The dicussion between single-cable and double-cable installations has been as hot a topic over the past five years as hand-braking vs. gravity-braking systems. So, which is safer? Why do cruise lines require double-cable installation? What are the pros and cons?
First, let me say that I have worked on, tested, and trained staff to operate both single cable and double cable systems. I have further trained on hand-braking and gravity-based systems.
Second, the views expressed herein are mine alone.
Third, each course is going to have to weigh the pros and cons for its course, make their own decision. My recommendation is that prior to making a decision, contact a professional builder, engineer, and analyze your market.
Fourth, it has been my experience in the industry (provided that the course was designed by a professional, engineered, and built to standards) that the greatest risk to clients is inadequate staff training and operations management and not course engineering.
Lastly, I am the managing partner of a single-cable, hand-braking course.
So, is a double-cable zip line system safer than a single-cable system? My answer: "Yes, No, Maybe, Probably Not"
propecia Common sense would tell us that two is better than one. Redundancy trumps simplicity. There is merit in this thought; however, analysis of zip line failures rarely conclude that the failure would have been solved by adding more cables. This is not to say that a second cable may not be necessary. There are certain types of installations where a second cable is necessary because the adequacy of a single cable is questionable or inadequate. In this case, two cables are installed not as redundant systems but as a single system composed of two cables. There are also new system designs that utlilize two cables to control direction of travel, provide a more consistent approach speed, create opportunities for conitnuous belay and so forth. Ultimately, what is important here is that system is engineered properly. But if the only reason to require a second cable is for redundancy, might I remind the user that wire rope is comprised of numerous strands and wires. A typical 1/2" 6x25 IWRC cable has 6 strands of 25 wires plus an independent core which generally consists of 7 additional strands of 7 wires. That's a 199 wires.
propecia The greatest concern about double-cable systems is that failure to engage both cables could result in injury or system failure. Double-cable system can increase the complexity into the transfer process for end-users and for guides. This should be considered and procedures should be implemented to reduce the risk of misclips. In systems where cables fail to meet the standard independently, the system should be proof tested to determine what would happen in the case of a misclip. Greater cable deflection often means that riders will ride lower in the corridor. This can expose riders to foreign and known objects that would not normally be in the flight path. Second, in the case of a misclip, what rescue procedures are required to stay within the working load limit of the system.
In the case of at least one accident where a double cable system failed in 2008, the findings concluded that the secondary cable was not properly installed nor was the secondary capable of independently supporting the anticipated load in a rescue scenario. It is the author's belief that should two cables be installed, the failure of one should not result in system failure. Both cables should independently be able to support the anticipated load inside the working load limit of the system.
Builders and operators should also have in place systems to deal with the increased risk of cable entaglement and improper clipping. Procedures should be clearly defined to handle misclips and to anticipate the consequences of possible misclipping which can in some systems include weld abrasion and damage to lanyards, and improper loading of equipment.
propecia In certain situations, it is benefitial to use two smaller diameter cables rather than one larger diameter (and heavier) cable to reduce the load on anchors. The intentional engineering of two smaller diameter cables might also be elected to allow the system to use lighter, commercially available trolleys rather than heavier trolleys, which until recently, have not been readily available to the mass market.
propeciaThe premise that redundancy equals increased safety is not always accurate. Proper engineering, staff training, and operational standards are the best means of reducing risk. As the result of a worker fall from heights in 2010 in North America, the local government attempted to push through regulations requiring a second cable for flexible life lines. In this case, the cable did not fail. The worker fell because he was not properly attached. Similarly, the cruise lines persisting demand to have a second cable has similar reactionary roots. Reactionary regulation and thought lead to systems that replace the demand for proper engineering with presciptionary principles that are not always true.
So, where does this leave us? In the author's opinion, the answer is simple.
1. Properly engineer and test your system.
2. Develop operating procedures speicific to your system.
3. Train your staff.
The safest system is the one that is best designed to fit your course and program design. Whether you have one, two, or twenty cables, the real question to ask is not whether two cables is better than one, but whether your system has been engineered, tested, and proven. While this requires more intentional thought and cost, it is time as an industry that we step back from prescriptionary practices and move to proper engineering and program design.
Over the past five years, advances in technology, new installation techniques, and creative implementation has led to the development of a myriad of new opportunities. Reactionary regulation and prescriptionary standards pose one of the greatest risks to future development. When in doubt, my recommendation is to solicit the opinion of numerous professional, create a strong network of advocates, and proceed with caution.