If the sprinkler goes off, you've got worse issues than the lack of a gasket on the panel. Don't worry so much about the weirdest possible thing that might go wrong and you'll live a long, stress-free life.
When in the planning stage, when working in dusty areas or where there is particles that may accumulate on top of the panels and such I tend to try not to go in the top. Although it is not always possible that is my first line of defence.
If you are worried about water or liquid (splashing or mist) then you are in a totally different context. The breaking of a water line is not considered part of the normal operation so it normally should not be considered. Equipment is installed according to it "normal working conditions". If we dealt with "what ifs" on every site, everything would be quakeproof, waterproof, sasquatch proof, etc.
If you use some kind of gasketed device, you have to remember that you will, in most cases, affect the mechanical connection for the bond between the connector (either cable or pipe) and the enclosure. You will likely require some sort of bonding bushing to be installed. Again not a big deal and low cost, but a pain if you forget them and need to put them on afterward.
I don't know the specific code or if there's one that exits but here in ontario specifically in the GTA we always use the o-rings when entering the top of a cabinet or panel...I'm actually piping all the mechanical rooms on a new tower in downtown Toronto and it's specd to use the o-rings when entering the top of electrical distribution equipment..
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