Mitsubishi TH-N12TP Thermal Overload Relay – TH-N Series

Mitsubishi TH-N12TP Thermal Overload Relay — Precision Motor Protection in Three-Phase Industrial Control Circuits

The Mitsubishi TH-N12TP is a bimetal thermal overload relay engineered for three-phase AC motor protection within the TH-N series product family. Its primary function in a motor control circuit is to monitor continuous current draw across all three phases and initiate a trip signal when sustained overcurrent conditions exceed the thermal capacity of the connected motor winding. Unlike electronic overload relays that rely on microprocessor-based sampling, the TH-N12TP uses a calibrated bimetal strip deflection mechanism — a deterministic, self-contained trip architecture that operates independently of external power supply or firmware state.

In a standard DOL (Direct-On-Line) starter configuration, the TH-N12TP mounts directly onto the load-side terminals of Mitsubishi S-N series contactors (S-N10, S-N12, S-N18, S-N21), forming a mechanically integrated motor starter assembly. This plug-in architecture eliminates inter-device wiring between the contactor and overload relay, reducing panel assembly time and eliminating a common source of field wiring errors in high-density MCC (Motor Control Center) builds.

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Technical Parameters

Hardware Logical Analysis

The TH-N12TP trip mechanism is built around a three-phase differential bimetal assembly. Each phase conductor passes through a dedicated heater element bonded to a bimetal strip. Under normal load, the strip deflection remains within a calibrated neutral zone. When current exceeds the set threshold for a duration consistent with Class 10 trip characteristics (trip time ≤ 10 seconds at 7.2× rated current), the accumulated thermal energy causes sufficient strip deflection to release the trip latch.

The phase loss detection function exploits the asymmetric deflection behavior of the three-strip assembly. When one phase is lost, the remaining two phases carry elevated current. The differential deflection between the active and inactive strips exceeds the mechanical threshold faster than a symmetric three-phase overload would, providing accelerated trip response to single-phasing conditions — a failure mode that causes disproportionate winding damage in squirrel-cage induction motors due to negative-sequence torque components.

The auxiliary contact block (1 NO + 1 NC) uses a snap-action mechanism driven by the trip latch. This ensures a clean, bounce-minimized contact transition regardless of the speed of bimetal deflection, which is important for PLC digital input modules that may interpret contact bounce as multiple trip events. The snap-action geometry also maintains a defined contact gap under trip conditions, providing reliable isolation of the control circuit.

The temperature compensation design of the TH-N12TP accounts for ambient temperature variation across its rated -10°C to +60°C operating range. The bimetal alloy composition and heater element geometry are selected to maintain consistent trip current accuracy across this range, avoiding nuisance trips in high-ambient enclosures or delayed trips in cold environments — both of which are failure modes observed in lower-grade thermal relays with inadequate compensation design.

The manual/automatic reset selector is a field-configurable mechanical switch on the relay body. In automatic reset mode, the relay self-resets after the bimetal cools below the trip threshold — suitable for unattended pump or fan applications where remote restart is acceptable. In manual reset mode, a physical button press is required after a trip, enforcing operator acknowledgment before motor restart — the preferred configuration for safety-critical or process-critical motor circuits.

System Integration Benefits

• Deterministic trip response: Bimetal thermal mechanism operates without firmware, eliminating software-related trip failures or update-induced behavioral changes in safety-critical motor circuits.
• Zero inter-device wiring: Direct plug-in to S-N series contactors removes terminal block connections between contactor and overload relay, reducing panel wiring labor and eliminating a documented source of loose-connection faults in vibration-prone environments.
• Phase loss protection without external CT: Integrated differential bimetal detects single-phasing conditions without requiring external current transformers or additional monitoring relays, reducing BOM complexity in standard motor starter designs.
• PLC-compatible auxiliary output: The 1 NO + 1 NC snap-action contact block interfaces directly with 24 V DC PLC digital input modules (Mitsubishi MELSEC iQ-R, iQ-F, Q series) for trip status feedback without signal conditioning hardware.
• Field-selectable reset mode: Manual/automatic reset selection without tools allows commissioning engineers to configure trip behavior at panel level to match process safety requirements without relay replacement.
• Compact form factor for high-density MCC builds: The TH-N12TP’s footprint matches the S-N contactor body width, enabling standard IEC MCC drawer configurations without additional horizontal spacing allocation.
• Ambient temperature compensation: Calibrated bimetal alloy maintains trip accuracy across -10°C to +60°C, supporting deployment in outdoor enclosures, foundry environments, and cold-storage facilities without derating adjustments.
• IEC 60947-4-1 Class 10 compliance: Standardized trip class enables direct substitution in IEC-compliant panel designs without re-engineering the motor protection coordination study, reducing retrofit engineering cost.

Quality Assurance & Global Logistics

Every Mitsubishi TH-N12TP unit supplied by siemensplc.com is sourced through verified supply channels and undergoes physical inspection covering label integrity, housing condition, terminal quality, and part marking verification against Mitsubishi Electric’s published dimensional and electrical specifications. Units are stored in ESD-controlled, humidity-monitored warehouse conditions in Xiamen, China, consistent with the storage requirements for electromechanical relay components.

Xiamen’s geographic position as a major South China port city provides direct access to international freight lanes serving Asia-Pacific, the Middle East, Europe, and the Americas. Standard shipment lead time for in-stock units is 1–3 business days from order confirmation. International shipments are accompanied by a commercial invoice, packing list, and certificate of origin. HS Code guidance and export compliance documentation are available upon request for customs clearance in regulated import markets.

All units are covered by a 12-month warranty from the date of shipment. Warranty claims are processed with photo documentation of the defect and unit label. Volume pricing is available for orders of 10 or more units. B2B purchase order procurement is supported for registered business accounts.

Contact Information

Email: [email protected]
WhatsApp: +86 18359268345
Web: siemensplc.com
Location: Xiamen, China
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