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| Relay in series with SSR as high current switch for longetivity ? |
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| duak:
To respond to 741: If safety aspects have to be considered to get agency approval, it's far easier if there's a relay or switch in series. When I worked on getting a laser based product approved in the last century we followed IEC-950. Apparently somewhere in it is something about fail safe design, single point failures and semiconductors failing shorted but I relied on our in house safety experts to guide me on the details. I went back to R&D before retiring so I don't know where it's all got to these days. I recall our experts were talking about how the regulators were trying to integrate firmware into IEC-950. I've got a few microwave oven repair stories. If there's one product or device where fail safe design is important this is it. Two of them had failures that were fail safe that inherently prevented hazardous operation. In one, the magnetron simply died. The second had disintegrated its HV rectifier so it also simply stopped working. (I made it work again by transplanting a replacement from the first oven.) Two more ovens had failures where hazard was prevented by a safety interlock. The first had its main switching thyristor short out so the magnetron ran as long as the door was closed. Fortunately, regulations require two door interlock switches to prevent hazardous radiation, although by running without a load, the oven may destroy itself. Another oven had a similar failure. It has a relay that seems to occasionally stick on but so far the door switches are fine. Neither of the last two ovens detected that there was a possible hazard - that was left to an alert operator. |
| fcb:
--- Quote from: 741 on February 16, 2020, 06:53:32 pm ---Presumably in some applications it would be a safety requirement the relay contacts are not bypassed? However, contacts can weld shut, and if the answer to that is to monitor for that situation (and then do what???), that covers a failed parallel SSR too. I am interested in this discussion in the context of https://www.eevblog.com/forum/projects/how-do-i-rate-a-ct-for-leakage-detection-use-also-a-dc-leak-detector-coil/, post #16. The video link's narrators suggests when contact weld is seen the "Zappi 2" PCB's parts step in to cut the current. I wondered how this could be, maybe the PCB just has another 'rarely opened' emergency series relay, or maybe an SSR? --- End quote --- Correct. There is a DPST relay just up from the LNE input connector. This will be in series with the contactors. You can see the footprint on the video for the second relay for L2/L3 when the PCB is populated for 3phase. |
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