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weird ways to get wasted - 1 22-Jul-2003 At 12:28:53 PM dodgy

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Yet more...
Here's a better explanation of the belay device's role in a "dynamic belay".
First assume that whatever device the belayer is usingis "locked off"; obviously with a Gri Gri this is "automatic". Also assume that the belay is tied down.
What will happen at the device is that there will be no slip until the device is overloaded.
Essentially the rope WILL slip at a certain load, in the case of an ATC it's around
2-4 kN, a Gri Gri about 9 kN. Obviously the numbers will depend on the diameter and age of the rope, the age and type of device and the belayer's "technique". With any device the smaller the diameter of the rope the lower the overload point. With a Gri Gri the device can only "clamp" a rope down to about 9.7 mm. (All this is probably good in the sense that smaller diameter ropes will receive lower loads). When the rope slips it acts as an energy absorber (in the same way as the rope stretching), so it effectively limits the maximum load in the system. It is almost impossible using dynamic rope to generate that sort of energy, so essentially a Gri Gri can be regarded as a static device. When other devices slip it is NOT a loss of control, the rope sliding through the device will use up enegy, and when the remaining fall enegy drops below the slip point the fall will stop (most times you probably won't even notice it).
This all makes sense when you look at the loads your pro can take, BD stoppers for example. #1 & 2 = 2kN. #3 = 5 kN. #4 & 5 = 6kN. So in theory, any of these, in an ideal placement, combined with a tied down Gri Gri belay, WILL FAIL. Yes I know there are lots more variables but I don't think I want (personally) to test them with my life (thanks anyway).
It's probably worth noting that this relates to the impact force of you rope too. A high impact force rope combined with a low slip device like a Gri Gri or stitcht plate on trad leads is a real bad idea.