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Xyspade t1_ixy3392 wrote

Reply to comment by cherlin in Not as old as a lot of the stuff on here, but a 6 plug wall adapter from RadioShack. Made November 20th, 1994. by 2XGSWsurvivor

It's not a fuse; fuses are protection for current (amps), surges are spikes in potential energy (volts).

It's a component (or multiple) called a MOV (metal oxide varistor). They divert surges to the ground pin, but somehow they also absorb the surges and fill to capacity eventually. I don't understand how they could do both because that seems contradictory, but I have yet to find a source that explains it properly.

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musclegeek t1_ixy4yln wrote

MOVs resistance changes inversely to the applied voltage. If the voltage across it increases the resistance decreases and vice versa. They are connected to gnd but have a very high resistance for the normal operating voltage. When the voltage spikes their resistance goes extremely low allowing current to pass effectively creating a short circuit to gnd.

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Xyspade t1_ixzbaia wrote

Thank you! What about when there's only one MOV between hot and neutral instead of a group of three, how does it pass that spike to ground? Or does it just pass it to neutral?

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musclegeek t1_ixzcolo wrote

Neutral is there specifically to handle small surges. Especially in wye configurations and neutral eventually goes to ground. Having it connected to both neutral and ground is a fail safe. No neutral goes to ground, surge too high for neutral goes to both or some large f-up voltage spike from neutral goes to local ground.

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PlasmaSheep t1_ixy63lt wrote

Put simply, the MOV acts as an open circuit until the voltage across live/ground or neutral/ground exceeds a "clamping voltage". The MOV then presents almost no resistance, making most of the current flow through it rather than through the device under protection. It gets "used up" because passing current through the varistor affects its structure and increases its clamping voltage.

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Xyspade t1_ixzb2h1 wrote

Ah, thank you! So when the protection light goes out, that's its way of telling you the clamping voltage is too high to be useful anymore?

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PlasmaSheep t1_ixzy297 wrote

That's right.

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Xyspade t1_iy06psp wrote

Makes sense then why people say they should be replaced every few years regardless of whether the indicator is still lit or not. Learned a thing today!

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