My friends always tell me to unplug my devices during a 'brownout' (it's what they call power outage here in my country. In some places they call it blackout). Why should I be? Is the voltage or maybe current surges up when the power goes on again?
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3\$\begingroup\$ To my knowedge, brown-out and black-out are different things. Black-out is when there is no voltage on the distribution network, brown-out is when the voltage is there, but lower than specification \$\endgroup\$– Joren VaesCommented Sep 2, 2017 at 13:23
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\$\begingroup\$ it depends on your utility quality \$\endgroup\$– D.A.S.Commented Sep 2, 2017 at 20:40
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\$\begingroup\$ One reason is that when the power comes back, if you have many devices connected, the combined inrush current to their power supplies may trip your fuse, and that can be quite annoying. \$\endgroup\$– DampmaskinCommented Sep 2, 2017 at 21:32
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3 Answers
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Some devices can be damaged by voltage that is below normal specifications but still present.
Typically this kind of device would contain a compressor or other kind of pump that has a large starting surge to overcome or get over some kind of mechanical torque "hump". Under brownout conditions the motor could stall and get very hot, possibly damaging it. The stalled motor has no back EMF so the current is high, and probably less cooling. So, refrigerators, freezers, air conditioners, dehumidifiers and that sort of thing could be at risk.
It's also possible that poorly designed switchmode supplies could be damaged- the input current and therefore some kinds of stress on the switching devices goes up when the input voltage is just above where it stops working.
This is mostly a developing country issue.. in most developed countries the power is either there and within specifications or (very rarely) gone entirely.
Power going off entirely is a different matter- if the reason the power went off is because there is a powerful electrical storm or because some 18-wheeler went off the road and took down the power lines, and they're dangling near some EHT lines, yes, it might be an issue. Personally I don't find the risk sufficient to bother doing anything.
answered Sep 2, 2017 at 19:24
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Like Spehro's compressor, some digital equipment can be damaged by brownouts, if badly designed, or in some corner cases.
Normally the power supply includes a "power good" detector which generates a reset signal which keeps the cpu in reset until the voltage is sufficient. If this is not implemented properly, then the cpu could get to execute code with a below-spec supply voltage, and this can result in erroneous results, which can be written to persistent storage like Flash or harddisk. Also, these storage devices will write false data if the supply voltage is too low, which can result in filesystem corruption, etc.
Also, computers tend to dislike being switched off by pulling the plug while the storage devices are working. Journaling file systems are supposed to prevent this, but they are not universal. A brownout with wobbly supply voltage could cause a device to go through many cycles of boot, crash, boot, crash...
For analog equipment, there can be corner cases when too-low supply voltage causes excess power dissipation in a device, for example because the circuit is kinda-half-working but the feedback loop is out in la-la land. There are many ways things can misbehave. In a normal power-up or power-down scenario, the nastiness does not last long enough to result in a blown part, but if it lasts a while...
So, yeah, a brownout will basically be a corner-case debugging session for everything that's plugged in. Since the conditions are abnormal, likelihood of finding a bug the manufacturer has inadvertently left in the product is higher than in normal conditions.
Also, switching power supplies are a negative impedance load for the grid: as the voltage gets lower, the input current increases. I guess with enough SMPS plugged in, and a bad enough grid, it could make it harder for power to come back.
Ironically, this happened here in 1978, as a major fault caused a blackout in the whole country. It took a while to fix, and since it was in winter, by the time it was fixed every heater, every boiler in every home wanted to turn on... So the grid had to start up into a load much higher than what it could handle. Utility circuit breakers blew all over the country. They had to coordinate the grid reboot, sector by sector, to have it come up.
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\$\begingroup\$ +1 for the badly designed reset circuit corruption-of-memory issue. All too frequent in cheap consumer goods. \$\endgroup\$ Commented Sep 2, 2017 at 23:38
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Brownouts deliver power at too-low voltage which can cause some machines to overwork themselves. Motors act weird on wrong voltage. Switch-mode power supplies are weirder.
Blackouts, or rather their repair, will involve throwing switches on high voltage distribution lines. if you see a telephone pole with a weird lever and linkage, follow it up and you'll see a goofy looking triple knife switch. Or the much larger oil-filled switches (for snuffing arcs) in switchyards.
Those big switches, and the reactivity of the load of a whole town or more, make absolutely enormous voltage spikes.
answered Sep 3, 2017 at 4:01