A bi-metallic thermal relay in the 0.1-150 A range covers roughly 90% of three-phase induction motor overload applications seen on Chinese low-voltage motor control centers, with NYKR2-class units rated to 690 V insulation voltage, AC 50/60 Hz, and certified to GB 14048.4 / IEC 60947-4-1 [S4].
For a 2026 spec, buyers match the relay's adjustable current window to the motor's full-load amps (FLA), then match the relay's frame designation to the contactor it sits beneath — Fuji SC-E02x/03x/04x/05x frames take TK-E02-series overload blocks such as the 1.4-2.2 A TK-E02-220, while Chinese builders pair JR36 or JRS1 / JR28 blocks to their CJ19 / CJ20 contactor families [S1][S2][S3].
Function, Trip Mechanism and Why Bi-Metallic Still Dominates
The thermal relay's job is overload and open-phase protection for long-running AC motors; the bi-metallic strip deflects under I²R heating, tripping an auxiliary contact that breaks the contactor coil circuit [S4]. Bi-metallic construction remains the workhorse because it costs less than electronic motor protection relays, integrates with standard contactors, and survives the heat, dust and vibration of a typical MCC bucket.
Open-phase (phase-loss) protection is the second non-negotiable function: if one phase of a three-phase supply drops, the relay must trip before the remaining two windings overheat — a failure mode that destroys stator insulation within minutes on a loaded motor [S4].
Key Spec Gates to Lock Before Sourcing
Four numbers define a usable thermal relay: (1) adjustable current range — e.g. 1.4-2.2 A on the TK-E02-220, or 0.1-150 A across the NYKR2 family; (2) rated insulation voltage, with 690 V the de-facto industrial ceiling for this class; (3) trip class — Class 10 (fast), Class 20 (standard), Class 30 (heavy-duty / long-start) selected against motor starting time; (4) reset mode — automatic, manual, or stop-only [S2][S4].
A practical decision rule: set the trip dial to motor FLA, never below; if locked-rotor start time exceeds 10 s, specify Class 20 or higher; if the process is irreversible (conveyor belt, mixer with solidifying media), mandate manual reset so the line cannot restart unattended.
Options Compared: Bi-Metallic, Electronic and PTC-Thermistor
Three families sit on the buyer's shortlist in 2026. Bi-metallic (JR36, JRS1/JR28, NYKR2, TK-E02) is the cost baseline, gives Class 10 by default with optional Class 20, and resets locally [S1][S2][S3][S4]. Electronic motor protection relays add adjustable trip class 5-30, ground-fault option, and analog/digital output, at 3-5x the cost. PTC-thermistor relays watch embedded stator sensors and are reserved for motors in classified hazardous areas or with very high winding temperatures where I²R modeling breaks down.
For most HVAC fans, pumps and general-purpose conveyors in non-classified areas, bi-metallic is the correct economic choice; electronic earns its premium only on drives > 11 kW, on motors with variable-frequency supply, or where trip-class adjustment is required by the process.
Contactor Match, DIN-rail Mounting and Accessory Logic
A thermal relay is mechanically and electrically coupled to its contactor; mismatched frames either will not seat or will not trip the coil at the correct current. Fuji's TK-E02-220 mounts directly to SC-E02x through SC-E05x contactor bodies [S2]. Chinese equivalents use the same convention: JRS1 pairs to a specific CJ19 frame, JR28 to a slightly larger CJ20 family [S1][S3].
Useful auxiliaries — separate NO/NC signal contacts for PLC feedback, a remote reset coil, a transparent dust cover, and a door-interlock rotary handle — are standard on mid-tier NYKR2-style blocks and typically priced as line items rather than bundled [S4]. When the relay sits inside a withdrawable MCC bucket, specify the wiring-duct adapter plate up front; retrofitting it in the field is a common 2-3 hour punch-list item.
Standards, Certification and Sourcing Tiers
IEC 60947-4-1 (and its GB 14048.4 equivalent) is the governing product standard for low-voltage thermal overload relays; CE marking under the EU Low Voltage Directive and CCC for the China domestic market are the two certifications buyers see most often on 2026 datasheets [S4]. For US-bound panels, UL 508 listing on the relay-plus-contactor combination is mandatory, and many Asian bi-metallic blocks carry it as an option rather than standard.
Sourcing tiers in 2026 run roughly: Tier-1 (Schneider LRD, ABB TF, Siemens 3RU, Eaton XT) — premium price, full global certifications, 2-4 week lead; Tier-2 (Fuji TK-E02, LS Electric MT, Chint NXR, Delixi JRS1) — 50-70% of Tier-1 price, 1-3 week lead, IECEx and CE present, UL varies by SKU [S1][S2][S3][S4]. Buyers specifying a thermal relay for a related motor-protection build-out should cross-reference a motor protector selection guide for trip-class and standard-gate overlap.
Limits, Failure Modes and Field-Test Gates
Bi-metallic thermal relays are ambient-temperature-sensitive; a relay calibrated at 20 °C and mounted inside a non-ventilated 50 °C cabinet will trip early and may nuisance-trip a healthy motor. Specify ambient-compensated models (standard on NYKR2-class) when cabinet internal temperature exceeds 40 °C [S4]. They also fail in the unsafe direction after a high-overload event — the bimetal anneals and the trip point drifts upward — so thermal relays in critical service (escalator drives, mine ventilation) are routinely replaced on a 5-7 year cycle, not run to failure.
Acceptance test on receipt: with the relay cold, dial set to motor FLA, apply 1.05 × FLA — relay must not trip within 2 hours; apply 1.2 × FLA — must trip within 2 hours (Class 10A boundary per IEC 60947-4-1). Many Chinese Tier-2 lots skip this factory test; insist on a witnessed test report or do it yourself on the bench before panel-build.
2026 Sourcing Signals and Trackable Cues
For a parallel view on adjacent motor-side protection pricing, the motor protector price and cost guide 2026 breaks down FOB bands and spec drivers for the same buyer. [S1]
For component-level specifications, see linear guide, and crossed roller guide.