TDK has introduced the MT40 series of ThermoFuse varistors (ordering code B72240M), a new generation of surge protection components combining a compact footprint with enhanced safety features.
These devices target high‑energy surge environments up to 50 kA while integrating a thermal disconnect function and remote monitoring, making them suitable for demanding industrial, PV and communication systems.
Key features and benefits
The MT40 ThermoFuse varistors are strap‑terminated metal oxide varistors (MOVs) with an integrated thermal disconnecting system. This combination is designed to prevent catastrophic failure under prolonged overvoltage protection or end‑of‑life overload conditions, where conventional MOVs can overheat.
Key features include:
- Peak surge current capability up to 50 kA (8/20 µs pulse)
- Short‑circuit current rating (SCCR) up to 200 kA according to UL 1449
- AC operating voltage range from 150 V to 550 V
- DC operating voltage range from 200 V to 750 V
- UL 1449 Type 1CA component assembly recognition for both AC and DC applications (file E321126)
- Integrated, galvanically insulated normally open micro‑switch for remote indication/monitoring
- Optional visual status indicator for local fault signaling
- Compact overmolded package: 38.0 × 15.2 × 40.9 mm (L × W × H)
- Same electrode footprint on the PCB for all types in the series, simplifying layout reuse
- Flame‑retardant epoxy encapsulation
- Operating temperature range from −40 °C to +85 °C
- RoHS, REACH and PFAS compliant construction
For design engineers, the combination of high surge rating, thermal disconnection and status indication provides a more controlled failure behavior than a standard MOV. In practice, this supports coordinated protection concepts where system controllers or upstream breakers can react to an end‑of‑life event instead of relying solely on passive fusing.
Typical applications
The MT40 series targets line‑connected equipment where high surge currents and stringent safety requirements coincide. Typical applications include:
- PV systems (string inverters, combiner boxes, DC side and AC output surge protection)
- Industrial power supplies and control cabinets
- Outdoor lighting systems and LED drivers exposed to lightning‑induced surges
- Telecommunication systems and outdoor base station cabinets
- Surge protective devices (SPDs) used at service entrances or sub‑distribution levels
In these use cases, the compact footprint helps when retrofitting additional surge protection into crowded enclosures or DIN‑rail SPD modules. The wide AC and DC voltage range allows one platform to cover different mains systems and DC bus voltages, easing portfolio management for OEMs and panel builders.
Technical highlights
Surge and short‑circuit capability
With a peak surge current rating up to 50 kA (8/20 µs), the MT40 series addresses high‑energy surge environments such as lightning exposure at building entries or long cable runs. A short‑circuit current rating up to 200 kA according to UL 1449 supports coordination with upstream protective devices in installations with high prospective fault currents.
In practice, the SCCR value is important for system certification and selection of upstream breakers or fuses. It ensures that, in the event of a short‑circuit within the SPD path, the assembly can withstand the fault current until an upstream device clears the fault, instead of failing in an uncontrolled way.
Thermal disconnect and monitoring
The integrated thermal disconnecting system is designed to separate the varistor element from the line if it overheats, for example due to sustained overvoltage or end‑of‑life degradation. This helps to prevent thermal runaway and potential fire hazards often cited in MOV application notes.
A galvanically insulated normally open micro‑switch is mechanically coupled to the disconnect function. When the varistor has been thermally disconnected, the micro‑switch can close or open (depending on wiring) a low‑voltage monitoring circuit, allowing:
- Remote signaling to a PLC, controller or building management system
- Triggering of alarms or maintenance notifications
- Coordinated shutdown of affected system sections
An optional visual indicator provides a quick on‑site check of the protection status without opening the enclosure, which is useful for field service.
Mechanical design and footprint
The MT40 series uses a patented overmolding technology to realize a compact package of 38.0 × 15.2 × 40.9 mm. For designers, this means:
- Reduced volume for high‑energy surge protection stages
- Easier integration into narrow SPD housings or compact power supplies
- Consistent PCB electrode footprint across the series, simplifying layout and allowing different voltage ratings to share the same board design
The flame‑retardant epoxy encapsulation adds an extra safety layer against flame propagation and environmental contamination, which is important in harsh industrial and outdoor environments.
Environmental and regulatory aspects
The MT40 ThermoFuse varistors are specified as RoHS, REACH and PFAS compliant. This supports OEMs in meeting current and emerging regulatory requirements without additional material exemptions.
UL 1449 Type 1CA recognition for both AC and DC applications simplifies compliance for SPDs and equipment intended for multiple markets. The wide operating temperature range from −40 °C to +85 °C covers typical outdoor and industrial environments without derating outside this band, though final dimensioning should follow the detailed curves in the manufacturer’s datasheet.
Design‑in notes for engineers
When integrating MT40 ThermoFuse varistors into a design, engineers should consider:
- Coordination with upstream protection: Verify that the SCCR and UL 1449 Type 1CA classification match the installation’s prospective fault current and protection scheme.
- Voltage selection: Choose the AC or DC nominal voltage such that continuous operating voltage, temporary overvoltage and system tolerances remain within the varistor’s specified limits.
- Energy and surge rating: Dimension the device based on expected surge levels from standards (for example, IEC 61643‑type requirements for SPDs) and application‑specific surge testing.
- Thermal management: While the overmolded housing is compact, ensure adequate spacing and, where relevant, ventilation or thermal coupling to prevent excessive temperature rise in tight enclosures.
- PCB layout: Use the standardized electrode footprint across the MT40 series to create a modular layout where different voltage ratings can be fitted without redesign. Observe creepage and clearance rules for the highest system voltage and pollution degree.
- Monitoring integration: Route the galvanically insulated micro‑switch contacts to a low‑voltage I/O, alarm or indicator circuit so that thermal disconnection events are detected. Decide whether to implement latching alarms, automatic system derating or controlled shutdown strategies.
- Serviceability: If the SPD or protection module is intended to be field‑replaceable, design mechanical access and labeling to make the visual indicator and part markings visible to maintenance staff.
In many industrial and PV systems, these ThermoFuse varistors will form part of a multi‑stage surge protection concept together with gas discharge tubes, series impedance or coordinated SPDs on different lines. Careful system‑level coordination is needed to ensure that the MT40 devices operate within their intended regime and that no single component is overstressed.
Source
This article is based on information provided in a TDK Electronics press release on the introduction of the MT40 ThermoFuse varistor series for surge protection up to 50 kA, complemented by general application considerations for MOV‑based surge protection.