During a full power outage, power to devices abruptly stops, which is not when potential harm actually occurs. The problem is when power is restored; this is when a surge can happen and result in irreparable damage. Black Diamond Plumbing describes a power surge as lasting for a short duration; when it happens, there’s a brief increase to the household’s current. A power spike, meanwhile, is a sudden, immediate, and intense burst(s) of electricity. Both are potentially bad for any device that remains plugged into a power source during an outage. Because of this, it makes sense to unplug items that would be expensive to replace, such as TVs, game consoles, computers, and single-room air conditioning units.
While large appliances like washers, dryers, freezers, and stoves are also susceptible to surge damage, unplugging them may be too difficult (but there are things you can do to avoid problems with these items). Additionally, switching furnace and central air conditioning units to “OFF” is advised because their electrical components can easily be damaged when power resumes, says Custom Air & Plumbing.
It’s also important to note that this action is recommended after an outage or brownout but not during a voltage sag, which according to Appliance Parts Pros, occurs when there is a rapid, brief demand on the electrical system due to an appliance, such as a vacuum cleaner, being turned on. A power sag may cause lights to flicker momentarily or appliances to lag until the draw on the system ends.