For questions regarding 3σ MBS or product performance, call us at 80. As of 2011, CMC Rescue products compliant with NFPA 1983 are marked with the actual 3σ MBS rounded down to the nearest kN. The standard requires the manufacturer to label the product with either the specified MBS or any higher number that is not more than the actual 3σ MBS calculated from the most recent test results. So, how strong is your carabiner? Just look for the marking right on the product. For example, a General Use carabiner must have a 3σ MBS of at least 40 kN (8,992 lbf). To establish an industry accepted factor of safety, NFPA 1983 specifies a minimum performance standard for rescue equipment. For testing rescue equipment the NFPA has selected a sample size of five.

The larger the sample size, the higher the level of confidence that any individual from the population will meet or exceed the calculated MBS. This provides an MBS that is very near the true lower limit of the population. The MBS for the entire population is then calculated by subtracting three times the standard deviation (3σ) from the mean result of the tested samples. To determine a product’s minimum breaking strength (MBS), a sample size is chosen and tested and the results analyzed. Sigma is the Greek letter used to denote standard deviation, a measure of how far a set of numbers (in this case, breaking strengths) is spread out around the mean. This would be an expensive project, and you’d end up with no useable units! The scientific solution is to use a statistical formula, referred to as three sigma (3σ). But to determine how wide that range is and, more importantly, what the lowest strength is, you would have to break every unit in the population. So a randomly selected sample from a population will perform within some range of break strengths. Yet, even with modern, high-tech alloys, two identical carabiners will break at slightly different forces. You might think that answering the question, “How strong is that carabiner (or other item of rescue equipment)?” would be relatively easy.

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