Object
This test method determines the ability of an optical fiber cable to withstand mechanical twisting. The primary objective is to measure any variation in optical power transmittance when the cable is subjected to external torsional forces. A secondary objective is to assess the potential for physical damage to the cable structure resulting from these stresses.

Sample
The test specimen shall be a section of optical fiber cable of sufficient length to allow proper clamping and twisting, as well as optical transmittance measurements in accordance with the detail specification.

Apparatus
The twisting apparatus shall consist of two cable gripping devices or clamps—one fixed and one rotatable—with adjustable spacing. The rotating clamp shall be connected to suitable torqueing equipment capable of controlled twisting.
The gripping devices shall:

• Hold the cable firmly in a straight line without inducing local deformation or crushing.
• Prevent slippage during twisting.
• Avoid any significant or measurable increase in attenuation during clamping.

If required, a tensile load may be applied to maintain cable alignment and compensate for shortening during twisting. One end of the fixture shall allow longitudinal movement to accommodate this effect.

The apparatus shall also include optical transmittance measurement equipment in accordance with IEC 60793-1-46, Method A (Transmitted Power) to monitor changes in optical performance during testing.

Requirements
Acceptance criteria shall be as specified in the applicable detail specification. Typical failure modes include:

• Loss of optical continuity
• Excessive increase in fiber attenuation
• Physical damage to the cable sheath or internal components