Electro-Voice 1252 - Manual (Page 135)

Chapter 12
Flicker
12.1: Overview
Flicker is the sensation that is experienced by the human visual system when it is subjected to
changes occurring in the illumination intensity of light sources. The primary effects of flicker are
headaches, irritability and sometimes epilepsy.
IEC 61000-4-15 and former IEC 868 describe the methods used to determine flicker severity. This
phenomenon is strictly related to the sensitivity and the reaction of individuals. It can only be
studied on a statistical basis by setting up suitable experiments among people.
12.2: Theory of Operation
Flicker can be caused by voltage variations which are caused by variable loads, such as arc furnaces,
laser printers and microwave ovens. In order to model the eye brain change, which is a complex
physiological process, the signal from the power network has to be processed while conforming with
Figure 16.1 below.
• Block 1 consists of scaling circuitry and an automatic gain control function that normalizes input
voltages to Blocks 2, 3 and 4. For the specified 50 Hz operation, the voltage standard is 230 V
RMS.
• Block 2 recovers the voltage fluctuation by squaring the input voltage scaled to the reference
level. This simulates the behavior of a lamp.
• Block 3 is composed of a cascade of two filters and a measuring range selector. In this
implementation, a log classifier covers the full scale in use so the gain selection is automatic and
not shown here. The first filter eliminates the DC component and the double mains frequency
components of the demodulated output.
The configuration consists of a .05 Hz Low High Pass filter and a 6 Pole Butterworth Low Pass
filter located at 35 Hz. The second filter is a weighting filter that simulates the response of the
human visual system to sinusoidal voltage fluctuations of a coiled filament, gas-filled lamp (60
W - 230 V). The filter implementation of this function is as specified in IEC 61000-4-15.
• Block 4 is composed of a squaring multiplier and a Low Pass filter. The Human Flicker
Sensation via lamp, eye and brain is simulated by the combined non-linear response of Blocks 2,
3 and 4.
• Block 5 performs an online statistical cumulative probability analysis of the flicker level. Block
5 allows direct calculation of the evaluation parameters Pst and Plt.
Q Flicker Evaluation occurs in the following forms: Instantaneous, Short Term or Long Term. Each
form is detailed below:
Electro Industries/GaugeTech Doc # E107706 V1.25 12-1