Mitsubishi Electric TH-N Series Thermal Overload Relays — TH-N12KP

Technical Dossier

Product Details And Specifications

Mitsubishi Electric TH-N Series: Comprehensive Thermal Overload Relay Range and Technical Overview

The Mitsubishi Electric TH-N Series thermal overload relays are bimetallic, current-sensing protective devices designed for use with the S-N and S-T series magnetic contactors. Deployed across global heavy industry — including petrochemical plants, nuclear auxiliary systems, offshore platforms, and continuous-process refineries — the TH-N range provides motor branch-circuit overload protection compliant with IEC 60947-4-1. The series covers trip current ranges from sub-1A to over 800A, accommodating motor control centers (MCCs) from 0.1 kW to several hundred kW. Its mechanical simplicity, manual/automatic reset options, and direct contactor-mounting design have made it a standard specification in Asian and global industrial installations for over three decades. As the TH-N series has entered its mature/end-of-life phase for several sub-models, long-term maintenance sourcing is a primary concern for plant operators running legacy motor control infrastructure.

The Evolution of TH-N Series Architecture

The TH-N series was introduced by Mitsubishi Electric as a direct-mount thermal overload relay companion to the S-N contactor lineup, replacing earlier standalone bimetallic relay designs that required separate panel mounting and external wiring. The original TH-N architecture used a three-phase bimetallic strip mechanism with a differential trip function to detect single-phase loss conditions — a critical feature for three-phase motor protection.

Early variants (TH-N8, TH-N12, TH-N18) were rated for use with S-N10 through S-N35 contactors and covered trip ranges up to approximately 18A. As industrial motor ratings increased, Mitsubishi extended the series upward with TH-N60, TH-N120, TH-N220, and TH-N400 to address larger drive applications. The KP suffix (e.g., TH-N12KP) denotes models with a built-in alarm contact output, enabling remote fault signaling to PLCs or DCS systems — a feature that became standard in process industries requiring SCADA integration.

Compatibility note: TH-N relays are mechanically interlocked with their corresponding S-N contactor frame size. Cross-frame mounting is not supported. For installations migrating to the newer S-T or S-V contactor series, the corresponding TH-T or TH-N (T-frame) relay variants must be specified. Mitsubishi has issued end-of-production notices for several TH-N sub-models; replacement cross-references to the TH-T series are available through authorized distributors and specialist spare parts suppliers.

TH-N Series Full Catalog & Functionalities (SKU List)

Thermal Overload Relays — Standard & KP (Alarm Contact) Variants:

TH-N8: Bimetallic overload relay, 1.6–8A trip range, S-N10 mount
TH-N8KP: TH-N8 with auxiliary alarm contact output for remote fault signal
TH-N12: Bimetallic overload relay, 3–12A trip range, S-N10/S-N18 mount
TH-N12KP: TH-N12 with alarm contact; 3–12A; direct mount on S-N10/S-N18 contactor
TH-N18: Bimetallic overload relay, 6–18A trip range, S-N18/S-N25 mount
TH-N18KP: TH-N18 with auxiliary alarm contact, S-N18/S-N25 frame
TH-N20: Overload relay, 10–20A trip range, S-N25 contactor mount
TH-N25: Bimetallic relay, 13–25A trip range, S-N25/S-N35 mount
TH-N25KP: TH-N25 with alarm contact output, S-N25/S-N35 frame
TH-N35: Overload relay, 18–35A trip range, S-N35/S-N50 contactor mount
TH-N35KP: TH-N35 with remote alarm contact, S-N35/S-N50 frame
TH-N60: Heavy-duty bimetallic relay, 30–60A trip range, S-N50/S-N65 mount
TH-N60KP: TH-N60 with alarm contact, for S-N50/S-N65 contactor frame
TH-N120: High-current overload relay, 60–120A trip range, S-N95/S-N125 mount
TH-N120KP: TH-N120 with auxiliary alarm contact, S-N95/S-N125 frame
TH-N220: Large-frame bimetallic relay, 110–220A trip range, S-N150/S-N220 mount
TH-N400: Extra-large frame relay, 200–400A trip range, S-N300/S-N400 mount
TH-N400KP: TH-N400 with alarm contact output, S-N300/S-N400 frame

Sourcing Hard-to-Find & Obsolete TH-N Series Parts

Multiple TH-N sub-models have been discontinued or placed on limited-availability status by Mitsubishi Electric as the product line transitions toward the TH-T and TH-N (new frame) series. For plant operators maintaining legacy S-N contactor-based MCCs, replacement sourcing requires specialist inventory channels.

Quality Control for the TH-N Series Range

Thermal overload relays require functional verification beyond visual inspection. DriveKNMS applies the following test protocol to all TH-N series units prior to shipment:

Trip Current Calibration: Each unit is tested at 100%, 125%, and 200% of rated current using a calibrated current injection test bench. Trip time is recorded and compared against Mitsubishi Electric published time-current characteristic curves (Class 10 / Class 20 as applicable).

Reset Function Test: Manual and automatic reset mechanisms are cycled a minimum of five times to verify mechanical integrity and reset reliability.

Alarm Contact Continuity (KP variants): For TH-N12KP and all KP-suffix models, the auxiliary alarm contact (97-98 NC / 95-96 NO) is verified for correct switching state before and after trip, using a calibrated continuity tester.

Single-Phase Loss Sensitivity: The differential bimetallic mechanism is tested for correct response to simulated single-phase loss conditions, confirming the phase-failure sensitivity function is operative.

Insulation Resistance: 500V DC megger test applied across all poles and between poles and frame, with minimum 10MΩ acceptance criterion.

All test records are retained and available upon request for quality-critical installations.