Littelfuse LST Series of SPD Thermally Protected Safety Varistors

Offering thermal disconnection function and electric arc shield for use in Type 1 and Type 2 SPDs

Littelfuse has introduced the new LST series of safety varistors which provide the advanced protection functions and high current capability required in the Type 1 and Type 2 Surge Protective Devices (SPDs) used in large industrial, commercial and residential electrical installations.


The new Littelfuse SPD Thermally protected LST series of varistors features the company’s proprietary thermally protected varistor TMOV technology. The TMOV technology’s built-in thermal-disconnect function and electric arc shield provide a high level of protection, preventing catastrophic failure and fire hazard even under the extreme circumstances of abnormal over-voltage conditions or the varistor’s end of life.


The LST varistors are available in versions with a maximum discharge current rating of 50kA or 75kA. Both versions share the same footprint and PCB layout, giving OEMs the flexibility to make two different SPD products with a single board design.


The varistors include a mechanically triggered AC- and DC-rated micro-switch with normally open or normally closed options for remote monitoring and status indication. Each LST product also features a clear indicator showing when it has reached its end of life and needs replacement.


  • 200kA short-circuit current rating
  • 20kA nominal discharge current
  • UL 1449 Type 4 recognized component assembly
  • Visual tab option to display working status
  • ≥100MΩ insulation resistance


  • Type 1 and Type 2 SPDs used in:
    • Residential, commercial and industrial buildings
    • Uninterruptible power supplies
    • Industrial motor control
    • Renewable energy systems
    • HVAC systems

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