MOV for arc suppresion

[fieldserveng]

Here's one for you. I am curious about the use of an MOV in parallel with a set of relay contacts as a form of arc suppresion for inductive loads. I have just discovered that the controller being used in an application is utilizing the arraingment that I have just mentioned. The MOVs are soldered onto the controllers mother board and are in parallel with the digital output relay's contacts. My understanding about MOVs is that their failure mode is to short circuit. I theorize that if the MOV fails and fails shorted the device being driven by this digital output will not be de-energized when commanded. In the application the manufacter is using a pre-packaged RC network in parallel with the inductive loads (interposing relay coils and some solenoids). The manufacter claims that in several thousand units they have never encountered the failure mode that I have a concern about.

Isn't this backwards? Wouldn't one specify an RC network in parallel with the relay contacts and a MOV across the inductive load?

 

[Skogsgurra]

You are absolutely right!

MOV failure mode is mostly short circuit. It is a foolish thing to put them across the contact.

Remove them and rely on the RC units across the load. The fact that "it has never happened" does not mean that it will never happen. It is certainly a risk.

 

[OperaHouse]

MOV's fail every which way. They can increase in voltage or decrease. That decrease will cause other problems. When in series with a load they are less likely to short catastrophicly. It is likely that this is an additional protective measure the manufacturer has added because he can't control how these are to be hooked up. I remember one customer that always used 240V relays on a 480V. The MOV voltage the manufacturer selected is likely much higher than you would usually see in that kind of service. The RC on the load should keep the MOV from ever operating. While an old practice, I am against any suppression in parallel with the contacts. That just sends noise into the line that can cause problems with newer electronics. Spikes should be absorbed at the source.