Life on the edge: testing the edge kit

Introduction

We have used an edge kit for several years. Over this time, I have never done any testing on the combination of Purcell Prusik (Prusik on a Prusik), Prusik hitch 6mm on 8mm cord and double overhand (scaffold) hitch onto a carabiner. There has also been an assumption made that the edge kit would provide some shock absorption if you did take a fall.

Over the last year, we undertook some drop and slow pull testing (n=126) in 3 different brands of cord with 3 different variations of a hitch with hand-tied and tensioned components.

  • Drop test factor 1 fall (dropped the length of Purcell Prusik) with a 100kg load 
  • Slow Pull tests at 100mm per minute for the complete unit and individual components: 8mm double overhand hitch on a carabiner, 6mm cord loop.
  • Brands tested of both 6mm and 8mm accessory cord: Korda’s, PMI and BlueWater.
  • Prusik hitch variations tested: 3-on-2, 3-on-3, and 4-on-2.

At the end of this post, there is a bonus detailed 240+ page report you can download with all the testing, graphs as well this summary of the testing.

Edge Kit Components
Edge Kit Components

Context and purpose

For backcountry rescue, each person has an edge kit: a small bag with 10m of cord with a pre-rigged Purcell Prusik. The edge kit allows everyone to be safe and independently mobile at the rescue site when rigging or undertaking the rescue operation.

The edge kit is ideally used either as a travel restraint or work positioning system. In either case, the use is to prevent a fall.

  • Travel restraint systems protect a person in a harness from approaching an unprotected edge, thereby preventing a fall from occurring.
  • Work positioning systems enable a person to work supported in a harness under tension in a way to prevent a fall.

Note: We decided to test the edge kit for the ability to catch a fall. Where a fall occurs by accident, would the edge kit prevent us from hitting the ground? The results that follow in no way imply that the edge kit should be used where a fall can occur.

Edge Kit Example
Edge Kit Example

What do we want to know?

  1. Which loop variation is ideal for the Purcell Prusik 6mm cord for slow pull?
    • Single loop with a bend (overhand rethread and double fisherman’s) or
    • Two loops with triple rethread (overhand or figure-8).
    • Which variation has sufficient breaking strength (ideally 10kN+)?
  2. Does the 8mm double overhand (scaffold) hitch onto a carabiner have sufficient breaking strength (ideally 10kN+)?
  3. Is there any significant difference in the type of Prusik tied for both slow pull for the complete edge kit?
    • Variations 3-on-2, 3-on-3, 4-on-2
    • Max force ideally more than 7kN
    • First slip is ideally more than 5kN
  4. How does the Edge Kit perform after a factor 1 fall (dropped the length of Purcell Prusik) with a 100kg load?
    • Impact force under 5kN ideally
    • Successfully catches the falling load
    • Is there any damage to the 8mm cord or 6mm Purcell (sheath exposed?)

Materials used

BlueWater 6mm and 8mm accessory cord

  • Breaking strength: 6mm = 7.7kN, 8mm = 14kN
  • Materials: Core/Sheath – Nylon/Nylon
  • Standards: EN 564 Accessory Cord
  • Manufacturer: Spelean Australia

Korda’s 6mm and 8mm accessory cord

PMI 6mm and 8mm accessory cord

Testing procedure

  • We used a repeatable test procedure.
  • New cord was used for the testing.
  • All knots, bends and hitches had hand tension. All strands were pulled tight and compact.
  • Mostly three tests were undertaken on each variation unless otherwise stated.

Slow Pull testing

  • A 100kN vertical testbed (60hZ) was used at Aspiring Safety, Christchurch, NZ.
  • Tests were completed between 12mm pins, 12mm carabiners or rope clamps as appropriate.
  • Testing speed was set at 100mm/minute.
Slow Pull testing aspiring safety

Drop testing

  • A vertical height rigged to 5m, 2 point anchor on a large tree, 8 strands of low stretch polyester webbing, Geraldine, NZ.
  • Rock Exotica Load Cell was used for recording max force on fast mode (500 samples per second)
  • All tests were completed between 12mm carabiners.
  • A test mass of 100kgs released using a 3 ring mechanism.
Edge Kit Drop Test Factor 1 100kg
Edge Kit Drop Test Factor 1 100kg Finish

Testing

6mm cord loops slow pull

This testing series concerns how you tie your Purcell Prusik loop. Either as two loops with a triple rethread or a single loop tied with a suitable bend.

Overhand triple rethread

6mm Kordas triple overhand rethread
Cord tested# testsMax force kN
Bluewater 6mm311.19
Kordas 6mm39.97
PMI 6mm38.57 

Figure-8 triple rethread

6mm PMI triple figure-8 rethread
Cord tested# testsMax force kN
Bluewater 6mm310.08
Kordas 6mm310.08
PMI 6mm39.23

 

Overhand rethread bend loop

6mm BW Overhand rethread loop

 

Cord tested# testsMax force kN
Bluewater 6mm310.98
Kordas 6mm311.72
PMI 6mm310.69

Double fisherman’s bend loop

6mm Kordas Double Fishermans Loop
Cord tested# testsMax force kN
Bluewater 6mm311.46
Kordas 6mm312.41
PMI 6mm311.48 

Double overhand (scaffold) hitch on a carabiner slow pull

Double overhand scaffold hitch carabiner
Cord tested# testsMax force kN
Bluewater 8mm310.16
Kordas 8mm311.28
PMI 8mm311.47 

Edge kit complete slow pull

3-on-2: 6mm Purcell and 6mm onto 8mm

Edge Kit Complete 3on2 test
Cord testedFirst slip kNMax force kN# testsComments
Bluewater 6mm5.537.842Broke at hitch one strand
Kordas 6mm2.735.773One test kept on slipping 2.06, one test broke at hitch one strand, one test stripped 8mm sheath
PMI 6mm3.805.853Broke at hitch one strand 

