New Industry Products

Littelfuse Debuts Varistor Line For Type 1 and Type 2 Surge Protective Devices

March 11, 2021 by Gary Elinoff

The new line is speced at maximum continuous operating voltages ranging from 150 to 690 VRMS and features a protective thermal disconnect function.

Based on Littelfuse’s thermally protected varistor technology (TMOV), The new LST Series is targeted use in Type 1 and Type 2 surge protection devices (SPD) found in both residential and in commercial or industrial applications.

Image courtesy of Littelfuse.
Image courtesy of Littelfuse.


Generally speaking, type 1 devices SPDs protect against external threats such as lightning, while Type 2 devices guard against internal threats, such as surges from inductive devices like motors tuning on or off.


Meeting UL1449

These devices are all spec’d at a maximum short circuit current rating (SCCR) of 200 kA. That’s the maximum overload that the device can safely mitigate. They also feature a nominal discharge current (IN) of 20 kA, which means fifteen 80/20 µs pulses of 20 kA. As per Littelfuse, these specifications will help SPD OEMs to “facilitate” obtaining UL1449 Type 4 recognition.

Another key specification for varistors is peak surge current (IMAX), which is the amount of current the unit can handle for one 80/20 µs pulse. Members of this series are rated for IMAX’s of either 50 or 75 kA. Designers unsure of the IMAX requirement for their specific design will take heart in knowing that the 50 or 75 kA versions sport the same footprints.


What’s all this 8/20µs Stuff, Anyway?

As we described in a previous article, An 8/20 μs pulse rises to its maximum in 8 microseconds. By the time 20 μs have passed, its intensity has receded to 50% of its maximum. 


Remote Monitoring

Of course, once its thermal disconnect is “blown”, the varistor is at the end of its useful life. The device features switch pins that can remotely indicate that situation in the eventuality that it occurs. There are also visual tabs located on the device that also indicate the device’s functional status.

Littelfuse also included what it describes as an electric arc shield to add further protection.


Device Availability

As mentioned earlier, all members of the LST family feature SCCRs of 200 kA and INs of 20 kA. The values below are from the series datasheet, and are clearly indicated by Littelfuse to be preliminary.



Voltage Protection

Rating (VPR)

in Volts

LST1505VL2NT1 150 50 600
LST1507VL2NT1 150 75 600
LST1805VL2NT1 180 50 800
LST1807VL2NT1 180 75 800
LST2755VL2NT1 275 50 900
LST2757VL2NT1 275 75 900
LST3205VL2NT1 320 50 1000
LST3207VL2NT1 320 75 1000
LST3855VL2NT1 385 50 1500
LSTV3857L2NT1 385 75 1500
LSTV4205L2NT1 420 50 1500
LSTV4207L2NT1 420 75 1500
LSTV5105L2NT1 510 50 1500
LSTV5107L2NT1 510 75 1500
LSTV5505L2NT1 550 50 1500
LSTV5507L2NT1 550 75 1500
LSTV6905L2NT1 690 50 2000


As per Boris Golubovic, Vice President, Marketing and Strategy at Littelfuse, "The LST Varistor provides several advantages when compared to other technologies on the market." He goes on to state that "By incorporating the LST in your latest design, you not only will benefit from its wide operating voltage range, high surge current rating, and complement of options for microswitch, your SPD will also be more robust to meet the protection demands of your customer’s applications." 



  • Residential, Commercial, and Industrial buildings 
  • HVAC systems 
  • Uninterruptible Power Supply (UPS) 
  • Industrial motor control 
  • Renewable energy 


Physical facts

  • Members of the LST Series operate over a temperature range of -40 to +85℃


Environmental Considerations

  • RoHS compliant
  • Lead-free
  • Halogen-free