Understanding and specifying crush performance for optical-fiber cables

The "standard" test procedure for crush performance leaves several key parameters up to the user. Here`s how to make sure the cable you`re ordering meets your requirements.

Todd Rhyne, Siecor Corp.

Crush performance is one of the primary mechanical characteristics that are routinely tested and specified by optical-fiber cable manufacturers. Crush testing determines the ability of an optical-fiber cable to withstand and/or recover from the effects of a compressive force. Although there is an industry-standard test method, a basic understanding of the test method and requirements is required to make a practical judgment about the crush performance of an optical-fiber cable.

Variable standard

TIA/EIA-455-41A, "Compressive Loading Resistance of Fiber Optic Cables" (FOTP-41), is the industry-standard test procedure that outlines the apparatus and proper method for performing crush testing. The testing apparatus consists of two flat contact plates, one of which is movable. The plates should be 10 cm (4 inches) in length with rounded edges. The contact plates are installed into a loading system that will apply the compressive load. The loading system applies the load at a constant rate, and the specified attenuation measurements are made.

It is important that cable manufacturers use this standard test method so that every manufacturer is performing the same test. This consistency gives customers an easy way to accurately compare cables from different manufacturers.

But FOTP-41 is only a test procedure and does not specify all the necessary information for performing the test and for determining the pass/fail criteria. It shows how to set up and perform the test but leaves the actual test-level determination to the user. Some of the key parameters that are not specified in FOTP-41 are the compressive load, the rate at which the load is applied, the amount of time the cable is under load, the number of fibers to monitor, and the attenuation measurements, if any, to be performed.

With these key parameters undefined, cable manufacturers typically specify crush performance in one of three ways. The manufacturer can subject the cable to:

an industry-standard compression force and measure the attenuation while the cable is under load and after the load is removed.

any compressive load and measure only the attenuation after the load is removed.

any compressive load and measure only the continuity of the fiber.

Siecor uses the first method to specify crush performance because we believe it is the most practical method of the three. By measuring the attenuation while the cable is under compressive load, the user will know how much attenuation the cable will exhibit if a compressive force is applied while the cable is in service. This information is critical in assuring that there will not be a service failure due to compressive load on the cable. The customer knows that if the cable is subjected to a compressive force, the loss budget will not be exceeded and the system will be undisturbed.

If the attenuation is measured only after the load is removed, the customer will know only how much attenuation will be left in the cable after being subjected to a compressive force. While this information is useful, it does not provide an indication of possible service failures due to a compressive load. There is no indication of how much attenuation is induced in the cable while it is under load.

If continuity is the only performance criteria, then a cable could exhibit 10-dB or greater attenuation after the crush testing and still be considered as passing. However, attenuation increases of 10 dB or more are usually much greater than the allowable loss budget of most systems. Therefore, a cable that passes this criterion might be unusable in a practical system.

Taking all these factors into consideration, it is clear that the method by which manufacturers apply FOTP-41 can have a significant impact on the reliability their cables exhibit in the field.

Apples and apples

Thus, when comparing specifications from various manufacturers, it is important to understand all of the pertinent criteria. If one manufacturer uses option 1 and another uses option 3, then the customer cannot make an "apples to apples" comparison. Furthermore, a very high load specification with option 3, for example, does not necessarily indicate a more robust cable than would a lower-load level with option 1.

So what is the best way for users to know and specify what crush performance they need from an optical-fiber cable? Before answering that question, let`s look at how FOTP-41 was developed. Originally, FOTP-41 presented the test apparatus and procedure for performing the test and referenced a Department of Defense specification (DOD-STD-1678). This specification was derived from a specialized optical-fiber cable that was deployed on the ground to connect a satellite downlink antenna to an equipment hut.

While this application is far from the typical application for optical-fiber cables--such as underground duct, direct burial, cable trays, and lashed aerial--it is the best way to specify crush performance. Users can specify FOTP-41 as the test method but also use a current industry standard to specify the pass/fail criteria. There are several industry standards that specify crush-testing performance levels. The Insulated Cable Engineers Association Inc. (ICEA) has two documents that specify requirements for optical-fiber cables. ICEA S-83-596, "Standard for Fiber Optic Premises Distribution Cable," and ICEA S-87-640, "Standard for Fiber Optic Outside Plant Communications Cable," can be used to specify crush performance for optical-fiber cables.

Both ICEA standards specify the compressive load, the rate at which the load is applied, the amount of time the cable is under load, the number of fibers to monitor, and the attenuation measurements to be made. These standards specify attenuation requirements while the cable is under compressive load and after the load is removed.

The table shows the attenuation requirements and compressive loads stated by ICEA for premises and outside-plant cables. These requirements provide the most practical way of specifying crush performance.

To see how the compressive loads specified by industry standards represent typical loads that the cable would be subjected to in field conditions, Siecor performed an experiment (see Lightwave, October 1997, page 46). The experiment was designed to measure the compressive load exhibited on a cable in direct-buried applications. Load cells were buried at the bottom of a trench 48 inches deep, 24 inches wide, and 11 feet long.

To simulate different burial scenarios, four different media were used to backfill the trench to the original ground level. The backfill options included excavated soil, sand, small gravel, and large rocks. In addition to being backfilled, concrete slabs were placed under the load cells to simulate a solid bottom. Also, one measure was made with a vehicle parked on the trench.

The observed measurements were all significantly below the loads specified in the ICEA document as well as other industry standards. The maximum compressive load for all scenarios was measured to be 33 N/cm. The average compressive load for all scenarios was 26 N/cm. Even allowing for variability in the experiment, the compressive forces measured in varying direct-buried applications were well below the values specified in industry standards.

Specifying the crush performance of an optical-fiber cable requires more than just specifying the load. Although cable manufacturers may use the same industry-standard test method (FOTP-41), the load applied to the cable does not fully characterize the cable`s resistance to crush. Pass/fail criteria must also be specified to ensure that the cable will perform in practical applications.

You make the call

The best way to specify the pass/fail criteria is to use current industry standards such as ICEA S-83-596 and ICEA S-87-640. These standards specify maximum attenuation changes for the cable while it is under the compressive load and after the load is removed, thus providing the best overall characterization of a cable`s crush performance. u

Todd Rhyne is an applications engineer at Siecor Corp. (Hickory, NC).