What is 3 Sigma MBS?

You might think that answering the question “how strong is my carabiner?” would be relatively easy. But even with modern, high-tech alloys, two identical carabiners will break at slightly different forces. So, breaking the five samples called for by the NFPA test method will provide five different results. At CMC Rescue we use modern statistical methods to ensure with extremely high confidence that even the weakest carabiner from the population will meet or exceed the performance criteria specified in NFPA 1983. NFPA 1983 specifies a minimum breaking strength reporting method for rescue equipment which is referred to as 3 Sigma (3σ), which ensures that at least 99.87% of the carabiners in the population exceed the specified strength.

 

NFPA 1983 specifies a minimum performance standard for rescue equipment. For example, a General Use carabiner must have a 3σ minimum breaking strength (MBS) of at least 40 kN (8,992 lbf). The standard allows the manufacturer to label the product with the MBS required by the standard or any higher number that is not more than the actual 3σ MBS calculated from the test results of the five samples. Beginning in 2011, all CMC products compliant with NFPA 1983 will be marked with the actual 3σ MBS rounded down to the nearest kN. As always, CMC regularly submits products to a third-party test lab (Underwriters Laboratories, UL) to ensure that current production samples of compliant products continue to meet the performance requirements of the standard.

 

Carabiner Performance Requirements

Technical Use “T” (Formerly Light Use “L”)

  • Major Axis 3σ MBS of not less than 22 kN (4,946 lbf)
  • Major Axis with gate open 3σ MBS of not less than 7 kN (1,574 lbf)

 

General Use “G”

  • Major Axis 3σ MBS of not less than 40 kN (8,992 lbf)
  • Major Axis with gate open 3σ MBS of not less than 11 kN (2,473 lbf)
  • Minor Axis 3σ MBS of not less than 11 kN (2,473 lbf)

 

Pulley Performance Requirements

Technical Use “T” (Formerly Light Use “L”)

  • 3σ MBS of not less than 18 kN (4,046 lbf)
  • 3σ MBS of a becket of not less than 11 kN (2,473 lbf)

 

General Use “G”

  • 3σ MBS of not less than 36 kN (8,093 lbf)
  • 3σ MBS of a becket of not less than 22 kN (4,946 lbf)

 

Descent Control & Rope Grab Performance Requirements

Technical Use “T” (Formerly Light Use “L”)

  • Rope grab devices must not show any permanent damage to the device or to the rope at 5 kN (1,124 lbf)
  • Descent control devices must not show any permanent damage to the device or to the rope at 5 kN (1,124 lbf)
  • If the descent control device has a passive breaking feature, the device shall not slip more than 25 mm at a force of 1.35 kN (300 lbf)

 

General Use “G”

  • Rope grab devices must not show any permanent damage to the device or to the rope at 11 kN (2,500 lbf)
  • Descent control devices must not show any permanent damage to the device or to the rope at 11 kN (2,500 lbf)
  • If the descent control device has a passive breaking feature, the device shall not slip more than 25 mm at a force of 2.7 kN (600 lbf)

Write a Comment

You must be logged in to post a Review Log In / Create Account