A voltage dip and a voltage sag are the same event: a short drop in RMS voltage, usually caused by a fault or a large motor starting somewhere on the network. “Dip” is the IEC and European term; “sag” is the IEEE and North American term. An interruption is the deeper case, where the voltage almost disappears. The difference matters because dips are the single most common reason production lines, drives and medical systems stop, yet they are short enough that nobody sees them happen.
This guide explains how each is defined, why two reports of the same event can look different, and how to tell whether your equipment should have survived it.
The definitions side by side
| IEEE 1159 | IEC 61000-4-30 / EN 50160 | |
|---|---|---|
| Name | Sag | Dip |
| Starts when | RMS voltage falls to between 0.1 and 0.9 pu | RMS voltage falls below the dip threshold, typically 90 % of declared voltage |
| Duration | 0.5 cycle to 1 minute | From when it crosses the threshold until it recovers above threshold + hysteresis (typically 2 %) |
| Becomes an interruption | Below 0.1 pu | Below the interruption threshold, typically 5 % or 10 % depending on the instrument and region |
| Sub-classes | Instantaneous (0.5–30 cycles), momentary (30 cycles–3 s), temporary (3 s–1 min) | Classified in a table by residual voltage and duration |
Sources: IEEE terminology overview, CSE, Schneider Electric EN 50160 note on dips, IEEE 1159 definitions, Power Monitors.
Anything longer than one minute is no longer a short-duration event. It is an undervoltage or a sustained interruption.
Residual voltage vs depth: the most common confusion
There are two ways to describe how bad a dip was:
- Residual voltage: the lowest RMS voltage during the event, as a percentage of nominal. “A dip to 70 %.”
- Depth: how far the voltage fell. “A 30 % dip.”
Both describe the same event. IEC reports and most modern instruments use residual voltage; many older reports and conversations use depth. Before comparing two reports, or a report with an equipment specification, check which one each uses. Mixing them up turns a mild event into a severe one on paper.
Why short dips stop long processes
A 100-millisecond dip lasts five or six cycles. That is enough to:
- drop out the contactor coils that hold a motor starter closed;
- trip a variable-speed drive on DC-bus undervoltage;
- reset a PLC or a controller with a weak power supply;
- abort an MRI or CT sequence, which then has to be repeated.
The process may take hours to restart even though the supply was back within a fraction of a second. That is why our guide on voltage sags and production stops starts with timestamps: matching each stop to a recorded event is the only way to prove the cause.
Where dips come from
- Faults on the utility network, often on another feeder or the transmission system, cleared by protection within a few cycles. These cause most dips and are outside your control.
- Large motor starts inside your own site, especially direct-on-line starts without soft starters.
- Transformer energisation and other large inrush currents.
- Faults inside the site, on another branch of your own distribution.
A recording that captures the waveform before and after the event helps tell these apart: a fault-driven dip is usually abrupt and short; a motor start pulls the voltage down more gradually and recovers as the motor accelerates.
Should my equipment have ridden through it?
Two reference curves answer that question:
- The ITIC (CBEMA) curve describes what typical information-technology equipment should tolerate: any dip shorter than 20 ms, a residual voltage of 70 % up to 0.5 s, 80 % up to 10 s, and 90 % beyond.
- SEMI F47 sets the requirement for semiconductor processing equipment: 50 % residual voltage for 200 ms, 70 % for 0.5 s and 80 % for 1 s (Advanced Energy application note). It is widely used as a benchmark for industrial controls too.
If an event was inside these curves and your equipment still tripped, the weakness is in the equipment or its settings. If it was outside, the supply or ride-through equipment is the problem. Our voltage sag ride-through checker plots any event against both curves.
How Inspector records dips
Inspector classifies dips, swells, interruptions and rapid voltage changes in line with IEC 61000-4-30 and stores each event with 5 cycles of waveform before and 5 cycles after. That shows not just the residual voltage and duration but how the event started and ended, which is what separates a utility fault from a motor start. With alerts by SMS, e-mail or app, the team knows about the event before the shift supervisor calls.
Not sure whether dips are behind your unexplained stops? Talk to an Inspector engineer or take the free assessment.
