A "Prusik" is a specific friction hitch, and it's very reliable but hard to operate. Other friction hitches have different properties. Complicating matters, every rope and cord combination is different, not to mention the number of wraps. The exact place where a friction hitch will slip or fail is an unknown, but the 0-6kn reference isn't accurate. Obviously, it held 1 or we would have slid off it. Where did you get that info? If i knew my friction hitch would slide if it saw 6kn, i believe that's might be good thing because the only way it could see that load was a big mistake for me: it could only happen from a fall with way too much slack.
As for your comment about a friction hitches that be broken under load being on the LOW end of the strength spectrum, it's too broad a statement. Where did you get that info? What hitches? Based on what test? For example, the JRB Ascender Hitch is remarkably easy to break under normal loads and astoundingly strong under a heavy load.
Has anyone done these kinds of tests with your favorite rope and favorite mechanical device? Why not? Why are we all guessing and speculating when another person reports a slip?
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