Thinking Critically About Life-Safety Carabiners

Choosing a carabiner for your climbing system, lanyard or redirect sling – all life-safety applications – seems like a simple process. Which brand, what style and, of course, which color do you like best? However, I’ve seen numerous instances of climbers using carabiners that do not meet the minimum safety standards for our profession. And occasionally, those choices have contributed to an incident that resulted in severe injury or death.

We can help prevent a failure of imagination by making the best decision and choosing carabiners that meet or exceed safety standards. Photo by Caleb Best, from

Why does this happen when the standards are relatively simple? One possible reason is that we (humans) have difficulty considering negative outcomes that are beyond our personal experience. This has been called a “failure of imagination,” and it was a factor in a famous disaster of the early NASA space program.

On January 27, 1967, a fire in the cockpit during a routine systems test killed all three astronauts aboard the Apollo 1
spacecraft. The cockpit was pressurized with 100% oxygen, and a frayed wire ignited a fire that quickly grew out of control and increased pressure inside the spacecraft. The astronauts on board were unable to open the escape hatch because it opened inward, and the increased pressure made that impossible. Nobody had considered this outcome, and that failure of imagination contributed to the death of the crew: Gus Grissom, Ed White and Roger B. Chaffee.

DMM’s Ultra O Durolock. Earlier this year, the Ultra O connector was DMM’s fourth locking carabiner to be paired with this gate option. Image courtesy of DMM.

Failures of imagination also can occur in our profession – see the TCI Magazine “Accident Briefs” in each issue. Perhaps you’ve experienced a precursor when speaking to a co-worker or employee about a safety hazard and they respond, “What do you mean? I’ve worked that way forever and nothing has gone wrong.” Of course, the important word missing from that sentence is… “yet.”

Following the Apollo 1 disaster, Gene Kranz, a famous NASA flight director, addressed the engineering team. He was angry that problems had been casually dismissed as “that can’t happen.” According to historians, he told the engineers:

It [is] not good enough to ask what you would accept. Instead … ask what action you would take to prevent the failure from ever happening. [Your answer] … should always satisfy one final question: What is the very best thing to do in this situation?

This principle applies directly to our decisions about life-safety carabiners. We can help prevent a failure of imagination by making the best decision and choosing carabiners that meet or exceed safety standards.

With that in mind, choosing an appropriate carabiner seems easy enough. Visit your favorite tree-climbing-gear retailer and make a selection. It would be logical to assume that every item in the “Climbing Carabiners” section is designed and rated for life-safety applications. Sadly, that’s not the case. The list of available products will include almost every carabiner-like object imaginable: double-locking aluminum, steel with screw gates, corner traps and pulley sheaves, plastic devices for arranging gear and carabiner-looking items with no gate at all. Each has a specific purpose, but many are not designed or rated for life support. So, how is one to know?

DMM’s Durolock gate operation is designed to minimize the chance of accidental gate opening. Image courtesy of DMM.

In the field of arboriculture, we have specific standards for the functionality and strength of life-safety carabiners. For tree workers and companies in the United States, these standards are documented in American National Standard ANSI Z133-2017: Safety Requirements for Arboricultural Operations, Section 8.2.8. While this short paragraph may not be riveting literature, it contains essential information to help us differentiate between life-safety carabiners and unsuitable alternatives. So, let’s break it down together. The first sentence reads:

“Carabiners used as part of a climber’s
work-positioning (suspension) system shall be self-closing and self-double locking and shall have a gate-locking mechanism that requires at least two consecutive, deliberate actions to unlock.”

Work positioning

It’s important to recognize that professional tree climbers are required to use fall-protection systems designed for work-positioning applications, sometimes referred to as suspension. These systems utilize semi-static ropes, harnesses and hardware to support the climber when moving through the canopy and to prevent falls greater than two (2) feet. Work positioning is very different from fall-arrest systems, such as those used by aerial-lift operators or recreational rock climbers that are designed to limit the force experienced by the user in longer free falls. Work-positioning equipment provides a more efficient and precise solution for moving into and through the tree canopy. The difference in these systems is the reason that maintaining tension in our climbing systems is so important. Excess slack can create a free-fall scenario that exceeds the rating and design parameters of our work-positioning systems.

Double locking

The locking function is a key characteristic of carabiners designed for arboriculture. Our life-safety carabiners must be self-closing and self-double locking. They must close and lock on their own – typically by a combination of springs – without any effort from the user. The purpose of these requirements is automatic function and redundancy to help prevent unintended opening of the gate. As you’d expect, an open gate dramatically increases the likelihood of becoming disconnected from your system. Plus, when the gate is open or not fully closed, the carabiner’s strength is reduced by roughly two-thirds, increasing the likelihood of failure under heavy loading.

A self-double-locking carabiner requires “at least two consecutive, deliberate actions to unlock …” before the carabiner can be opened. This commonly entails sliding the gate barrel up/down and then rotating laterally. Manufacturers of quality gear offer various locking mechanisms that meet this standard, some using the common push-twist design, while others incorporate button or pin mechanisms.

“ISC’s (International Safety Components’) Quadlock barrel is available on a range of aluminum and steel carabiner shapes, but I think the most popular version for the arb industry is the KH204 HMS (shown here),” says Carly Jones, ISC’s marketing & business development manager. “All aluminum Quadlocks have purple barrels. This image shows the hinge-pin area and how the Quadlock barrel is shaped in order to give the ‘turn, lift, turn, push open’ sequence.” Photo courtesy of ISC.

