In a normal oven - gas or electric - when you set it to the BROIL setting it turns on the top heating element, otherwise it is in BAKE mode and will only light up the bottom element. However, if you are using a counter-top toaster oven, then it might turn on both elements.
What brand/model number oven are you having problems with??? Can't really give you a specific answer without a little more information.
"It ain't what you don't know that gets you in trouble. It's what you know for sure that just ain't so." - Mark Twain