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There, I've said it (well, wrote it) 3 times.
Redundancy is key in many industries, and is absolutely key in safety-critical industries like tree work and forestry. There are different facets to redundancy- for example, there is critical equipment. It's better for a tree crew to have multiple saws in the truck. We don't need ten, but it is good to have more than one. For example, we carry a climbing saw (MS 201), and large saw (MS460) and a third, middle saw. The MS 260 ( our middle saw) can be hauled into the tree in a pinch (and is often used as the wood diameter becomes larger) and can also be used effectively on the ground. If the climbing saw quits, we're still able to work. If the big saw quits... we're still able to work.
Now, the most important area where redundancy is important, in tree work, is safety. In particular, our climbing systems, on which one directly relies, require redundancy in order to keep the arborist safe. In the first case, redundancy comes through engineering and materials.
How is that? Well, let's say a climber and gear weights in at 200 pounds. The worst case being climbing above an SRT canopy anchor. Calculating that scenario- a Factor 2 fall of a 200lb climber (with gear) results in 6.9KN (or appx 1550 lbs of force). That's with a completely static rope, on a completely static anchor (hypothetically) and the minimal payout of rope with a maximal fall distance.
Now, climbing ropes have a Maximum Breaking Strength (MBS) or about 23 KN, which give us a 4X redundancy factor in terms of materials and engineering.
Having said that, the engineering qualities of the rope are not the whole story. A climber relies on both the rope and an anchor point for safe ascent and descent. An anchor point in a tree can be estimated for soundness, but it cannot be tested fully. We can ascertain that a 4" diameter tie-in point for the climbing line is likely weaker than tying in 10 feet below that point, where the wood is 10" in diameter. The problem with that, however, is that optimal movement through the canopy is achieved with a high tie-in point. There is also the question of tree species (eg oak vs poplar) and tree health. While a declining tree might be relatively safe at a lower tie-in-point, it can get pretty dicey at the tips.
In these cases, we choose redundancy of climbing systems to increase our safety margins as much in possible. We achieve this by always operating at least two safety lines- both the high climbing line, and a lanyard. The lanyard prevents ever being in a 'free climb' position within the tree, and a lanyard must always be set whenever any cutting, whether handsaw or chainsaw, is to be done.
Another climbing system redundancy is the use of a second tie-in-point on a separate spar within the tree as the main climbing point. This permits two ropes, which in the case of wide spreading trees, can actually assist in moving through the canopy. On occasions where two ropes slow the work too much, especially in the case of more upright trees with less canopy spread ( excurrent ) we then backup the tie in point with a lower tie-in-point anchor that then snakes up the tree and connects to the friction saver. In the case of the main tie in point failing (where the wood is at it's narrowest) the lower tie-in-backup in much thicker wood will more than likely hold a fall. While this isn't a completely guaranteed solution, careful selection of the tie-in-backup can significantly reduce the chance of catastrophic tie-in-point failure.
