My friend who owns an electric oven says that for broiling the oven door stays ajar. Is leaving the door ajar while broiling common practice applied to ALL electric ovens? (My electric oven has no instructions.)
I'm sure each oven is different. You should check with the manufacturer of your oven. See if they have a website or 800 number if you don't have the manual. Most of them are happy to send you one for free if you just ask.
I read somewhere that the door is left open to allow any steam generated in the cooking process to escape. Otherwise you don't get the full effect of the high/dry heat necessary for broiling.
My oven has a pretty efficient venting system that pumps out a lot of steam when I roast anything. I still leave the door open when I broil even though the instructions don't tell me to. (Open, to me means the first few inches, not full open.)
I would expect different rules to apply to convection ovens like Jill's where air is mechanically circulated in the oven. If the door were left open all the hot air would be pumped into the kitchen resulting in an uncomfortable environment and higher energy bills.
Cchiu has the only safe answer. Whether the door should be left open or not varies by manufacturer. I saw both systems in my search for new kitchen appliances. Like Jill Reichow, for example, I just acquired a Dacor, and they are quite insistent that the oven door be closed while broiling, and warn that you can roast off exterior parts above door if it is left open.