Voltage Sags - Neighbors

Voltage sags are a very normal occurrence on the power lines. The closer a neighbor is to the customer, the more likely sags caused by the neighbor may disrupt equipment in the facility.

Sags occur when there are changing currents interacting with system impedance. Loads in the neighbor's facility or office must be large enough and changing enough to affect the voltage feeding the customer's facility or office. Quite often this occurs when a panel, office, or whole building trips a breaker. When someone throws the breaker back on, a sag results.

If shared wiring is present, then simple devices like copy machines and printers may cause similar concerns at turn on.

When a sag occurs, the electronic device's power supply uses some of its stored energy to compensate for the loss of input voltage. If enough energy is lost due to the sag, then the power supply can no longer keep the system operating, and it shuts down. Since the switch is still on, as soon as the sag is over and the voltage has returned to nominal, the system turns back on, or restarts.

The lost energy from the sag may not be enough to shut down the device, but only enough to confuse the digital components. If this happens, then a lock-up may result.

Most of the time neighbor-induced sags are difficult to identify. If they can be determined, then one solution may be a simple procedural change at the neighbor's site to prevent the radical load changes or breaker trippage.

Once again, if neighbors are creating these disturbances, it is a good idea to request the utility's input on how to resolve the situation.

If the voltage instability cannot be stopped at the source, then voltage regulators must be used to prevent the problems.