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I recently attended an electric utility training course discussing relay protection practices. One topic that came up was a fuse saving scheme where a protective reclosing device would momentarily interrupt power to save a fuse from blowing.

My question is how standard electronic devices would react to a very momentary interruption. The instructor said that if power was interrupted for 18 cycles or less on a 60 Hz system that the digital clocks would not reset and blink. I was unable to find any reference to back up this assertion. Are any of you aware of any design standard that would substantiate this claim?

Our practice has always been to push the outage out to the furthest point with a sustained outage rather than "blink" more people than necessary.

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    \$\begingroup\$ He must have been referring to something very specific. If you turn off power to most digital devices for 0.3 s, things will go very wrong. \$\endgroup\$ – Marcus Müller Aug 9 '18 at 13:04
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    \$\begingroup\$ @MarcusMüller Not so, for example most PC's will tolerate 500mS+ before a reset. In the world of switching regulators the voltage range from operating to brownout is quite large. For a vast number of smaller electronic appliances that have switching supplies, they are very tolerant of both low voltage and short duration supply loss. \$\endgroup\$ – Jack Creasey Aug 9 '18 at 16:11
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Its product dependent.

When the power is off, only thing that device can rely on is internal capacitors. Time that device may keep the voltage will depend on rated power, actual power consumption, and device topology.

Say, you are having a 5V 200A power supply. If you power a LED from it, it may stay lit for many seconds after power-off. In the other hand, if you put high load on the same power supply, reset may happen even if few cycles were missed.

Most of the devices have different capacitors for different power rails, in your case, there might be a capacitor that keeps clocks running for some time.

Even if you skip a few cycles, many devices will go crazy. You need to use UPS to prevent that

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It depends on the thing. Nearly all devices have at least power supply filtering capacitors: these store enough energy to run the device for some amount of time. If the power is out for less time than it takes to deplete this stored energy enough to go below the minimum working voltage for the device, then the device can "ride though".

The trouble is the power consumption, energy stored in the capacitors, and the minimum voltage required by a device varies. Some devices will be fine for many seconds, others may reset on the slightest interruption.

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