The National Electrical Manufacturers Association ( NEMA ), who is largely responsible for setting the standards for plug and receptacle configurations, does its best to ensure that the correct plug is inserted into the appropriate receptacle, but by sheer determination it may be possible to thwart their best effort. By sheer force it would be possible to destroy the plastic preventing the bent blades from being inserted into the wrong type of receiver and connect different amperages/voltages together with dangerous results.
The very common marine shore power plug rated for 30A/125V (NEMA L5-30) has a portion of its grounding blade bent inward, and the also common 50A/250V (NEMA SS-2) has both of its ‘hot’ lines blades bent slightly outward with the ground on the sleeve. If you were to force these two very different connectors together by attempting to plug a 30A/125V shore power cord into a 50A/250V receptacle there is one of three possible outcomes, none of them, unfortunately, include the safety ground on the 50A/250V from coming into contact with any path to ground.
Two of the three scenarios require reorientation of the 30A/125V ground blade by bending the locking tab outward to appear similar to the ‘hot’ receiver of the 50A/250V receptacle; the other two blades are straight, so minimal force will only be necessary to get those to insert. Electrically the outcome is that the ground aboard that vessel is ‘hot’, which goes beyond foolishness into the realm of downright dangerous. Every piece of grounded equipment on board would be energized, and the AC potential in the vessels ground system will try to find a path back to ground through the water, which could be lethal for any person in the water.
In addition to energizing the ground conductor the fate of the other two lines in the first two scenarios results in a reverse polarity situation in one instance, and the other instance things may appear deceptively normal. The third scenario does not require reorientation of the ground blade, so it may be the path of least resistance for anyone foolhardy enough to attempt to make this ill-advised connection.
In the third scenario, the ground on the vessel’s system is fed the neutral shore line. Neutral on board becomes hot, and the hot on the vessel is connected to the other hot line from the 50A/250V shore side power. Every piece of electrical equipment that is designed for 125VAC would be fed 250VAC. Most equipment is not designed to withstand voltages in excess of their design parameters, and the result would most likely be a catastrophic failure.