Last Updated on November 17, 2022 by Mary Pressler

The Difference Between Brownouts and Blackouts

What is a Blackout? 

A blackout is a complete interruption of the power supply, and all appliances and equipment that depend on electricity are deactivated. 

What is a Brownout?

An electricity brownout is different than a blackout. In a brownout, the voltage provided by the power supply drops below its standard value, but the electric service is not interrupted completely.

Reduced voltage may seem better than having no voltage, but brownouts can be worse than blackouts in many cases. Electrical devices are designed for a specific voltage range, and they may suffer damage when exposed to a voltage that is too high or too low. This risk is not present during a blackout, since there is no voltage at all.

What Is the Difference Between a Brownout and a Voltage Sag?

In some buildings, you may have noticed that the lights dim down for a few seconds when a large motor activates. For example, this may happen when starting a central air conditioner or a centrifugal pump. These quick voltage drops that only last for a few seconds are called sags.

When starting an electric motor, the inrush current can be 5 to 8 times higher than the normal current, and this will cause a voltage sag if the motor is large. Electrical engineers recommend inverter drives, which start motors by ramping up the voltage gradually, instead of applying the full voltage at once. This starting method reduces the inrush current, eliminating sags.

A brownout is an extended event, on the other hand, lasting for several minutes or even hours. When a building is affected by a brownout, the cause can be external or internal:

  • When a brownout affects a large area, the most likely cause is a power grid overload. The voltage drops because electricity generation cannot keep up with consumption, and electric companies may also lower the voltage intentionally to reduce power demand.
  • Brownouts that affect individual buildings are clear warning sign of electrical issues. For instance, if too many appliances share the same circuit, the resulting overload can cause a localized brownout.

When a brownout is due to external causes, you should focus on protecting your electrical devices. On the other hand, if the brownout cause is inside your property, you can get an electrical inspection to find a solution.

Which Devices Are Most Vulnerable to Power Failures?

The effects of brownouts are not the same for all electrical appliances. Incandescent lights are only dimmed without damage, and actually that is where the term “brownout” comes from. The effect is similar for resistance heaters, which experience a reduction of their heat output.

Other devices are more vulnerable to brownouts, in particular those with electric motors or electronic components:

  • Electric motors are very susceptible to low voltage, since it causes them to draw more current and overheat. A motor that is constantly exposed to low voltage will suffer a drastic reduction of its service life.
  • Electronic components can also suffer damage with low voltage. Even if there is no damage, a brownout can cause incorrect operation by altering the voltage pulses used in control signals.

Electricity companies are aware of the negative effects of brownouts. When they are caused intentionally, the decision is a last resort to prevent infrastructure damage or major blackouts.

In general, you should avoid exposing your electrical appliances to brownouts, especially those appliances with motors or electronic components. You can do this by installing a brownout protection device called an UPS. 

Protecting Your Appliances from Brownouts

Just like a circuit breaker protects against excessive current, an undervoltage relay can disconnect your devices when the voltage drops too much. However, these relays are only recommended for sensitive appliances, since they add costs to your electrical installations.

brownout protection device

Purchase an UPS or brownout protection device.

Uninterruptible power supplies (UPS) often have voltage regulation capacity, and they can keep a stable voltage output even when the voltage supply from the power grid fluctuates. UPS devices are normally used to ensure a stable power source for critical devices, such as data centers that host important applications.

Installing many under-voltage relays and UPS units at home can be very expensive. These measures are normally used only for sensitive devices and critical applications. Many home devices are controlled with switches, and leaving them in the OFF position provides effective protection against brownouts.

Some electronic devices remain in a “standby” mode when not in use, and they are vulnerable to brownouts unless they are unplugged. You can connect several electronic devices to a power strip with a switch, and disconnect them all at once when a brownout occurs.

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