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EEVblog 1500 - How an AC Automatic Transfer Switch Works

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Automatic AC transfer switches are pretty cool devices. A look at what they do, a practical demo, teardown, and then reverse engineering to explain how it does it.
Bonus look at how my automated home battery backup system will work.

Hi Dave, interesting to see inside one of these, I have seen ones where the switch contacts are actuated by a motor before, but not one with solenoids like this. It looks like some effort has been taken to prevent the mains and grid terminals ever being bridged, in order to guard against backfeeding.

One thing that wasn't clear from the video is what would happen if the mechanism jams, either due to an internal fault, or by the actuator being held externally. Will the microswitch still shut off the relevant solenoid? I very much doubt they are rated for continuous duty, I can see the magic smoke escaping (or worse) if they are left energised. Hopefully there are some thermal fuses at least?

I think those metal bars you saw are for arc quenching.

That braided cable in the "tranfee" seems uncoated, not good for humid or near the sea environments. The "transfer" switch seems to have a better solution.


There is one problem with (many) freezers and fridges you should be aware of:

If the power disappears while the compressor runs and comes back immediately, the compressor may not be able to restart against the gas pressure, and stalls with power applied.

For reliability, you should put a "delayed-on" timer between your transfer-switch and the compressors, so that when power goes, it stays gone for at least a minute or two, before the compressors are restarted.

Does the auto switch meet the Australian standards and   will your house insurance cover you when installed?


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