Understanding Long Term Flicker Severity: how twelve measurements over two hours reveal voltage quality

What is the value derived from twelve measurements over two hours called?

If you’ve ever stood in a room with flickering lights or noticed a lamp that seems to surge in brightness every so often, you’ve brushed up against a very real part of power quality. In the world of power substations, there’s a tidy way to quantify those flickers over time. The value you get from twelve measurements taken over two hours is called Long Term Flicker Severity. In short: Long Term Flicker Severity, often abbreviated as Plt, is the metric that captures how voltage fluctuations look when you watch them across a meaningful stretch of time.

Let me unpack what that means and why it matters.

What flicker actually is, and why we care

Flicker isn’t just a dramatic word you hear in electrical talks. It’s a real phenomenon that shows up as temporary changes in light intensity caused by quick shifts in voltage. When a large, fluctuating load—think big motors starting and stopping, welding equipment, or certain kinds of lighting like dimmable LEDs—drags the voltage around, lamps respond with brightness that seems to “flicker.” For people and processes that rely on steady illumination, flicker can be more than a nuisance. It can affect precision work, comfort, and even productivity.

There are two related ideas you’ll see tossed around in standards and engineering briefs: short-term flicker, measured by Pst, and long-term flicker, captured by Plt. Short-term flicker looks at a brief window—typically 10 minutes—enough to catch quick disturbances. Long-term flicker, on the other hand, surveys a longer horizon so you can see patterns that emerge only when you watch for hours. That’s where our twelve-measurement, two-hour approach comes in. It’s the big-picture view.

Why twelve measurements over two hours?

Why not just take one measurement and call it a day? Because flicker isn’t a one-and-done event. It ebbs and flows with load changes, equipment cycling, and even the time of day when certain devices are more active. A single snapshot can mislead you, making a system seem steadier—or more erratic—than it truly is. By sampling every ten minutes for two hours, you capture a representative cross-section of the disturbance environment. The method is simple in spirit but powerful in practice: it reveals how often and how strongly the light and voltage dance over a meaningful period.

In practical terms, you take twelve Pst values, each representing a ten-minute window, and you combine them in a defined way to produce Plt. The result isn’t just an average of flicker; it’s a seasoned summary that weighs larger flicker events more heavily. The standard approach used in many power-quality frameworks calculates Plt by aggregating those twelve Pst values in a cubic manner and then taking a final cubic root. The math keeps the emphasis on more intense flicker episodes, which is exactly what engineers want when they’re sizing mitigations or planning improvements.

What Plt tells you about a substation and its loads

Long Term Flicker Severity isn’t a vanity metric. It’s a practical signal about how the network behaves under real-world conditions. If Plt is high, you’ve got to pay attention to the kinds of disturbances that cause light intensification or dimming over time. In factories with sensitive lighting, in offices with large windowed spaces relying on consistent illumination, or in retail environments where display lighting matters for mood and sales, a high Plt can translate into uncomfortable glare, misread readings on gauges, or subtle interference with sensors that require stable lighting conditions.

A few concrete drivers of higher Plt include:

• Nonlinear loads and rapidly changing currents, such as welding power sources, arc furnaces, or certain inverters.

• Heavy motor start-ups that cause voltage dips or swells as equipment ramps up.

• Dimmable lighting circuits that interact poorly with irregular voltage.

• Uneven distribution of loads across phases, creating localized voltage swings.

• Inadequate system design for harmonics and transient events.

On the flip side, a low Plt suggests the voltage environment is smooth enough that lighting and sensitive equipment won’t suffer noticeable flicker across a typical working period. That kind of stability is a big deal in precision manufacturing, medical facilities, and data centers, where consistency is part of the product.

How it’s measured and what that implies for design

Measurement of flicker follows well-trodden paths in electrical standards. The basic idea is to quantify how much the voltage deviates from its nominal value and how that translates to perceivable light fluctuations. The Pst measure captures the instantaneous flicker effect within each ten-minute window. Then, by rolling up those twelve Pst results with a cubic aggregation, Plt yields a single figure that reflects the long-term behavior.

A few practical notes you’ll hear in the field:

• The two-hour window isn’t arbitrary; it’s chosen because many lighting and electrical processes exhibit patterns that repeat or persist over a couple of hours. If you only looked at a shorter period, you might miss a recurring disturbance.

