Breaking Strengths and Safe Working Loads
Strength ratings on rigging or safety equipment are typically given as either a Minimum Breaking Strength (MBS) or Safe Working Load (SWL). Occasionally you will also see WLL (Working Load Limit), but that's basically the same thing as a SWL. The ratings mean different things and can be confusing, so here's the inside info.
Minimum Breaking Strength: This one is pretty self-explanatory; it's the minimum amount of weight you would have to have on the product to make it fail completely. These ratings are assigned conservatively; we recently pull tested a handful of our lanyards that have a 7,000 lbs break strength rating and none of them broke with less than 8,500 lbs.
Safe Working Load: This refers to the max amount of weight you could put on a piece of equipment day-in and day-out without damaging it. Depending on the type of material and what it's used for, this figure is usually 1/3 or 1/5 of the breaking strength. So if you see a 3,500 lb SWL lanyard and you're concerned that it doesn't meet the 5,000 lb MBS requirement for commercial applications, don't sweat it! That lanyard is sure to have a breaking strength of at least 10,500 lbs.
One last note: Many ratings on safety gear are expressed as kilonewtons instead of pounds. A kilonewton (kN) is roughly the equivalent of 225 lbs, so when you see a 22kN rating on a zip line carabiner, just imagine 22 burly dudes hanging from the carabiner before it snaps. That's about 4,950 lbs.