Jim, Ken, is correct in that when MOV's fail, they fail short. That is why you see a circuit breaker or fuse in front of the MOV's. If your safety ground is defective or does not return to the neutral side of the power source, then your safety ground raises to line potential when the MOV's short line to safety ground, with significant current available. In a case like this the failure does not trip the circuit breakers or fuses in the electrical panel.
We had a such a failure here (multiple failures were to blame) and the only thing damaged were 26 MOVs (in 26 electrical strips) that were line to safety ground. None of the computers plugged into the strips were damaged, even though fuses blew to protect the MOVs (after letting the smoke out of the MOVs), but left the computers powered up. None of the computers had MOVs line to safety ground. In our case the cubical area is three phase (115VAC 60Hz). One cubical was miss wired, which put the neutral to safety ground MOV in the none fused part of the electrical strip, in to the line side of the circuit. The main electrical box for the area was not properly grounded, nor was the safety ground tied to the neutrals. So when the MOV failed it raised the safety ground to the line potential of phase A, which put 230VAC across all the MOV's on Phase B and Phase C. As Murphy would have it the MOV that failed had a unintentional heatsink and survived longer than it should have. All the other MOV's did not have heatsinks. The reality is you should never really need a MOV from line to safety ground (or neutral to safety ground). It is the difference in potential that destroys semiconductors. In most designs the semiconductors are line to neutral not line to ground. So as long as you have one across line to neutral you should be OK (not to mention the filters that assist). The past discussions on this subject can be found in our archive on : https://www.ieeecommunities.org/emc-pstc?go=t907166 Jim Jim Bacher, Senior Engineer Paxar Americas, Inc. e-mail: j.bac...@ieee.org From: owner-emc-p...@majordomo.ieee.org [mailto:owner-emc-p...@majordomo.ieee.org]On Behalf Of Ken Javor Sent: Thursday, December 11, 2003 11:08 PM To: Jim Eichner; 'EMC-PSTC - forum' Subject: Re: MOV's to ground Not a legal answer, but I have seen a failure mode for MOVs where they partially conduct line current. This was in power strips. My house electrical system took a lightning hit. Nothing was damaged but the MOVs, including all the stuff in the house not connected to MOV "protected" power strips. The power strips with bad MOVs were quite hot (thermally) due to the "fault current" (high impedance short to ground). The MOVs were tiny, I think larger ones might have fared better. No doubt there is a spec covering how many Joules/Watts an MOV must be able to absorb without damage. This message is from the IEEE EMC Society Product Safety Technical Committee emc-pstc discussion list. Visit our web site at: http://www.ieee-pses.org/ To cancel your subscription, send mail to: majord...@ieee.org with the single line: unsubscribe emc-pstc For help, send mail to the list administrators: Ron Pickard: emc-p...@hypercom.com Dave Heald: emc_p...@symbol.com For policy questions, send mail to: Richard Nute: ri...@ieee.org Jim Bacher: j.bac...@ieee.org All emc-pstc postings are archived and searchable on the web at: http://www.ieeecommunities.org/emc-pstc
