Hi Robert,
My MOV story... Our Midway Line opened in the fall of '93. Between then and the summer of'96, I lost alot of ROW (right of way) SCADA(Supervisory Control And Data Acquisition) RTUs (Remote Terminal Unit) power supplies. The failure mode always involved losing the power mosfet &, guite often, the 400V, 8A bridge rectifier. There was never any damage to input hash filters, decoupling caps or even the onboard fuse. When I finally had enough, I worked with our electricians to install MOVs L-N, L-G, & N-G. Our RTUs use NON-5 fuses and the line MOV is installed on the load side of the fuse. The MOV I used appears to be out of production as Harris, like many others, seems to have gotten out of the semi business. The devices I used were V130LA20B, 130VRMS, 70 Joule red discs. I used SPC teflon tubing (TTI-S18) & ring lugs to make the shortest lead length assemblies practical for going between fused L & N. A 2nd assembly had 2 MOVs with one lug each tefloned & lugged and the other leads tefloned and crimped in one end of a butt splice. The other end had a length of green THHN that they were asked to terminate to ground as short as possible. I have had very few failures(if any) of these ROW located supplies in the 9+ years since.
This is where I'm going to vent a little because I can see folks mentioning that this is DIY. Well I need to know how many times I'm supposed to let a poorly protected product blow up before I do something about it??? Your furnace mfg won't do anything to better protect their (YOUR) control board BUT, they'll be more than happy to sell you a new one. I had a Remote TBC controller blow up right out of the box. The builder made several mistakes, including fusing the neutral. He didn't see a problem with that.
One of our 2 RTU mfgs is a Canadian company. A surge that would enter a status termination card, would propagate and take out most or all of the other status cards, AC & DC fed switchers, and their surge suppression scheme. I wrote a failure analysis report with suggestions for a fix. They proposed a fix that wouldn't fix anything. I finally built a prototype that solved the problem and had a pc board shop make a run of them. Since then, the guys have been coming back with one blown status card and at worst, one blown protection card. The last particularly nasty substation hit that I personally repaired, took out one status card and vaporized foils on my card, but it stopped there.
So in short, DIY isn't necessarily a bad thing if there is sound reasoning behind it.
Sorry for being so verbose but since this is theory & apps, I'm hoping you'll let me slide.
Joe
