Surges

[Guest trn_psycho]

This really isn't an AB question, but y'all have always come through in the past for me so... I've got a heating station that is powered by a SLC 505. It's been working great without problems. Recently though, we added onto this system and placed a redloin controller in the mix. Here's the scoop. If one of the heaters goes out on the PLC side, nothing happens. We replaced the heater and everything is fine. HOWEVER, if one of the heaters on the Redlion side goes out, it takes the solid state relay with it. I know that it can't be the controller causing this problem, but I'm at a loss. Both are hooked up the same, 24v from the controller turns on a SSR (277V) that fires the heater. The SSRs are a different brand though, so the tolerances may be different. The new SSRs were supposed to have build in surge protection, but I'm thinking they don't. Where/what type of surge protection would y'all recommend adding to this setup?

[paulengr 44]

Surge protectors protect against momentary spikes. There are 2 that I like: MOV's (metal oxide varistor). Cheap and simple. Basically, it's a variable-voltage resistor, although it has more in common with a semiconductor device physically. Above a threshold as the voltage increases, the resistance decreases. Reaction rate is very quick but it will still let through very high voltage spikes such as lightning to a degree. GDT's (gas discharge tubes). Above a threshold voltage, the gas inside the tube flashes into plasma and it goes into 100% conduction until the arc extinguishes. No leakage resistance to worry about. In areas where I've had lightning trouble (RS-485 cabling that runs outdoors over a few miles), this has to be the most effective device I've ever had. MOV's just don't cut it compared to what these things do, although they are a bit more expensive. HOWEVER, you are talking about a sustained short circuit. That's not a surge, that's a short circuit. Surge protectors are designed to handle surges that clear, not continued short circuits. The device you need is a "fast blow" fuse. Since you're talking about protecting a semiconductor device, you probably need a semiconductor fuse which trips in a small fraction of a cycle. Standard "fast blow" fuses trip in about 1/4 cycle. Circuit breakers take at least a couple cycles (even the newer "fast" ones). For the money, I'd probably try a "fast blow" fuse first because they are available for pennies compared to the more expensive and rare semiconductor fuses. Bussmann is a major fuse vendor. They have a huge book (about 150 pages) for download on their web site that does an excellent job of explaining what type of fuse to use and where. It does have some "advertising" claiming that fuses are superior to circuit breakers. For your application, this is true. When it comes to distribution circuits however, the answer is "it depends".