Why do I keep burning-out heating elements?

Yesterday I discovered that, for the second time in four months, I burnt out my heating element.

I usually brew on the stove in the kitchen, and since my stove-top burners aren't very powerful, I usually supplement them with a immersed electric-heating element (1500-2000W) that I place in the kettle while the burners are running below. This has worked very well for me, and I've managed to reach the kind of vigorous boil I could never achieve with burners alone, but I'm beginning to wonder if this is somehow extra-stressful on the heating elements, or did I just luck on two bad ones? For the record, the element is completely immersed, and both times the element did not fail during the brew, but rather didn't work the next time.

I wonder if boil bubbles from the bottom of the pot are causing an almost dry fire situation. You may be better off lowering the watt density by going with a larger 5500 watt element and running it 120v.

Hmmmm. Seems doubtful, as the element is inserted diagonally in the pot, so the density of bubbles would change along the length of the element. Two things this brings to mind, though:
1. The tip of the element does touch the metal of the pot.
2. The element is a 220V element (as is my entire house current)