Here's the usual configuration for the sliding X when I do it with one continuous sling (click on image to see it full-size). I consider this safe:
Notice the two non-extending figure eight knots tied at either side of the power point. Also notice the twisted loop of just one of the sling sections at the power point.
If the sling breaks above the non-extending knots on either side, I'm OK. The knot on that side catches me, and doesn't let me fall beyond that knot. This is why there's a twist in one sling at the power point. Without this twist, if the sling breaks on one side above the knot, you will fall.
How about if there's a cut below the knots? Suppose section 1 of the sling is cut, but section 2 of the sling still holds. This is still OK; section 2 keeps me from falling. So this section of the sliding X is redundant. Obviously if both 1 and 2 are cut, we're dead, but all setups have a problem like this.
BTW, typically I'll have this sling attached to a bolt on one side, using a girth hitch, and a biner clipping it to a second bolt on the other side. This creates two legs to the anchor; if either one breaks we're still safe.
Three-sling sliding X
Suppose you have two slings that are not long enough to reach over a ledge, and you want to extend them with a third sling, using a sliding X. You want to do this safely so as to mimic the above situation. I would do this by tying a figure eight on a bight at each end of the middle sling, and attaching those bights to the two other slings using biners:
In the photo, I've got a fake "cut" in the sliding X and it's clear this is safe, I won't fall. Since this is a sliding X, if the cut had occurred above the biners on either side, I'd still be caught by the figure eight on a bight on that side, assuming I had put the little twist in at the power point.
Three-sling sliding X - unsafe way
If I didn't tie two knots at the end of my red sling, the sliding X would be unsafe as seen here (even with the twist):
Oops! This photo makes it clear that this version of the sliding X is not redundant. Cutting a single strand of my third sling will allow me to fall. So always look for the non-extending knots, even when using multiple slings.
I like the sliding X because the location of the power point is easily adjustable - no retying the knot to even out the anchor on each side.
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