You've hit on a key point: it's likely a combination of liability and potential for issues, though the issues might be subtle. Here's a breakdown of why Legrand likely requires power to be off when adjusting the pot, and why you might have gotten away with it on Lutron

1. Liability (The Primary Driver):
CYA (Cover Your Assets)

This is the biggest factor. Electrical safety is paramount for manufacturers. If something goes wrong while someone is fiddling with a live dimmer, Legrand doesn't want to be held responsible. By stating power must be off, they create a strong legal defense.

Varied User Skill Levels

They have to assume the user is not an electrician and may not understand basic electrical safety. Touching the wrong thing inside a live dimmer can be dangerous.

2. Potential Technical Issues (While Less Likely, Still Possible)



Arcing and Interference

Adjusting a potentiometer (pot) while it's under load (with power flowing through it) can cause tiny arcs inside the pot. These arcs can: Create electrical noise that interferes with the dimmer's operation, or even with other electronics nearby (especially smart dimmers). Over time, damage the potentiometer itself, leading to erratic dimming or failure.

Inrush Current

Some dimmers, especially electronic ones, can experience inrush current when the power is turned on. Adjusting the pot at that precise moment might cause the dimmer to misinterpret the setting and behave unpredictably.

Component Sensitivity

The adjustment pots are often connected to sensitive electronic components within the dimmer's circuit. Adjusting the pot with power on could theoretically introduce transient voltage spikes that might stress or damage those components, especially if the pot is adjusted quickly or aggressively.

Feedback Loops

Dimmer circuits are often designed with feedback loops to regulate the output voltage and ensure smooth dimming. Adjusting the pot while the dimmer is active could disrupt this feedback loop, leading to instability or damage.

Why You Might Get Away With It (Sometimes)



Lutron Design

Lutron products generally have a good reputation for robust design and build quality. It's possible that their dimmers are designed with more insulation, better component protection, or more robust potentiometers that are less susceptible to damage or interference from adjustment under load.

The "Lucky" Factor

You might have simply been lucky. The effects of adjusting a pot under load might be cumulative, meaning the damage (if any) occurs gradually over time. You might not notice any immediate issues.

Type of Load

The type of bulb or fixture connected to the dimmer can also affect the risk. Incandescent bulbs are generally more forgiving than LEDs or CFLs, which have more complex electronic ballasts.

Quality of the Potentiometer

Not all potentiometers are created equal. Some are built to handle more current or voltage than others. If a dimmer has a higher-quality pot, it may be less susceptible to damage from adjustment under load.

Margin for error

Dimmer components could be rated for higher voltages and currents than needed for standard operation.

Bottom Line



Follow the Manufacturer's Instructions

The safest and wisest course of action is always to follow the manufacturer's instructions. Legrand specifically tells you to turn the power off, so you should do so.

Risk vs. Reward

While it might be tempting to ignore the warning for convenience, the potential risks (even if small) aren't worth it. You could damage the dimmer, create a safety hazard, or void the warranty.

Lutron Might Be More Forgiving, But Still Not Recommended

Just because you haven't had problems with Lutron dimmers doesn't mean it's a safe practice. Their design might be more tolerant, but you're still taking a small risk.*

Think About Alternatives: If repeatedly turning off the power is a huge inconvenience, consider using a clamp meter or multimeter to measure the power consumption to the load, and use that as a guide to fine-tune the trim pot. Alternatively, install an accessible switch near the dimmer for easy on/off control during adjustment.In short, prioritize safety and follow the manufacturer's guidelines. It's not worth risking damage, injury, or voiding the warranty for the sake of a few minutes of convenience. Flag for review