Thermal overload relays rated for AC motors up to 690 V with temperature compensation are the baseline motor protection on mining conveyors and crushers, with NXR-class devices citing automatic compensation across ambient swings [S2].
For mining, the relay question is not which brand is loudest, it is which trip class, current range, and protection logic match the actual starting and running profile of a belt drive or a crusher motor on a VFD-fed long cable run [S4].
Where a Thermal Relay Fits on a Mining Motor
A thermal overload relay sits downstream of the contactor and upstream of the motor, modelling the heating effect of running current and tripping before winding insulation exceeds its Class B (130 °C) or Class F (155 °C) limit; GEYA and CHINT both position their TORs as motor protection against overload and phase failure [S3][S2].
Selection Criteria: Current Range, Trip Class, Reset
The NXR thermal overload relay series covers AC motors up to 690 V, with the NXR-38 size cited as a typical frame, and offers temperature compensation plus manual and automatic reset options to match site philosophy [S2].
Current range is set by the heater element or electronic setting, and must bracket the motor's full-load amperes (FLA) at the worst-case voltage; running a relay too far above FLA starves protection, too far below it causes thermal trip on every start [S3][S2].
Trip class (Class 10, 20, 30) sets the cold-start trip time at 6 × FLA, with Class 10 standard for normal motors, Class 20 for high-inertia loads, and Class 30 reserved for very long starting profiles such as apron feeders and large crushers [S4].
Reset mode is a process decision, not a relay one: auto-reset on a tail conveyor is fine, on a crusher drive it is a safety hazard because a stalled rotor can re-energise into a jam [S3].
Conveyor vs Crusher: Different Protection Logic
Conveyors need start supervision, locked-rotor detection, and underload or belt-tear detection, because loss of load often indicates belt tear, broken coupling, or chute blockage, and the relay should alarm or trip before downstream damage [S4].
Crushers present the opposite problem: high rotational inertia and drastic load changes mean the relay's thermal curve, stall window, and phase unbalance thresholds must match observed start profiles, including ore hardness and feed conditions, or the motor trips on a normal start [S4].
NewElec's MA Series is cited as proven for high-demand motor protection covering start supervision, unbalance, and stall logic, while NewCode and NewFeed are positioned for code-defined and feeder-specific protection schemes respectively [S4].
Comparison: TOR Options for Mining Duty
Across the main types specifiable today, the decision breaks down on four axes: current and voltage envelope, trip-class flexibility, integration with the contactor, and VFD/EL coordination. [S3]
On a 690 V / 38 A frame such as the NXR-38, the relay covers a typical 18.5–22 kW motor and pairs with a matching NXC or NC1 contactor for a clean direct-mount assembly [S2].
For higher-current crusher duty in the 100–630 A range, a separate-mount electronic motor protection relay such as the MA Series, or NewCode for code-defined protection, gives the thermal modelling, stall window, and PQ trending that a basic bimetallic TOR cannot [S4].
On VFD-fed conveyors, the relay should be installed on the line side of the drive only when used for upstream protection; on the motor side, VFD output filters and VFD-rated EL relays are required to avoid nuisance trips, and the TOR itself can be replaced by the drive's electronic thermal model (I²t) set per motor nameplate [S4].
Thiim's mining relay portfolio takes a different angle, combining current, phase-failure, and residual-current relays to protect motors and cables against overload, phase imbalance, and insulation faults, with voltage and frequency relays handling grid-side quality on mobile mining substations [S5].
Who a Thermal Relay is For, and Who Should Skip It
A thermal overload relay is the right call for any direct-on-line or star-delta started AC induction motor in the 0.1–630 A range, on a fixed-frequency supply, where overload and phase-loss protection are the main concerns [S2][S3].
It is the wrong call for servo-driven actuators, DC motors, or soft-starters running in bypass mode past the ramp, where the I²t model in the drive or soft-starter already handles thermal protection, and a bimetallic relay on the motor side will fight the electronics and trip early [S4].
On VFD-fed pumps and conveyors below 7.5 kW where the drive has an electronic thermal model set to the motor FLA, adding a separate TOR on the motor side is generally unnecessary and creates nuisance-trip risk; the drive itself is the thermal relay [S4].
Limits, Failure Modes, and Common Commissioning Mistakes
The most common failure mode is trip-class mis-selection: a Class 10 relay on a high-inertia crusher trips every start, a Class 30 relay on a small fan motor lets the winding cook before trip [S4].
The second is ambient-temperature derating forgotten in the field, a relay calibrated at 20 °C will trip early in a 45 °C switchroom unless it carries an electronic temperature-compensated element, which both NXR and GEYA TORs claim as standard [S2][S3].
Standards, Sourcing, and What to Verify
Thermal overload relays on mining motors in most jurisdictions are specified to IEC 60947-4-1 for motor starters, with ATEX category 2 (2014/34/EU) certification required for Group I M2 mining equipment in the EU, and IECEx for equivalent international sites; the relay's ambient-compensation range, trip-class setting, and short-circuit coordination with the upstream breaker or fuse must be supported by the manufacturer's certificate [S4].
For coal and combustible-dust atmospheres, verify the relay enclosure rating (typically IP55 or higher) and that any associated EL relay is matched to the system voltage, fault level, and VFD topology, because the wrong combination either trips or fails to coordinate [S4][S5].
Specifiers should also confirm the relay's setting range covers the motor FLA at the worst-case supply voltage, and that the contactor-relay assembly is listed as a coordinated pair by the same manufacturer, which simplifies short-circuit and overload certification [S2].
Related Spec Maps
For deeper detail on relay-current matching and trip-class to contactor selection, see Thermal Overload Relay Selection: Current Range, Trip Class, and Contactor Match. [S3]
For vendor-level sourcing, including MA, NewCode, and NewFeed, see Motor Protector Suppliers: 2026 Vendor Map and Selection Specs.
For background on the protection device class itself, the thermal relay encyclopedia entry covers bimetallic, eutectic, and electronic trip principles.
Track, on 2026-09-04, the IEC 60947-4-1 amendment cycle and any ATEX Group I M2 test-report updates from NXR-class manufacturers, plus NewElec and Thiim product bulletins for new conveyor-side EL zoning modules; these are the most reliable signals for the next 6 months.
For component-level specifications, see mining dump truck, and thermal imager.