Voltage sag is a temporary reduction in supply voltage, typically lasting from a few milliseconds up to several seconds. It is one of the most common power quality issues in electrical systems and can significantly impact the performance of connected equipment.

Unlike a complete power outage, voltage sag does not interrupt the supply entirely. Instead, the voltage level drops below its normal operating range before returning to standard levels. Even brief voltage sags can disrupt sensitive electronic systems.

Why Voltage Sag Matters

Voltage sag can lead to a wide range of operational issues, particularly in environments that rely on stable and continuous power. These include:

• Unexpected shutdown of equipment
• Malfunction of control systems and automation
• Data loss in computing and communication systems
• Reduced efficiency or performance in motors and drives

In industrial settings, even a short voltage sag can result in production downtime or system faults, making it a critical consideration in power system design.

Common Causes of Voltage Sag

Voltage sag is typically caused by sudden increases in electrical demand or faults within the power system. Common causes include:

• Starting of large motors or heavy loads
• Short circuits or faults on the network
• Sudden load changes within a facility
• Grid disturbances or switching events

These events draw a high current momentarily, causing the voltage to dip across the system.

Where Voltage Sag Occurs

Voltage sag can occur across a wide range of applications, including:

• Industrial automation and manufacturing systems
• Data centres and IT infrastructure
• HVAC and building control systems
• Lighting and LED installations
• Renewable energy and power distribution systems

Any application that relies on consistent voltage levels can be affected.

Voltage Sag vs Voltage Drop

Voltage sag is often confused with voltage drop, but they are not the same.

• Voltage sag is temporary and usually caused by sudden events
• Voltage drop is a continuous reduction in voltage due to resistance in cables or components

Understanding this difference is important when diagnosing power issues.

Managing Voltage Sag

To minimise the impact of voltage sag, engineers may use:

• Uninterruptible power supplies (UPS)
• Voltage regulators or stabilisers
• Proper system design and load balancing
• Power conditioning equipment

These solutions help maintain stable output and protect sensitive equipment from disruption.