Static and low stretch ropes are designed for radically different uses than dynamic ropes and therefore have significantly different properties.
Static and low stretch ropes are designed for rappelling, ascending, lowering, and raising operations where the load is supported by the rope and there should never be any slack in the system. These types of uses are common in rescue, rope access, tree work, caving, etc.
Dynamic ropes are designed to absorb shock loads when the slack comes out of a system, as with a falling lead climber in recreational climbing or mountaineering.

Because of these differences, the testing procedures for static and dynamic ropes are very different.
One thing that both types of testing have in common is that the test destroys the rope.
Ropes tested are considered a representative sample of the ropes actually sold and put into use.
In both types of testing, multiple samples are tested to gauge the consistency of the results.

“developed test criteria”

The Cordage Institute in the United States has developed test criteria for static and low stretch ropes.
In this test, rope strength is testing by wrapping the ends of the rope around 4-inch (or greater) bollards, and pulling at about 6 inches per minute.
A load cell measures the force applied to break the rope.

Testing to a common industry standard test allows the relative strength of different ropes to be compared.
It does not predict the actual load that would necessarily break a rope in a real application.
That would depend on many factors including; the condition and previous use of the rope,
the knots used to tie the rope, the shape of the surfaces that the rope runs across (tighter bends and sharp edges reduce the breaking strength),
and the rate at which the load is applied.

Even though traditional nylon kernmantle low stretch and static ropes stretch very little,
they do have some ability to absorb energy in a system. They do stretch a bit.

According to Cordage Institute standards (CI 1801), any rope labeled as ‘Static’ will have an elongation of <6% at 10% of it’s MBS.
A rope labeled ‘Low Stretch’ should have an elongation of 6-10% at 10% of it’s MBS.
These and other factors should be taken into account when choosing and rigging a static or low stretch rope.
Rope rigging training should include discussions of limitations and capabilities of different types of rope.

“ropes are tested by counting the number of times”

Dynamic ropes are tested by counting the number of times they can safely stop a falling test weight when dropped from a specified height.
Also, the rope must stop the falling test weight without exceeding a specified load (12kN).
That is, dynamic ropes have to be able to stop a falling climber but not stop the climber so fast that injury results.
This ability of dynamic ropes to “gradually” stop a fall are the main reason that static and low stretch ropes must never be used in applications where slack may develop in a system. Testing dynamic ropes by pulling them on a tester until they break is not practical due to the high amount these ropes stretch before breaking.

Be sure to use the right rope for your application.

It is important to note, however, that not all Cordage Institute standards
are relevant to Life Safety ropes. For example, CI-1201 – CI-1313 standards
are relevant to COMMODITY ropes, and not to LIFE SAFETY ropes.

“Resulting in much greater confidence”

The Cordage Institute standard that applies to Kernmantle Life Safety ropes is CI-1801.
This is important, because the tolerances, test methods, and reporting requirements in CI-1801 are much more stringent than in non life-safety standards.

For example, commodity rope standards typically derive an inferred diameter based on mass per unit length,
and an inferred strength based on diameter… whereas the life safety rope document dictates
very specific test methods for actual diameter (important for equipment compatibility)
as well as actual strength testing of multiple samples. CI-1801 also specifies
that the reported minimum breaking strength (MBS) be at least 3 standard deviations
below the mean of all test results… resulting in much greater confidence in those numbers.

CI-1801 is used as a reference document and as a baseline for other standards related to specific work at height.
While people who sit on such user based standards committees, such as NFPA, ASTM, ANSI, SPRAT, etc,
tend to be experts in those areas, it is recognized that deferring to rope industry experts – such as Cordage Institute –
is the prudent approach when it comes to testing for ropes. Therefore these industry standards usually reference
the CI-1801 document as a baseline for testing ropes for their application, but then place some
additional use-parameters on those ropes.

For example, while CI-1801 covers a wide range of rope diameters and properties,
NFPA 1983 narrows the range of what they accept to only those above a certain minimum breaking strength,
for specific applications. You may also find ANSI Z359 marks on ropes.

CI-1801 also clarifies the difference between what is known as a “static life safety rope”
(less than 6% elongation at 10% MBS) as compared with “low stretch life safety rope” (6-10% elongation at 10% MBS).

“EN 1891 is a European standard”

Another standard to which low stretch life safety ropes are sometimes marked is EN 1891.
This is a European standard. The US government does not officially recognize EN standards
(nor the CE mark) but on some occasions the EN standards and CE mark
may be accepted by private companies within the USA. This standard specifies slightly different
test methods and performance requirements than CI-1801, so the two are not the same.
For example, EN 1891 does not acknowledge ‘static’ rope at all – which can be problematic for rescuers
and others for whom low bounce and consistent positioning of loads is important; still, EN 1891 is an acceptable standard for some life safety applications.

In short, when specifying ropes for life safety applications, always and only accept ropes that meet standards
that are specifically designed and intended for life safety use.  Standards used for commodity ropes simply
do not contain specific enough guidance nor tight enough tolerances on test methods and performance requirements
such as strength, diameter, and elongation.

Presenter: Loui McCurley | Presentation | 
This presentation covers rope-based fall protection for industry.