My Friend Flicker

13 September 2019


Flickering lights. It’s a classic horror movie hoax, but are ghosts really to blame?

Unfortunately (or maybe fortunately) no, flicker is a power quality phenomenon at its core. However, today is Friday the 13th, so talk about some great planning on our part, right? Just kidding. This was an accident, but we will totally take the credit.  

Now, back to the regularly scheduled blogging with our good friend, flicker. The IEC and IEEE, our electrical knowledge powerhouses, define flicker as “the impression of unsteadiness of visual sensation induced by a light stimulus whose luminance or spectral distribution fluctuates with time”. Despite the fancy verbiage we just threw at you, the concept of flicker is simple, and you already know exactly what it means when the lights are “flickering”. At least, we are assuming you’ve experienced that phenomenon before. If not, where do you live? We want to know your secret.

Without us (humans), flicker wouldn’t even be a problem, since its directly related to our perception of incandescent light bulbs. So, go us! While flicker and voltage fluctuations are not synonymous, the criteria for controlling small changes in voltage is usually our perception of light flicker. Does that make sense? Maybe not yet, so let’s get into it even further.

What’s a voltage fluctuation?

Voltage fluctuations are changes in voltage magnitude over varying periods of time, which can cause periodic changes in lighting output. The magnitude and frequency of the voltage will determine the severity of the flicker that we see. Studies have shown that if you are using incandescent lightbulbs at 115V/60Hz or 230V/50Hz, voltage fluctuations at 8Hz are visible – thus, you’ll see that pesky flicker. But, if the frequency of the fluctuations goes above or below 8Hz, the voltage variations need to be greater in order to see a change in the lighting.

How can we measure flicker?

Again, we refer to our old pals at the IEC (specifically, IEC6100-4-15) for their methodology. To measure flicker, you simply compare the instantaneous voltage to the average rolling voltage and calculate the deviation between the two. Then, the deviation is multiplied by a value in a weighted curve, based on the sensitivity of the human eye at 120V/60Hz or 230V/50Hz. When it comes to flicker, we are all celebrities, since the whole concept is based on what WE can see. When things get tough, just remember that you are the apple of flicker’s eye. How special is that?

Okay, back to the math. That final value that you calculated is called a percentile unit (or flicker percentile unit). The percentile units ultimately go through a series of vigorous statistical analysis in order to calculate two important values. If the phrase “vigorous statistical analysis” doesn’t bring images of numbers frantically bench pressing, doing crunches, and swinging some kettlebells, where is your brain even at right now? Okay, maybe it’s just us. Fine.

But what about those two values?

Oh, it’s probably time to introduce you to short term flicker (Pst) and long-term flicker (Plt). Short term flicker is calculated based on the flicker percentile unit and a 10-minute interval, whereas long term flicker is calculated directly from the Pst (or short term flicker) and is based on a two-hour interval. Big difference.

Flicker is like most of life’s irritations and nuisances. While some of us are extremely bothered by certain situations and phenomenon, others can continue without batting an eye. It’s the same for flicker. Some individuals will be beyond annoyed by a long term flicker (Plt) value of 1.0, while others can tolerate much higher levels of Plt with no reaction. It all depends on the person.

Why does this even happen?

Well, we already know that flicker is caused by voltage variations. So, what you’re really trying to ask is what causes these fluctuations in voltage? Ugh. There are so many possibilities, including (but not limited to): the operation of ovens, welders, cranes, mills, arc furnaces, inrush currents from loads turning on, poor wiring, and bad connections.

So, what can we do about it?

Well, if you are in the power quality industry, we can give you some flicker recommendations.

In the United States, the IEEE (IEEE1453), has quite a few recommendations for your low voltage systems. You ready?

  • Short term flicker (Pst) should be less than or equal to 1.0 for 95% of the time
  • Long term flicker (Plt) should be less than or equal to 0.8 for 95% of the time

Oh wait, there’s more!

  • The planning levels for medium voltage systems are 0.9 for Pst and 0.7 for Plt
  • The planning levels for high voltage systems are 0.7 for Pst and 0.6 for Plt

For our friends across the pond, you’re in luck - we have some guidelines for you, as well. The European standard (EN50160) requires long term flicker to be less than or equal to 1.0 for 95% of the time, under normal operating conditions.  

Now, you can be a flicker educator, too. Next time you watch a scary movie (maybe even tonight), be sure to de-mystify the concept of flickering lights for all your family and friends. They’re going to love it. And, as spooky season (more commonly known as October) is quickly approaching, we know you will be the life of every Halloween party with your recently acquired flicker knowledge. Please, let us know how it goes!