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Purcell vs. Contact sling

Discussion in 'Tech Tips and Gear' started by Sonny Lawrence, Feb 27, 2018.

  1. Sonny Lawrence

    Sonny Lawrence

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    I have been thinking about situations where I use a Spectra sling at an anchor and have people clip into it as a safety while waiting to get on rappel. A Spectra sling by itself is probably not a good idea. It is very strong but very static. They can break when used in the wrong fashion. Ice climbers use energy absorbing devices such as a Yates Screamer. I thought of tying my own small load limiter. For canyoneering this could be a small, compact, cheap device. I would clip it into the anchor first, then add a Spectra sling for all to use.

    Paul Stovall and I did some backyard drop testing. We approached a fall factor 1.7 to 2 with 0.667 kN weight. We tied one of our old search and rescue 8 mm nylon prusiks as a Purcell prusik (3 on 2 configuration) load limiter. These are the typical prusik loops used in technical rope rescue. They are used in tandem to catch large loads, usually at a fall factor of 1/3. We clipped it in line with a Mammut Contact 8 mm Spectra sling. First, we dropped with the Purcell/Contact combo. The second drop was with just the Contact sling. Our hypothesis was the Purcell would be a load limiter protecting the Contact. We thought the Contact might break without the Purcell.

    For the combo: the peak force at the anchor was 7.7 kN. The Purcell slipped until it hit the double overhand bend that forms the prusik loop. Nothing broke. The Contact sling was intact. The prusik suffered a bit of heat. It was fused into one knot!

    For the Contact sling by itself, the peak force seen by the anchor was 9.5 kN. It did not break. However, it suffered tiny damage to the material where it looped over the carabiner. In the attached photo small portions of material are seen sticking out. However, the weight bounced. Once the system came to rest, it was cross loading the gate by the Contact sling

    We decided the small Purcell prusik might not offer more than psychological protection to a person clipped in. The peak forces in the event of a fall with either configuration might be too high. A person could suffer harm or the anchor could be ripped out. Plus, a much longer Purcell prusik would be needed to avoid the prusik hitting the knot. At some point it becomes a cumbersome tool to carry on a harness for the rare times it would be used.

    Cross loading the carabiner gate is a potential disaster. They typically break at 7 kN. The forces in our tests were greater than that.

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    • cross load.
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    • load limiter.
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    Skyloaf, SARguru and Brian in SLC like this.
  2. hank moon

    hank moon lovely ligatures

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    AFAIK testing has shown that Prusiks do not reliably function as load limiters. Sometimes, yes, perhaps more often than not, but other times they grab and do not slip. If a reliable load limiter is desired, avoid the Prusik.

    If you do repeat that testing, consider testing w/wet and wet/dirty rope and Prusiks.

    Possibly of interest (and good references therein):

    http://itrsonline.org/wordpress/wp-...rek_Friction-Hitches-for-Technical-Rescue.pdf
    http://itrsonline.org/PapersFolder/2006/Gibbs2006_ITRSPaper_PurcellPrusiks-LanyardsPartII.pdf
    Last edited: Feb 27, 2018
    Skyloaf likes this.
  3. Sonny Lawrence

    Sonny Lawrence

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    As a belay device for rescue loads it has limitations. Our county is in the process (as is much of the US) of switching from main and belay ropes to a twin tension rope system. However, Purcell prusiks are commonly used as one of two attachment points to an edge attender's safety line. The second is a hard tie in.
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