Flicker (voltage fluctuations) — NEN-EN 50160

A horticultural business with assimilation lighting driven via dimmers or frequent on/off switching, or with welding equipment and large motors that start often, can cause visible light flicker on the grid. NEN-EN 50160 ("Voltage characteristics of electricity supplied by public electricity networks") sets a quality limit for this, separate from the requirements for harmonics/THD (see the harmonics & THD guide) and voltage unbalance (see the voltage unbalance guide).

What flicker is

Flicker (voltage fluctuation) arises from rapid, repeated small variations in voltage amplitude — often caused by loads that switch with a fluctuating power draw (welding equipment, motors with frequent start/stop, dimmers, large compressors). The human eye is sensitive to the resulting variation in light intensity from incandescent and some LED lighting, which is perceived as annoying "flicker."

The measurement quantities: Pst and Plt

Flicker severity is expressed in two quantities:

Pst (short-term severity) — measured over a period of 10 minutes.
Plt (long-term severity) — composed of twelve consecutive Pst values, measured over 2 hours.

The standard's requirement

Under normal operating conditions, NEN-EN 50160 sets a limit on the long-term flicker severity Plt: for 95% of a week, it must not exceed 1. The standard does not set a separate limit value for the short-term quantity Pst itself — Plt is the representative indicator for conformity.

Causes and practical relevance

Arc welding — strongly fluctuating power during the welding process.
Frequently starting large motors/compressors (for example cooling or climate installations with many start/stop cycles).
Dimmers and phase-dimming-controlled assimilation lighting — depending on the dimming principle, this can introduce measurable flicker.

Note: whether a specific load actually leads to a Plt exceedance depends on the power of the load relative to the short-circuit power of the grid at that connection point — this must be measured/calculated per situation, not assumed based on the load type alone.

Difference from THD and voltage unbalance

Flicker, harmonics (THD) and voltage unbalance are three separate NEN-EN 50160 quality criteria, each measuring a different phenomenon: flicker concerns rapid amplitude variation (perceived as light flicker), THD concerns distortion of the voltage waveform, and unbalance concerns asymmetry between the three phases. An installation can comply with the standard on one criterion while having problems on another.

Common mistakes

Attributing visible light flicker to a defective luminaire or dimmer without checking whether the cause lies in the supply itself (for example another load on the same circuit/connection causing the voltage to fluctuate).
Confusing flicker with voltage unbalance or THD — these are three separate measurements with their own causes and their own solutions.
Assuming that every fast-switching load by definition causes a Plt exceedance — the actual impact depends on the ratio between the load's power and the grid strength at that point.