The purpose of IEC 60794-1-2 Method F5 A/B test is to determine the ability of a cable to block water migration along a specified length. Compliance shall be checked on samples of cable using one of the two following methods (F5A or F5B). Method F5A tests for water migration between the outer interstices of the optical core and the outer sheath, whereas method F5B tests for water migration over the entire cross-section designed to be water-blocked.

The apparatus consists of a vertical pipe containing water at height of 1 meter. The pipe is connected at its bottom to flexible tube that allows watertight connection to the cable under test. The cable sample is laid flat and a suitable arrangement is made in order to determine if water leaks from its exposed end.

The sample is supported horizontally and a 1 meter height of water is applied for 24 hours. A water-soluble fluorescent dye or other suitable coloring agent may be used to aid in the detection of water seepage. To pass the criteria, no water leak should detected from the exposed cable end.