*** Prussiks for Rescue Rigging ***

From: budt3@earthlink.net
To: sar-l@islandnet.com
Subject: Prusik discussion

I've been trying to keep up with all the traffic here about the prusiks.

Although I don't have exact numbers in front of me now, I know that in
the slow pull and drop tests, the prusiks held up much better than
mechanical devices.

1) in haul systems (either as a point of attachment to the main line or
in the haul system itself) either device may cause a catastrophic
failure during a haul. but think of this... if your litter gets stuck
and you continue to attempt to raise until failure... with a mechanical
device you will either completely cut, shear the sheath or bind the rope
into the device. all with little notice... once that occurs you have a
real problem. but, with my experiences with prusiks, they will continue
to squeeze the rope it is tied around until it starts to slip. only when
it slips a considerable amount will the necessary heat needed to fuse it
to the main line develop. and even then, the failure usually occurred
near the double overhand bend in the prusik, not the hitch itself.

2) in belay systems, forget it. the only experience my team has is that
when a mechanical device is shock loaded (ie: used as the unloaded belay
then suddenly loaded) it sliced right thru the line it was on.  tandem
prusiks on the otherhand held rescue loads (600lbs) with little slippage
and no apparent damage to the ropes (using an LR hitch tied with 9mm low
stretch rope, 8mm prusiks, 3 wrap, 1 hand width apart).

3) everyone has opinions on the different belay devices, especially the
munter hitch and the Tandem Prusik Belay (TPB). there are safe
applications for both in certain situations.  a few years back a few
people from the ROCO camp tried to prove that the munter hitch can catch
a rescue load as a belay device. they did write an artice to prove that
it could, but under careful and detailed questioning at NATRS 94, Mike
Roop was unable to answer questions about the testing procedure that
folks had.  since no one in that room had ever been able to catch a
rescue load with a munter hitch, it was agreed that the way the testing
was set up (ie: angles, numerous areas the rope contacted providing
added friction, etc) affected the results of the test in favor of
proving what the authors already decided, that a munter hitch can catch
a rescue load.

4) another thing to ponder: there has been some testing by the British
Columbia folks on gripping ability.  and it was proven that MOST of the
rescue population would be unable to hold onto a moving 7/16 inch low
stretch kernmantle (w/wo gloves) if the rope was moving/accelerating a
(x) ft/sec. (I'll try and find my copy of the test and post more
specifics.) so think about that when you are planning on catching a load
with a munter hitch, stitch plate, etc.

that's all for now... i think that should give you alot to think about
over the holidays....

"Stay alert, stay alive, don't let a per-diem drive."  (old EMS
humor...)

Happy Holidays from myself and the gang at Central Jersey Tech Rescue

Bud


Subject:         Re: Prusik discussion
  Date:         Thu, 26 Dec 96 11:54:09
  From:         lclem@pmi.pmi-petzl.com
    To:         sar-l@islandnet.com


In response to all of the "RAH RAH" about Prusiks and the "HISSS"ing about
mechanical rope grabs, I would encourage caution in interpreting "TEST
RESULTS" in such black-and-white terms. I would also encourage further
exploration before making definitive statements about "will work" and
"won't work".

As all of you know, every scenario, every rescue, is different. Some teams
prefer to have a "cookbook method" in which the words "never" and "always"
appear often. This is the concept which separates the "cooks" from the
"chefs". A chef spends years studying the effects of every variable and
perfecting the combination of tools and techniques. His "experiments" occur
in a controlled environment (like when he's not cooking for the Prince of
Siam) but those experiments allow him to excel in his command performances.

I contend that "never" and "always" and standardization in general
undermines the potential for being better (case in point: Denny's
Restaurant!). Conversely, studying the variables and possibilities in a
controlled environment allows us to apply the most appropriate and
efficient tools and techniques for the situation at hand.

In testing which I have performed I have found that tandem prusiks can
actually FAIL the main line by "pinching" it until either the sheath or the
entire rope separates. Further, I have test data which shows CERTAIN TYPES
of mechanical ascenders slipping rather than failing.

The moral? Never say never. And Never say always. And, neither "mechanical
devices" nor Prusiks should be lumped into all encompassing categories.
Their performance varies according to type of device, material & diameter
of prusiks and rope, and other conditions.

The reality of the situation is that most of us just don't have many
main-line failures which result in the deployment of our belay systems. If
we were really failing main lines that often, we'd be fixing our main-line
problems rather than tinkering with the belays!! In fact, having worked
with a very busy team in the Colorado Rockies since the early 80's, I can
tell you that we have NEVER had an unintentional main-line failure.

