That’s right. It doesn’t necessary have to be half a second, second or two should be fine.
There are two likely failure points here for the simple XT-90s loop key melting:
1 - not plugged all the way in. That small resistor doesn’t stand a chance of handling the full load. Poof! Especially when new these suckers are TIGHT and can be tough to seat fully.
2 - they fail. simple. They are made cheaply and if one misses a good QC and has an issue - it’ll still fail if everything else is still correct. My assumption of your issue @DeathCookies.
Orientation should not matter. Period. If you did not fully kill it and swapped orientation and it worked i would then suspect some soldering issue or similar. But most of the time it will die if it goes - and a new XT-90s that works in the other orientation may seem like the fix, but i’d be willing to bet you a shiny nickel if you swapped it - it would still work.
Still one of the easiest and cheapest ways to suppress the spark. Definitely worth getting a few spares just in case.
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(Yes, I know this is an old thread, but I thought this would be some important advice):
As suggested by @chinzw, if you use a loop key, make sure it cuts the positive side! I was having an issue with a separate circuit connected to the battery where some MOSFETs weren’t turning off, and it was because of the common ground issue that you get if you have a disconnect on the negative side. Finally figured out that was the issue today, so for anyone else out there planning on using some external circuitry, just be advised that you should install a loop key on the positive side.