There are a wide ranges of causes for brownouts. Just like blackouts, overloads on the electrical system can trigger a brownout, as the generating facility is unable to provide enough power. It can also occur when events such as storms disrupt the distribution grid, or when there are other problems in the system. Brownouts can last for a few seconds or a few hours, depending on the type of brownout and how quickly a power utility can get full power running again.
In some cases, a brownout is actually deliberate, in which case it is known as a voltage reduction. Voltage reductions are undertaken when utilities sense that a disruption in the grid may lead to serious problems. Rather than instituting rolling blackouts, the utility may temporarily cut voltage to some customers in an attempt to stabilize the grid and to allow reserves of power to accumulate again.
During a brownout, utility customers should turn off appliances like computers, as the irregular power supply can damage them. It is also a good idea to turn off lights, leaving one on to alert the customer to the restoration of full power. You may also want to check with the neighbors; if your house is the only one out, there may be a problem with the power supply in your residence, and you should take a look at your circuit board and breakers. You may have inadvertently caused a brownout by overloading your home's capacity to carry power, for example.