Voltage dips and interruptions (NEN-EN 50160) — classification and the difference with flicker

The guide on flicker/voltage fluctuations covers the long-term flicker severity Plt (NEN-EN 50160), and the guide on harmonics & THD covers the THD limit from the Netcode Elektriciteit. This article covers a third, separately classified type of voltage phenomenon from the same standard: the brief voltage dip and the interruption — two terms often used interchangeably in practice, but which NEN-EN 50160 defines precisely and distinctly.

The definition of a voltage dip

NEN-EN 50160 defines a voltage dip as a sudden drop in the RMS voltage to a value between 90% and 1% of the declared voltage (Un), followed by recovery of the voltage after a short period. The standard applies two boundaries:

the dip starts as soon as the RMS voltage falls below the 90% threshold of Un;
the dip lasts from 10 ms up to and including 1 minute.

As soon as the residual voltage during the event drops below 1% of Un, the event is no longer classified as a dip but as an interruption — the 1% boundary is the dividing line between "the voltage sagged sharply" and "the voltage actually disappeared".

Short versus long interruption

An interruption (voltage <1% Un) is in turn subdivided by duration:

short interruption: up to and including 3 minutes;
long interruption: longer than 3 minutes.

This 3-minute boundary matters because a short interruption is usually the result of an automatic reclosing attempt by a protection device in the network (for example after a transient fault that clears itself), while a long interruption generally requires manual intervention by the network operator.

Why there is no fixed compliance limit here, unlike THD and flicker

The THD limit from the Netcode Elektriciteit and the Plt limit of 1 from the flicker guide are both expressed as a value that may not be exceeded during 95% of a measurement week — a statistically reproducible limit for a continuous, stationary phenomenon. Voltage dips and interruptions, however, are stochastic, locally determined events (lightning, a tree branch on an overhead line, a switching action elsewhere in the network): NEN-EN 50160 therefore gives no fixed maximum limit for dips and interruptions that the network operator must guarantee, only informative, typical annual values — the standard recognises that the number and severity of dips varies greatly by location (for example a rural network with overhead lines versus an urbanised underground network) and cannot be enforced centrally the way THD can.

The ITIC/CBEMA curve: equipment sensitivity, not a grid standard

To assess whether a voltage dip of a given depth and duration causes problems for sensitive equipment (process control, servers, PLCs), the ITIC curve (formerly CBEMA curve) is often used in practice — a chart from the American Information Technology Industry Council that plots the region within which information-technology equipment should tolerate a voltage deviation (as % of Un, plotted against duration) without failing. It is not a European grid standard and not part of NEN-EN 50160 itself, but an industry guideline used to assess whether the dips a grid actually delivers fall within the immunity of the connected equipment — two different things that are often confused.

Practical relevance

When there is a complaint about equipment failing on "a small voltage dip" while the network operator reports that NEN-EN 50160 was not violated, that is not a contradiction: the standard sets no fixed maximum limit for dips in the first place. The follow-up question is then whether the equipment itself has sufficient dip immunity (for example checked against the ITIC curve or the manufacturer's specification), and whether additional provisions (UPS, dynamic voltage regulator) are needed — see also the guide on UPS battery types for bridging longer interruptions.

Common mistakes

Treating a dip and an interruption as the same phenomenon — the distinction lies at a residual voltage of 1% Un: above that it is a dip, below it an interruption.
Expecting NEN-EN 50160 to set a hard maximum limit on the number or depth of dips, as with THD or flicker — for dips and interruptions the standard only gives informative, typical annual values.
Mistaking the ITIC/CBEMA curve for a European grid standard — it is an American industry guideline for equipment sensitivity, not part of NEN-EN 50160.
Confusing a short interruption (≤3 min, often an automatic reclosing attempt) with a long interruption, which usually only ends after manual intervention by the network operator.