Voltage Sags & Swells

Sags and swells are short duration changes in the RMS level of the voltage. They typically last for less than a few seconds. Basically, sags and swells occur whenever there is a sudden change in the load current. Ohm's Law tells us that changes in the voltage occur when a changing current interacts with system impedance. If there is a sudden increase in current due to a load turning on, then a sag will result. On single phase circuits, a corresponding neutral-ground swell also occurs. If a load turns off, it produces a momentary increase, or swell, in the voltage. The greater the system impedance, the greater the magnitude of the disturbance.

Swells, including N-G swells, can easily damage equipment. Power supplies controlling all manner of devices are the most common victims of a swell.

Sags, however, do not directly cause damage. They initiate problems indirectly. For example, a sag may cause a laser CNC machine to restart, thus damaging the product it was making.

When internal voltage instability is a problem, the first step in solving it is to identify what is failing or being damaged and why. Where is the load causing the changing current? If that load or group of loads can be located, then a solution may be as simple as relocating either the victim or the culprit.

Since sags and swells are closely dependent on the wiring impedance, another possible solution is to increase the wire size in the distribution path. Increasing the size decreases the impedance, ultimately reducing the magnitude of the disturbance.

If these steps are not possible, then voltage regulation must be introduced to maintain voltage stability and protect equipment.

Voltage regulators may be stand-alone devices, or used with other technologies such as transformers to add to their capabilities. Once again, please remember that to properly provide a solution means you must understand the problem and ensure the solution is appropriate.