It is easy to become complacent when it comes to plugging an electrical cord into a socket. This convenient arrangement for making an electrical connection has become such a casual part of our lives. It is commonplace to just plug-in if it fits and sometimes, with a little encouragement, it can be made to fit.
At home when we need to plug in a lamp that does not quite reach the closest receptacle, an extension cord can be employed to manage the connection. The two blade lamp plugs easily and safely into the two slot receiver of the extension cord. Sometimes that device that does not quite reach has two blades and a pin, but the extension has only two slots. With a little encouragement the third pin can be made to slide unconnected beneath the extension cord’s receiver body. No harm done, except if there was a short in the device when that third connection (safety ground pin) should come into service to provide a safe path to ground and prevent the device from becoming energized.
NEMA (The National Electrical Manufactures Association) has been around for almost a century. They have standardized the configurations for plugs and receptacles to try and prevent dangerous and harmful connections being made between dissimilar voltages and/or amperages; sometimes despite their best efforts, the configuration is “close enough” that it can be “made to fit” with sometimes dangerous or disastrous results.
We have seen 30A cord sets plugged into 50A receptacles and recently 125V system plugged into a 208V receptacle. In both cases, thankfully, no persons were harmed (although there was a very real possibility of this happening), and in both cases there was extensive equipment damage.
If you have any doubts about the connection you are about to make, many NEMA plugs and receptacles have molded into their ends, by convention, a description of the voltage and amperage for which the plug or receptacle is designed. If they do not match, do not make the connection. The possibility of a damaging, or potentially lethal, situation developing is almost guaranteed.