Hi, electrical engineer here.
This is perfectly normal and nothing to worry about.
Many wall warts for all sorts of different electronics will spark when you plug them in (though it depends on the specific outlet and the motion/angle you have it at as you push it in, so it won’t necessarily consistently).
This happens with just about anything that is always on, things like power bricks, wall warts, appliances, and will happen with things like lamps if they are turned on with a light bulb installed when you plug them in.
The Mercury X adapter is a little beefy and has some inrush current from having equally beefy input capacitors. The spark is actually a sign that they didn’t skimp on build quality of that power brick. They went with large and low ESR capacitors, both of which indicate higher quality.
Rejoice in the sparking, be not afraid. It is no more powerful or dangerous than a jolt of static electricity from your finger to a door knob (or unlucky other house member).
If you don’t believe me, just inspect the prongs of the adapter. If it was a true spark (which involves metal vaporizing and being ejected into the air), the evidence of such will be clearly visible as small pitted feature somewhere on the prongs. Sparks like this are what typically ignite vapors, because the hot tiny pieces of metal have enough heat capacity to potentially act as an ignition source.
Arcing, which is what you’re seeing and calling a spark, it’s just dielectric breakdown of air (aka ionization). This is the same thing as a static electricity arc, or at the extreme end, a lightning bolt.
The distinction matters because arcs that size are so brief and the heat is contained in a very tiny channel of air with so little heat energy that it is far less likely to be able to serve as an ignition source.
I’m not going to tell you that it couldn’t ignite IPA fumes or anything. But it has a similar chance of igniting fumes as any static discharge (should you call socks, well, socks? Or foot-deployable murder tubes?), literally every time you turn any light switch on or off (which creates a spark too, though you usually can’t see it. Sometimes you can feel it though if your hand is wet!), any appliance or device with a relay inside (it will “click”. Fridges, home audio receivers, etc).
My point is that for the arcing you’re worried about to be a real threat, you will already have had to failed to safely handle the IPA in the first place, and there are countless sources including totally unpredictable ones like static discharge that are just as likely to serve as ignition sources.
Is it a danger? Yes, if you are handling IPA with totally inadequate ventilation, spilling it into an outlet at the same time you’re plugging something in, or you have tiki torches in your office/printer garage.
IPA fumes are heavier than air and can’t ignite until it hits concentration of at least 2%. The only way you’re going to hit that is if you have no ventilation in a very small room that you left large containers of IPA open for an hour ahead of time. It would be quite unpleasant to breathe in this room.
As long as you’re not doing this inside your car with all the windows rolled up and doors closed, as long as you keep the IPA liquid 30cm/1 foot from potential ignition sources, you’re good. Remember, all the warnings say KEEP AWAY from ignition sources, which means an actual distance and it’s not a very large one. No electronics within a foot, I think that’s pretty easy to achieve.
One other thing to put your mind at ease: IPA is dangerous to your health from inhalation far, far earlier than it is a danger due to being flammable. You will be breathing fumes at very uncomfortable concentrations that are strong enough to cause nearly immediate physical effects (particularly central nervous system ones) well before fume concentrations can present an ignition risk.
Remember, people literally put this stuff in spray bottles and produce large clouds of IPA droplets for use as a general cleaner or disinfectant (or use cleaning products containing it) in almost every home out there frequently and regularly without ever burning down their homes or causing an explosion.
Also keep in mind that the only combustion products of pure IPA are water and carbon dioxide, making it quite safe for a fuel burned indoors. Which people very much do
People are literally setting fire to IPA inside their homes on purpose and it’s less dangerous than a gas stove frankly.
Just use a modicum of common sense and you’ll be fine. Don’t worry about the spark and just keep it a foot away from your (air tight, sealable) 5 gallon vat of it and don’t let it sit overnight with the lid off (you’ll smell it if you do though so it’s a hard mistake to make).
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