Surge & voltage
Over / Under Voltage Protector Settings Calculator
Set a starting over-voltage and under-voltage window and reconnection delay for a voltage protector, based on the nominal voltage and how sensitive the load is. Confirm the final values against the equipment ratings and local rules — these are engineering starting points, not fixed settings.
How the window is chosen
A voltage protector disconnects the load when the supply stays outside a set window for longer than a delay, then reconnects after the voltage recovers and a reconnection delay. Sensitive loads use a tighter band (about 90–110 %), standard loads a moderate band (about 85–115 %), and robust loads a wider band (about 80–120 %). Over-voltage damage develops faster than under-voltage, so its disconnect delay is shorter. A compressor or refrigeration load needs a longer reconnection delay so pressures equalise before restart.
Frequently asked questions
Are these values a standard?
No. They are common engineering starting points. The correct window comes from the connected equipment’s tolerance, the site’s supply behaviour, and the applicable installation rules. IEC 60038 defines the nominal voltage the window is built around.
Why the reconnection delay?
Immediate restart after a dip can stress motors and compressors, and rapid short-cycling damages them. A delay of tens of seconds to a few minutes lets the supply settle and compressor pressures equalise.
Does this replace an SPD or a breaker?
No. A protector handles sustained over- and under-voltage only. Transient surge needs an SPD, overcurrent needs a breaker, and earth leakage needs an RCD.
Specify the protector
With the window and reconnection delay set, choose a single- or three-phase protector with adjustable thresholds and delay.
Starting points for guidance only. Final thresholds and delays must be verified against the connected equipment ratings, the site supply, and local rules by a qualified person.