⊕ Rope used for rescue, rope access or work at height should be specifically designed and intended for life safety use.

⊕ Tested and marked in accordance with a relevant standard.

⊕ USA most respected life safety rope standards (NFPA 2500 and ANSI Z359 and ANSI Z459) refer back to Cordage Institute Life Safety Rope Test Methods (CI1801).

  • Unique method of reporting Minimum Breaking Strength.
    • break testing at least five samples of rope
    • calculating three standard deviations below the mean of those results.

⊕ EXAMPLE: 5 samples from five different lots of rope

  • Lab wll pull test the rope to failure.
  • Inevitably slightly different results with each test.
  • what is the RIGHT number to report?

⊕ Actual numbers Scenario

  • (6500) pounds
  • (6750) pounds
  • (6675) pounds
  • (7005) pounds
  • (6925) pounds

⊕ What number do we report?

  • best result?  7005 pounds force.
  • worst-case? 6500 pounds (about 500 pounds less than the best result)
  • What aboutnatural variation in materials testing – are these reliable?

⊕ Cordage Institute test method (and the NFPA and ANSI standards that refer back to it)

  • mathematical approach to ensure statistical significance
  • Other Standdards(Euro-norms or CE) do not use this method

⊕ “3-sigma MBS” method.

⊕ At least five tests from different manufacturing lots, very specific test method

  • calculate the MEAN AVERAGE
  • SUBRACT three standard deviations from the mean
  • 7% confidence

⊕ Practical view, sample set.

  • add up the sum of all our test results (33,855)
  • THEN we divide by the number of tests (5)
  • Average – 6771
  • which is six thousand seven hundred seventy one
  • Calculate standard deviation from mean average (200lbf)
    • symbol called SIGMA.
  • multiply Standard Deviation by THREE…. 3x 200.98 = 602.94
    • refer to this as THREE SIGMA
  • SUBTRACT that 3-Sigma number from the MEAN AVERAGE
    • 6771 – 602.94 = 6168
    • 900 pounds less than the best test
    • 332 pounds lower than even the worst test

⊕ THIS WILL affect your safety factor calculations!

  • 3s mbs is a number that you can rely on
  • 7% confidence

⊕ NOT all standards use this method

  • even within Cordage Institute, only the Life Safety Rope standard uses it
  • many standards don’t even specify how the reported MBS is supposed to be derived
    • only that the strength of tested samples needs to be greater than X

⊕ How do YOU use MBS figures?