a spearfisherman
I don't know in years but you can leave it plugged in for over the winter and it be fine. Just make sure you use a three prong extension cord when plugging it in. The block heater should be a three prong plug as well.
Is it a plastic prong sealed together
Sure i do
The 3rd round plug is the ground connection.
There is not enough information stated as to what you are trying to do.
The equipment grounding conductor is attached to the rounded prong in a three prong plug. Electrical work is dangerous, call a qualified electrician to install. Improper wiring could lead to personal property damage, you or someone else being injured or killed!
My question is WHY did you replace a four prong dryer plug with a three prong! 220 volt Electric dryers require two hot legs, a neutral and a system ground wire. Sounds as though you shunted one of the hots or the neutral. You need to install a four prong plug of the same configuration and wire it exactly as the original.
A three-prong 120 Volt will work just fine.
tri = latin for three. (just a hint)
The third prong is to protect from shock. If there is any way for you to come in contact with a conducting surface on the appliance you have the third prong.
On a three prong plug there will be a neutral blade, a "hot" blade and a ground blade. The neutral blade is wider that the "hot" blade. In North America the ground blade is "U" shaped.
There are 3 prong and 2 prong sensors and depending on your year and model either three or four separate sensors.