3-on-3: 6mm Purcell and 6mm onto 8mm

Edge Kit 3on3 Purcell Testing
Cord testedFirst slip kNMax force kN# testsComments
Bluewater 6mm6.357.712One test broke at hitch one strand and one test stripped 8mm sheath
Kordas 6mm5.608.173One test broke at hitch one strand and two tests stripped 8mm sheath
PMI 6mm5.507.073Broke at hitch one strand

4-on-2: 6mm Purcell and 6mm onto 8mm

Edge Kit 4on2 testing
Cord testedFirst slip kNMax force kN# testsComments
Bluewater 6mm4.757.162Broke at hitch one strand
Kordas 6mm5.037.893Broke at hitch one strand
PMI 6mm5.706.833One test broke at hitch one strand and two tests stripped 8mm sheath 

Edge kit complete drop test (factor 1, 100kg)

Note: the 6mm Purcell Prusik was attached to the 8mm cord as high as it could, hard against the double overhand (scaffold) hitch. 

3-on-2: 6mm Purcell and 6mm onto 8mm

Edge kit 3on2 factor 1 drop test
Cord testedMax force kN# testsComments
Bluewater 6mm4.902Caught load. Glazing on Purcell. Significant glazing 6on8. All Prusiks fused.
Kordas 6mm3.523Load hit ground. Significant glazing on 8mm sheath. 6on8 Prusik failed. Purcell Prusiks fused.
PMI 6mm4.713Caught load. Glazing on Purcell. Significant glazing 6on8. Sheath failure 1 test 6mm. All Prusiks fused. 

3-on-3: 6mm Purcell and 6mm onto 8mm

Edge Kit 3on3 Drop Test Fall Factor 1
Cord testedMax force kN# testsComments
Bluewater 6mm4.792Caught load. Glazing on Purcell and 8mm cord. All Prusiks fused.
Kordas 6mm4.493Caught load. Glazing on Purcell and 8mm cord. All Prusiks fused.
PMI 6mm4.713Caught load. Glazing on Purcell and 8mm cord. All Prusiks fused.

4-on-2: 6mm Purcell and 6mm onto 8mm

Edge Kit 4on2 Drop Test Factor 1
Cord testedMax force kN# testsComments
Bluewater 6mm4.982Caught load. Glazing on 8mm cord. 6on8 Prusik fused. Purcell Prusik not fused.
Kordas 6mm4.313Caught load. Glazing on Purcell and 8mm cord. All Prusiks fused.
PMI 6mm5.083Caught load. Glazing on Purcell and 8mm cord. All Prusiks fused.

Analysis

6mm cord loops slow pull

  • Consistently across all the brands two loops tied with triple rethreaded bends (either overhand or figure-8) had some or all of their failures below 10kN with some being well below (8.5kN)
  • Consistently across all the brands single loops tied with a bend (either overhand rethread or double fisherman’s) had all of their failures well above 10kN.
  • Recommendation: Tie a single loop with a suitable bend to start the tying of a Purcell Prusik.

Double overhand (scaffold) hitch on a carabiner slow pull

  • Consistently across all the brands a double overhand (scaffold) hitch on a carabiner had all of the failures above 10kN.
  • Recommendation: double overhand (scaffold) hitch on a carabiner is suitable for use on edge kits.

Edge kit complete slow pull

3-on-2: 6mm Purcell and 6mm onto 8mm

  • The testing was inconsistent between brands with sometimes working and other times slipping at really low loads (2.73kN)
  • Recommendation: 3 on 2 not recommended for use on an edge kit without specific testing for a specific brand.

3-on-3: 6mm Purcell and 6mm onto 8mm

  • The testing was consistent between brands with first slip over 5kN and breaking over 7kN
  • Recommendation: 3 on 3 recommended for use on edge kits.

4-on-2: 6mm Purcell and 6mm onto 8mm

  • The testing was inconsistent between brands with first slip some under 5kN and some breaking under 7kN.
  • Recommendation: 4 on 2 not recommended for use on edge kits without specific testing for a specific brand.

Edge kit complete drop test (factor 1, 100kg)

3-on-2: 6mm Purcell and 6mm onto 8mm

  • The testing was inconsistent between brands with sometimes working and other times the load hitting the ground. All tests had major glazing of the sheath of the 8mm and one test the 6mm sheath had disappeared.
  • Recommendation: 3 on 2 not recommended for use on an edge kit without specific testing for that specific brand.

3-on-3: 6mm Purcell and 6mm onto 8mm

  • The testing was consistent between brands with force between 4.5-4.8kN
  • Recommendation: 3 on 3 recommended for use on edge kits.

4-on-2: 6mm Purcell and 6mm onto 8mm

  • The testing was inconsistent between brands with some tests the force over 5kN.
  • Recommendation: 4 on 2 not recommended for use on an edge kit without specific testing for that specific brand.

Conclusions

After 126 individual tests there is a recommended sweet spot for consistent performance for Edge Kit setup across all the brands tested:

  1. Tie the Purcell Prusik (6mm cord) initially as a single loop with a suitable bend (10kN+ slow pull).
  2. Tie the 8mm cord onto the carabiner with a double overhand (scaffold) hitch (10kN+ slow pull).
  3. Tie the Purcell Prusik hitch with a 3-on-3 and attach to the 8mm cord with a 3-on-3 Prusik hitch (7kN+ slow pull and 4.5-4.8kN factor 1 drop test 100kg).
Edge Kit Components

Grant at Over The Edge Rescue.

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