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Every now and again we read about blackouts, where the voltage of the power grid drops and the infrastructure is paralyzed. But have there ever been cases where the power grid experienced a massive overvoltage? What kind of damage would that do?

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    \$\begingroup\$ Every lightning strike causes overvoltage. Damage could be anything from none, to destruction of generation and comsumer equipment, distribution network and explosion of transformers - oil-filled are particularly spectacular. Other sources of network fault can also cause transients. \$\endgroup\$ – Transistor Aug 24 '19 at 22:37
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    \$\begingroup\$ My brother had fried Fridge , stove and all small appliances get fried from 3 phase imbalance upon power restored in Africa once in a home. \$\endgroup\$ – Tony Stewart Sunnyskyguy EE75 Aug 25 '19 at 0:03
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    \$\begingroup\$ If you design equipment to be connected to the mains, then you have to assume frequent 1500V spikes, and insulate accordingly. \$\endgroup\$ – Neil_UK Aug 25 '19 at 4:56
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There are three main effects,

  • Blackouts: a total loss of power, generally due to physical damage or a tripped breaker
  • Brownouts: this is where a substation is overloaded to the point where its output voltage falls due to excessive loading, but not for long enough to trip the breakers
  • Load spikes: this is where some very large load switches off, so for the small period it takes the power stations to correct for this, the voltage can overshoot the nominal voltage

They are slightly interconnected:

  • a lot of brownouts imply you're due for a blackout if they don't fix the issue,
  • a blackout, e.g. an entire suburb losing power all at once, can cause a load spike, and
  • a brownout is a sign that the load spikes are significant

To actually damage things, generally you're going to need to trim down the margins of your particular device. If there is a lot of grid-tied solar installations in your area this can raise your mains voltage very close to the maximum, meaning any load spikes end up higher in peak voltage, and more likely to damage things. Generally the amplitude of this is limited.

On the other side of the fence, lightning is a very large amount of current. Even the clouds moving overhead can change the relative voltage of your mains ground by a few hundred volts if your country does not require any in-house earth strap/ground rod.

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It's possible in ice storms or with other destruction of equipment (say a truck runs into a pole) for high voltage wires to come into contact with low voltage wires, causing a power surge.

For example this incident affected dozens of residential power consumers.

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