• The measurement setup typically uses synchronized sampling across the same phase or phases, ensuring you’re comparing apples to apples when you compute Plt.

• Different regions and industries may have recommended or mandated thresholds for Plt. When a plant or utility sees Plt crossing a threshold, it often triggers investigations into power quality improvements.

So why not rely on instantaneous voltage readings? Because a snapshot doesn’t tell the whole story. You might be lucky and catch a moment of calm, or you might stumble into a moment of peak disturbance. When flicker is a regular feature—when a two-hour view shows a pattern rather than a blip—you’re dealing with a reliability and comfort issue rather than a one-off nuisance.

Real-world implications and how to respond

Understanding Plt isn’t just about ticking boxes on a spec sheet. It helps you decide where to put effort and money to improve the power environment. Here are a few practical paths professionals often explore:

• Load balancing and phase distribution: Even out the demand across phases to reduce voltage swings.

• Harmonics management: Clean up non-sinusoidal currents that can aggravate flicker effects.

• Power conditioning: Filters or dynamic voltage restorers can smooth out the voltage waveform seen by sensitive equipment and lighting.

• Lighting design tweaks: Choose lighting that’s less sensitive to voltage fluctuations or add stabilizing ballast components where needed.

• Strategic placement of large loads: Stagger startup sequences or separate heavy loads to avoid simultaneous disturbances.

These moves aren’t about rewriting the laws of physics; they’re about tuning the system to behave more predictably under typical operating conditions. And that predictability matters—both for the people who work around the substation and for the devices that rely on steady illumination and stable voltages.

Common misconceptions to keep in check

• Plt is a single, one-time number that represents every moment of the day. Not true. It’s an aggregate built from twelve ten-minute windows. It’s a broader brush stroke, not a snapshot.

• A high Plt means a project is doomed forever. Not at all. It signals where attention should go, and often practical fixes can bring Plt down significantly.

• Plt equals the same thing as Pst. They’re related, but they measure different horizons. Pst is about the short term; Plt is about the long term. Both play a role in a full power-quality picture.

A friendly analogy to wrap your head around it

Think of Pst as a heartbeat monitor for brightness, watching a single breath in a crowded room. Plt, by contrast, is like watching the room over an evening—notes of excitement, quiet moments, and the occasional jolt when a door slams or a machine starts up. Taken together, they tell you not just how lively a room is in a moment, but how that liveliness settles into the evening rhythm. In power terms, that means you’re more informed about how lighting and equipment will behave day in and day out.

A quick recap

• The value derived from twelve measurements over two hours is Long Term Flicker Severity, or Plt.

• Plt captures how voltage flicker behaves across a meaningful, longer window, not just a quick moment.

• It’s calculated from twelve ten-minute Pst values using a cubic aggregation, giving more weight to stronger flicker events.

• This metric matters because flicker affects lighting quality and the reliability of sensitive equipment in commercial and industrial settings.

• When Plt is higher than desired, engineers explore load management, harmonics control, power-conditioning options, and lighting design tweaks to smooth things out.

If you’re navigating the world of substation design and operation, Plt is one of those practical tools that bridges theory and real-world impact. It’s not about chasing a perfect score; it’s about ensuring the lights stay steady where people work, machines run, and processes stay precise. And that steady glow—well, it’s something most of us notice, even if we don’t name it in a report.

A last thought to keep in mind

When you hear someone mention long-term flicker severity, remember they’re talking about a disciplined, time-aware way to measure how the electrical system behaves under typical operating conditions. It’s not just about one number; it’s about understanding how a system behaves over a meaningful slice of time and using that insight to keep power delivery comfortable, reliable, and robust. In the end, that’s the kind of clarity every engineer and operator can appreciate, whether you’re in a busy industrial setting or a quiet office with a glow from the lamps overhead.

If you want to connect the dots further, consider looking into how flicker metrics relate to other power-quality indicators and what practical mitigations teams deploy in response to elevated Plt. It’s a multi-tool mindset: push for stability, watch the patterns, and shape the system so the day-to-day work—from drafting to data collection—gets the dependable light it deserves.