Voltage Flicker

Voltage flicker is the visible fluttering of lighting caused by repeated, rapid variations in supply voltage. It is not a single event like a sag or a swell but an ongoing fluctuation, typically from loads that switch or vary their current draw cyclically. The human eye is most sensitive to these fluctuations in a specific frequency band — roughly a few to a couple of dozen cycles per second — which is why flicker is defined in terms of human perception rather than raw voltage change alone.

The name is almost too literal, which is exactly why people underrate it. Flicker is not primarily about your lights being annoying; it is a visible symptom of a supply that struggles to hold its voltage steady the instant a load changes, and that same underlying weakness stresses everything else sharing the circuit. Because the human eye happens to be exquisitely sensitive to certain rates of light variation, ordinary incandescent and even some LED fixtures effectively act as free, always-on diagnostic instruments reporting on the health of your feeder. When they flutter in time with your equipment cycling on and off, they are announcing a real electrical condition, and a miner is far wiser to listen to that signal and investigate the supply than to simply shrug and change the bulbs.

How flicker is measured

The IEC 61000-4-15 flickermeter model, also adopted in IEEE 1453, produces two severity indices. Pst, the short-term flicker severity, is evaluated over a ten-minute window, with a value of 1.0 representing the threshold of perceptible annoyance for a typical observer. Plt, the long-term severity, is derived as a cubic mean of consecutive Pst values over a two-hour period, capturing sustained nuisance rather than a brief disturbance. Keeping Pst at or below 1.0 is the usual planning target for a well-behaved connection, and utilities use these indices to judge whether a given load is a good neighbour on a shared feeder.

Why the eye, not the meter, defines it

Flicker is unusual among power-quality metrics because it is defined by human perception, not by a fixed voltage threshold. A given percentage of voltage variation is annoying at around eight to ten fluctuations per second — where the eye is most sensitive — and barely noticeable at much higher or lower rates. The flickermeter model deliberately weights the measurement to match that human sensitivity curve, which is why two disturbances of identical voltage magnitude can produce very different flicker severities. This is also why flicker is a complaint-driven phenomenon: it is measured the way it is because its whole significance is how it looks to the people living with it.

Relevance to mining loads

A miner running at steady state produces very little flicker, because a constant hashing load draws a constant current. The flicker risk comes from equipment that cycles or ramps — cooling fans and pumps switching under thermostat control, immersion-cooling gear, and especially staggered start sequences where blocks of hashrate come online in steps. Each of those current steps briefly tugs the supply voltage, and if the tugs land in the eye's sensitive band, nearby lights flutter. On a strong grid connection the effect is negligible; on a weak, high-impedance rural feeder it can become genuinely perceptible.

What it tells a home miner

For a home or small-site operator, visible flicker is a useful diagnostic, not just an annoyance. Lights that dim in sympathy with your miners switching on are the feeder telling you it is undersized or heavily loaded for the current draw you are imposing — the same weakness that shows up as sagging voltage under load and stressed power supplies. The fixes are the familiar ones: spread start-up in time so current steps are smaller, put large cyclic loads on their own circuit, verify wiring and connections are sound, and where the supply itself is the limit, involve the utility before you add more machines. Flicker sits alongside the other voltage-fluctuation phenomena tracked under power quality, and like harmonics it is most pronounced on shared, high-impedance circuits.