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MOV fire risk

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5U4GB:

--- Quote from: Kleinstein on December 11, 2023, 11:56:13 am ---MOVs get increasing leakage when getting old. So they may slowly run hotter and hotter and thus can overheat without blowing a normal mains fuse. The MOV is usually not flamable (after all it is metal oxide), but the hot PCB and neighboring parts may catch fire.

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I've always wondered about that, ten-cent MOVs get added to cheap power strips so the sellers can double the price by touting the "surge protection" it offers (the random-trigger incendiary functionality gets thrown in for free), but a while back I scored some NOS Islatrol IE-series DIN-mount hardwired power filters for a fraction of their pretty hefty new price, and a major design element was a "MOV array" which I wouldn't have expected in something that's rated for continuous operation over (at a minimum) its ten-year warranty period.  However I'm not that familiar with the design requirements for these things so maybe they're using them in some way that doesn't cause, or isn't affected by, their degradation over time.

Siwastaja:

--- Quote from: T3sl4co1l on December 19, 2023, 02:50:12 pm ---Very rarely, it can be much worse: since the supply is a 240VCT transformer, if the neutral comes loose for some reason (say, improper, faulty, or damaged wiring), more load on one side than the other will shift the neutral voltage, browning out one side and overvolting the other.  UL 1449 for example covers this.

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Neutral or PEN faults are, if not common, very not-non-existing in the countryside here, and a regular-ish issue on TN-C-S distribution systems. Most of the electronics, lightbulbs etc. blowing up is least of the problems; dying is worse, much more dangerous is that PE and N are connected together at the distribution box, so with phases present without the PEN conductor, now every "earthed" metal surface is at a possibly high potential, through the low impedance of the connected loads. Instructions how to handle this situation go along the lines, "when you see some of your lightbulbs glowing dim and others very brightly, you are experiencing a neutral fault. Take a long non-conductive object, for example a log of wood, to carefully turn off the main switch in your metal-case distribution box. Don't touch any device or any metal object, or you die. Alternatively, go sit in the middle of the room and sob."

Siwastaja:

--- Quote from: coppice on December 20, 2023, 12:31:39 am ---
--- Quote from: SiliconWizard on December 19, 2023, 10:56:42 pm ---Yes, twice the current is like not negligible.

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Its questionable if they have twice the current. The weak load that's possible from a US socket means they don't have most of the high consumption appliances, like 3kW kettles and 2.5kW hair dryers that are normal in 220-240V mains countries.

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This. Current is not twice, because all large loads in the US use 240V sockets. The opposite point can be made; German Schuko is officially rated to 16A, but fires are reported with sub-par sockets and plugs over 8A. Here, this is all we have, and 3kW loads must use this. In the US, 3kW loads use the same voltage, but plugs and sockets rated to actually survive 16A, and much more.

We mitigate by avoiding loads >10A which can be plugged in by end user completely; we just call an electrician every time we want to install a trivial thing like electric stove, to make a permanent connection - or do it illegally ourselves.

Siwastaja:

--- Quote from: CosteC on December 19, 2023, 07:52:29 pm ---True, yet USA split phase or two phase system is even more peculiar.

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Split phase is NOT two-phase, and it's not that peculiar. There is a reason why the term split phase has been coined as a separate thing from two-phase. Two-phase systems have usually 90 deg phase difference so they produce a rotating vector, while a 180deg phase shift produces nothing useful. Split-phase is just a dual voltage single-phase system. AFAIK, two-phase systems aren't used for distribution anywhere, at least not in large scale, because of the awkward dimensioning of neutral wire for double the phase current; 3-phase distribution avoids that. But we still see two-phase design in stepper motors, some small BLDC fans, and in capacitor-run "1-phase" induction motors.

Marco:
Two phase is an ancient and irrelevant historical artefact, forcing a differentiation between two phase and dual/split-phase has long become knee jerk pedantry rather than needed for clarity.

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