Because of the load force, we generate (50 pounds) and in turn the pulling force a MAS creates there is something called the "Rule Of 18" that we must adhere to. Basically, it's a guide limiting the amount of rescuers used to pull on a haul system. Here's how it works. Let's take a 4:1 MAS. We'll take the amount of rescuers used and multiply that number by the first number in the MAS 4:1. If we have four rescuers we would multiply 4X4 which equals 16. Sixteen is under 18 so we are in good shape. If one more rescuer were added to the team, we would exceed our rule of 18 (5X4=20).
Here is an example of why we conform to the "Rule Of 18." Your rescue team is performing the rescue of a victim located in the bottom of a storage tank. You've rigged all the necessary rigging, secured a bombproof overhead anchor and have assembled nine of your strongest rescuers to perform the raising operation. The MAS you chose to use was a prepackaged 4:1 and the victim weighs approximately 250 pounds. The 4:1 MAS will now take that load and through the magic of physics make it 62.5 pounds. To make this load move we will have to generate a minimum pulling or input force of 62.5 pounds Now remember when I explained that a human on average can generate a pulling force of 50 pounds -- well this 62.5 pound load should easily be moved by two rescuers being their total input or pulling force is at least 100 pounds.
Here is where you will understand why we have the "Rule Of 18." We have nine rescuers ready to haul this load and nine rescuers multiplied by 50 pounds of input force each will generate a input or pulling force of 450 pounds. If any of the victim's limbs or packaging equipment should become caught on an obstruction during the hauling process, there is a good chance the team above won't feel that resistance and the end result would be minor or severe injuries to the victim and/or gear damage with possible failure.
Taking this example into account, during your size-up you will need to calculate the MAS needed based on the victim or "load" amount and the amount of personnel on scene. Standards dictate that when hauling a victim or "human load," because of the force generated by a 5:1 MAS is the maximum MAS that can be used. During this size-up you also need to take into account the amount of rope you will need to use to build your system. Let's look at an example. A 4:1 MAS that is 30 feet above a victim will need at least 120 feet of rope to create the system plus additional rope to reach the area you will be hauling from. Like every rescue operation proper planning and a solid skill base is are a necessity.
Progress Capture Device
Now, let's look at another component of a MAS. It's called a progress capture device (PCD). The job of a progress capture device is to hold the load when you need to reset the haul system as well as catch the load in the event the rescuers lose contact with the rope. This in no way replaces the independent belay system you will need to construct. Let's take a second and talk about the job of the PCD. If you built a 4:1 MAS that measured 20 feet in length from pulley to pulley, the stroke of that system is probably about 18.5 feet because you must subtract for the knot and size of the pulleys being used.
Once the system reaches its maximum stroke, it's time for the PCD to do its job. In the example (see Figure 5), twin triple wrapped prusiks are used. These prusiks will be locked allowing us to reset the system or "stretch" it out to its original size or stroke. Something I must cover is the positioning of the tandem prusiks. If the prusiks are positioned improperly, they will not do their job. Let's briefly talk about how a prusik hitch works.
If you look again at Figure 5, you will see a prusik hitch applied to a main line. The friction caused by the rapid sudden movement of the main line will cause the prusiks to move forward and lock. The triple wrap of the prusik hitch causes a torsional pressure on the rope thus causing the main line to stop forward movement. Proper placement in the system is vital. The prusik hitch should be on the load side of the system. What that means is the rope that is connected to the load. If you're unsure of the placement, give the system a short haul and look at the movement of the ropes. On most systems the last rope moving towards the load when the system is released is going to be your target rope. Systems such as a 4:1 for example will have a slightly different placement of the prusiks simply because of system reset needs.