New Industry Products

Bourns’ New Surge Protectors Tackle MOV Degradation Issues

July 30, 2021 by Gary Elinoff

The new IsoMOV protectors combine MOV and Gas Discharge Tube (GDT) protection in a single component

Bourns IsoMOV hybrid overvoltage surge protectors combine metal oxide varistors (MOVs) with GDT protection to guard the MOV against the cumulative effects of leakage current.

The IsoMOV series of circuit protectors. Image courtesy of Bourns

The combination of the MOV and GDT into a single, integrated hybrid device enables the GDT to shield the MOV from the leakage current that often leads to premature failure.

As described by Tomaž Bojko, Product Line Manager for Overvoltage Protection Solutions at Bourns, "In our interconnected world, devices need to maintain maximum uptime. Because of this, many designers specify MOVs rated for much higher voltages and surge currents in order to ensure they are never stressed to their breaking point. Others have placed GDTs in series with MOVs to eliminate leakage currents and extend the life of the MOV. The efficient design of the Bourns® IsoMOV™ protector allows designers to do away with larger, generally lower-performing, and often more expensive design alternatives."

What is a MOV?

As described in a Bourns White Paper, an MOV is a bidirectional device that begins to “clamp down” on voltage after it exceeds the MOV’s specification. It does not enter a short circuit state, but the higher the voltage goes above the spec, the more current that will be shunted through the device. The power that the MOV must dissipate is, of course, the voltage times the current.

MOVs are characterized by having leakage currents that occur well below their specified clamping voltage. The cumulative effects of the leakage current are damaging to the device. Worse, every time the MOV “clamps”, the leakage current increases, accelerating device failure.

What is a GDT?

GDTs, once they are presented with a voltage high enough for device triggering, “crowbar” the voltage, dragging it down to about 10 volts. Thus, with less voltage, it will dissipate far less current. But, unlike MOVs, recovery is a slow process.

IsoMOV Combines MOV and GDT

The gas discharge tube portion doesn’t conduct unless its trigger is surpassed. Thus, there is no pathway for the debilitating effect MOV’s leakage current, protecting that portion of the IsoMOV.

Image courtesy of Bourns New Product Brief

Symbiotically, the MOV prevents the follow-on current that follows after a surge from damaging the GDT. The combination allows designers to opt for a device that accurately reflects the expected challenges with a smaller sized device, saving space and expense. 

The Three Series of Devices

The IsoM3, IsoM5 and IsoM8 series of devices are described in detail in their datasheet. All units feature a maximum leakage current of 10µA.



Nominal Capacitance @20kHz


Imax For One Operation

Maximum Continuous Operating Voltages 


Maximum Continuous Operating Voltages 



30pF 6,000 amps 175 through 320

225 through 415


40pF 10,000 amps

175 through 555

225 through 745


50pF 15,000 amps

250 through 555

320 through745


AC Line Protection:

  • Industrial equipment 
  • White goods
  • Fire alarm systems
  • LED lighting
  • High value consumer goods

DC Line Protection:

  • Power supplies
  • Solar Inverters
  • Distribution systems 


The units operate over a -40 to +125℃ temperature range.

Regulatory and Safety

UL 1449 Type 5 recognized


RoHS compliant