Note: Carabiner designs that use magnetic and/or “pinching” lock mechanisms, such as the Black Diamond Magnetron and Skylotec Pinchlock II, are generally considered to NOT meet ANSI Z133-2017 requirements.

Petzl’s Bm’D carabiner has the TriAct-Lock automatic-locking system. Photo courtesy of Petzl.

For additional security, several manufacturers now offer designs that require additional actions – beyond the minimum of two – to unlock the carabiner, such as the DMM Durolock and ISC Quadlock. You also will see carabiners with a thicker gate, often called an ANSI gate in reference to the ANSI Z359 fall-protection standard, such as the Petzl Bm’D. These have a stronger gate that resists inward crushing pressure, and are intended for specific applications, such as life-safety connections in an aerial lift. Both concepts meet and exceed the Z133 safety standard.

The standards for life-safety carabiners in arboriculture closely align with those of industrial rope access. However, they are very different from other professions and recreational activities. For example, the National Fire Protection Association (NFPA) only requires that a life-safety carabiner include a manual locking mechanism, either sliding or screw-type. This design closes on its own but the gate does not lock automatically. Standards for carabiners for recreational activities, like rock climbing and mountaineering, are provided by the International Climbing and Mountaineering Federation (UIAA, for its French name, Union Internationale des Associations d’Alpinisme). In these recreational settings, life-safety carabiners may be either locking or non-locking, depending on the application. Explanation of the reasoning behind these standards is beyond the scope of this article.

The key differentiator of life-safety carabiners for arboriculture, compared to other professional and recreational applications, is that they are self-double locking. When the gate is closed, it “requires at least two consecutive, deliberate actions to unlock” before the gate can be opened.This requirement protects climbers and helps prevent failures of imagination.

5,000 pounds

Strength is another defining characteristic of carabiners for arboriculture. Section 8.2.8 of Z133-2017 continues with this statement:

“A carabiner shall be capable of withstanding a 5,000-pound (22.24kN) load along its major axis, with the gate closed without breaking or distortion sufficient to release the gate.”

You may be wondering, “Why 5,000 pounds?” The simple answer is that this was the rating of rope snaps adopted from other industries in the early days of modern arboriculture. But let’s dig deeper into why that number makes sense. First, a few definitions. The “major axis” of a carabiner describes pulling end to end. This is the strongest orientation and the “direction-of-load” a carabiner is designed to support. The 5,000-pound figure refers to the minimum breaking strength (MBS) of the carabiner. The second number in parentheses (22.24kN) is the metric equivalent. The MBS is determined by the manufacturer and identifies the amount of force (load) above which an item of equipment is expected to fail when it is brand new. In other words, applying a load greater than the MBS is expected to break that carabiner on the very first day of use. Normal use of the carabiner – or damage due to misuse – will reduce that breaking strength. And remember, all carabiners and life-safety equipment must be properly configured and compatible with other elements of your fall-protection system.

A carabiner (or any equipment) is not intended to be loaded at or near its MBS. This will lead to failure after only a few cycles. Therefore, we must understand the working load limit (WLL). This is the intended load the item will confidently support for many thousands of cycles. Curiously, the WLL is not marked on most carabiners. The WLL is generally based on a safety factor (also called design factor). For life-safety equipment, the safety factor is often calculated at 10:1. Using that ratio, a carabiner with an MBS of 5,000 pounds will have a WLL of 500 pounds (2.22kN), or one-tenth of the MBS.

Now, let’s apply those numbers. An adult tree climber weighing 230 pounds with clothing and equipment has the potential to exert a 500-pound load on a life-safety carabiner in a short fall, i.e., less than 3 feet (see the “Work positioning” section). This type of loading may occur multiple times in a day. Therefore, when we recognize that a carabiner will regularly experience a load of 500 pounds, this working load limit makes sense. Then, the 10:1 safety factor leads us right back to the minimum breaking strength of 5,000 pounds, as required in the standard.

Make the best decision

Climbing trees professionally is a challenging and rewarding career. It also has unique hazards not present in other work-at-height environments. Among others, we are the only climbers who suspend ourselves from another living organism.

Remember that equipment retailers sell carabiners for many applications with vastly different safety requirements. Many of these carabiners do not meet the standards of Z133-2017 Safety Requirements for Arboricultural Applications.

When choosing life-safety carabiners for arboriculture, take responsibility and make the best decision. Don’t let a failure of imagination cause you to accept a lesser alternative.


To learn more about the early NASA spaceflight program and the Apollo 1 disaster, pick up a copy of Apollo 8 by Jeffrey Kluger, 2017, Picador, at your local bookstore.

Thank you to Taylor Hamel, DMM professional – technical representative, for providing input for this article.

Craig Bachmann is a Certified Treecare Safety Professional, Certified Arborist/TRAQ, Certified Tree Worker – Climber Specialist and an experienced safety/skills trainer. He is also an event head judge for the International Tree Climbing Championship. He operates Tree133, a TCIA member tree care company based in Seattle, Washington, and regularly speaks at industry conferences.

This article was loosely based on his presentation, “Gear Inspections,” made during TCIA’s Virtual Summit ’21, which took place in January of this year. Click below to listen to an audio recording of that presentation.

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