In reply to phil456:
If I remember correctly, there had been an accident with friction (heat) induced failure mentioned in Pit Schubert's books (founder and former head of DAV safety commission), alas not translated into English as far as I know. Let me check my library - one climber was rope soloing some 12m above deck with prusik instead of a (then used) ascender on his rope. Not a very safe practice... At the crux he neglected to push the prusik up the rope (since he "had his hands full"), creating some slack in the system, and finally falling, shock loading the prusik severely, breaking it. Schubert mentions the fall as ~4kN load, while the prusik cord was assumed to hold ~6kN. Theoretically it should hold (under the best circumstances), but with subsequent testing, DAV found out that even in smaller falls, the prusik knot on the same cord could slip along the rope up to half a metre before locking fully, creating enough friction and heat to stress the cord up to the point of failure. On a drop tower with 80kg dead weight, the cord tested (5mm) broke in the knot (friction heating) with certainty on 2.5m falls (not that much slack at all). The amount of slip before locking, how much friction and heat is generated during the slippage of course depends on how the prusik is tied, friction with the rope, and is hard to quantify. Obviously, nobody in their right mind would rope solo with prusiks only, especially with slack involved, but it happened. So keep your prusiks tight and tidy, and watch the slack