That said, it is clear that we all still seem to feel the need for belays,
and based on hearsay and lectures many of us have come to employ tandem
Prusiks for the job.

Some time ago (like 1990-ish) at a Rescue conference/competition in Oakland,
I was facilitating the testing of "Manual Belays". That is, using belay 
devices with a human as a backup to a mainline. What we were testing was
the ability of rescuers to hold the load with different devices as well as
the forces incurred. As a backup to the belay, we were using "tandem
Prusiks" to prevent the load from hitting the ground should the belayer
fail to hold it. (this based upon testing in which Dill, Larsen, Thorne,
Murray and others had been recently engaged)

During one drop in which the belayer failed to make the catch, the load
dropped onto the Prusiks and the Prusiks SNAPPED with hardly a squiggle on
the load cell! I can tell you, all of us just stood there, stunned, in full
silence for about 10 seconds before the incredulous "WOW"s and "OOOH"s
started. The drop was not only within the belay competence test method used
by Prusik proponents, there was also some force absorbed by the belayer
before the Prusiks ever deployed. What happened?

Realizing that I'd possibly been taking a little too much on faith, I began
to look a little more closely at the tandem Prusik question. Here are some
of my observations:

1) First, I feel that it is critical to note the test conditions under  
which Prusiks are said to "perform better than mechanical devices".

     a)  The BC Belay Competence Drop Test which consists of a 200kg
     (440lb) or 280kg (617lb) test block falling 1 Meter (3.3 ft) on 3
     Meters (9.9 ft) of rope. This translates to a fall factor of .33. Most
     static rope manufacturers recommend a fall factor of no more than .25
     - a figure which NFPA 1983 also cites.

     b) The stopping distance for this 1 Meter (3.3ft) fall must be less
     than 1  Meter. (total distance travelled, about 6.6 ft)

     c) The belay unit extension must be within 40 cm (nearly 16").

     d) The maximum arrest force permitted in the test is 15 kN
     (approximately 3375lbf). This translates to no more than 6kN (approx
     1350 lbf) for the individual rescuer, assuming the forces are
     distributed evenly.  This is one half the force deemed acceptable by
     OSHA and UIAA/CE. (12kN or around 2700lbf)

     e) Deformation or damage to the rope or belay unit is permitted, so
     long as it is not "severe".

     f) The standard test is performed in a "straight line" with no edge.


2) When I look at these factors and compare them with my own systems and
preferred techniques, and what is acceptable to me, I find that my needs
differ from the parameters set forth  in the test. This is why publication
of test methods and performance data is  so much more important than the
simple "pass/fail" information. I do not dispute the validity of the tests
as performed, but their "pass/fail" designation is less important to me
than the data itself.

For instance, looking at the Larsen/Dill tests and using the test
parameters outlined above, it was found that a belayer can arrest the load
with an ATC - except that there is 50-60 cm extension rather than 40.

With Tandem Prusiks, using one specific brand with another brand of rope,
extension will also be in the 50 to 60 cm range. (sorry for not identifying
brands, but that was 6 yrs ago and the info would be obsolete now.. anyway,
the point of this exercise is not to name names but to encourage you to
EXAMINE data and TEST YOUR OWN systems...)

On other Tandem Prusik tests, if the prusiks were not tight enough the load
simply would slip to the ground, the Prusiks never engaging...

Results also vary according to whether the rope comes off a flaking shelf
(creating a ~~| sort of diagram) or rests on the ground, forming a  /^\ at
the belay.

As for Gibbs and Rescucenders... despite the fact that they are not
designed by the manufacturers as "belay devices", each performed well
using certain types of ropes but had catastrophic failures when other
types of ropes were used.


3)  Finally, if you are still reading, I would point your attention to an
article in the UK based "Technical Rescue Magazine" which took the
published Prusik testing of 6 years ago (Larsen, Dill) and examined
developments in the industry with respect to belaying. The point is well
made in the article that "Strictly speaking, all belay devices for a full
rescue load are being operated outside of mnufacturers recommendations."
That said, the article goes on to examine the performance of a number of
MECHANICAL devices, some automatic, some requiring action of a belayer,
and many of which performed admirably!

If you have not seen the article I would encourage you to inquire of Mr
Ade Scott at  pushdtp@argonet.co.uk

Happy Holidays!

Loui Clem


!! The words and opinions expressed above are my own, and do not
necessarily reflect those of my employer, PMI-Petzl Dist.  !!

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