Diagnostic Tool — send a nameplate photo, get a coordination proposal in under a minute.
- Sales and delivery records — recent shipments of LRD01, 3RU2116, and 193-EC series.
KOEED Engineering Desk
Industrial automation editors at KOEED. We write about PLC sourcing, motor-branch coordination, and legacy system support across Allen-Bradley, Siemens, Schneider, ABB, and Eaton. Reach the team at Moritta@KOEED.COM .