We are facing an issue in our protection circuitry. For the below DC design MOV between Earth is getting damaged in the site.

MOV 2 & MOV 3 in the circuit is failing in the site.

System is powered from UPS with Centre point earthed.

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MOV is having a continuous DC voltage of 380V & Spark-over voltage of GDT is 230V/s & Break-over voltage 450V/Us.

We have also one more design with GDT connected in series with MOV2 & MOV3 to Earth & This design has no MOV damage issues.

So my Doubt is :

What could be the reason of MOV failure in the circuit without GDT protection. ?

Whether GDT is blocking the high Impulse voltage so that MOV is safe without failure?

GDT used - http://www.digikey.com/product-detail/en/bourns-inc/2035-23-SM-RPLF/2035-23-SM-RPLFCT-ND/1220355

MOV used - 14D471K from bourns

  • \$\begingroup\$ Is there a good reason why the GDT is in series with the MOV? Links to parts used are probably going to be helpful (GDT and MOVs). \$\endgroup\$ – Andy aka Jan 12 '17 at 14:08
  • \$\begingroup\$ Varistors are not a good choice for a DC circuit, unless it is pulsing (rectified but not filtered) DC. In an AC circuit, there are zero-crossing points where the voltage is zero and the MOV can return to the open state. \$\endgroup\$ – Mike Waters Jan 12 '17 at 15:44
  • \$\begingroup\$ You need proper overcurrent protection in both the + and- lines and large enough MOVs. Why are there GDTs in series with them? \$\endgroup\$ – Mike Waters Jan 12 '17 at 16:22
  • \$\begingroup\$ @Andyaka Included parts used. \$\endgroup\$ – Akhil Joy Jan 13 '17 at 6:35
  • \$\begingroup\$ @MikeWaters MOV used is having Max Continuous voltage of 380VDC. Normally GDT will prevents the leakage current which flows through the MOV. But here it is not the leakage current which is damaging the MOV because the MOV damaged are found to be completely burnt. \$\endgroup\$ – Akhil Joy Jan 13 '17 at 6:40

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