working on a soft starter for conveyor belt motor,unit is a 480 volt 3-phase 100 hp the problem is that the epoxy encapsulated snubber filter ,mounted about 1 inch from the anode and cathode heat sinks of one of the thyristors caught fire the flame was large enough to travel and discolor the line phase connection and cable,app. 12 inches above it.I think the 3 snubbers should be relocated ,I am concerned that a really bad situation could have happened here as the other phases are about say, an inch apart.May I ask the opinion of the forum on how best to approach this potential hazard and prevent it from causing harm to both man(first) and machine?
The snubbers need to be as close to the SCRs as possible, the farther away you put them, the less they attenuate and the more susceptible they are to failure. Also you have to remember that they are at line voltage, and people are used to thinking small components like resistors and capacitors are low voltage electronics so they often mistakenly assume they are relatively safe to touch. Having them right inside of the heat sinks means they tend to stay away anyway, putting them out somewhere else increases that chance of contact.
Re-designing the equipment is not a great idea in my opinion, you don't know all of the issues that go into the component and design decisions. The fact that yours caught on fire is likely due to a defect, that happens, or a serious voltage spike. But in 25 years of building and supporting soft starters in the field for manufacturers, I have seen RC Snubbers fail less than a dozen times, it's just not that common of a thing. Usually the resistor was bad or a connection was bad, but sometimes if it sat un-powered for a long time, the caps fail when re-energized. If it was an older one with electrolytic caps, they tended to fail messily.
By the way when the RC Snubbers fail, if the soft starter run for a long time afterward, you could have some incremental damage to the SCRs.