Service Manual

GSM-205-423
Connecting fibre optic cables
31st Oct 01
Inst. 2–44
Installation & Configuration: Horizon macro indoor
CONTROLLED INTRODUCTION
68P02902W08-B
Minimum bend radius
All optical fibres have a minimum bend radius. This represents the smallest
circle that is allowed to be formed from a loop of fibre, that is how tight it can be
coiled, looped or bent.
CAUTION
Under no circumstances should fibres be bent tighter than
the minimum bend radius.
If fibres are bent tighter than the minimum bend radius then two possible effects
can happen:
 Excessive light loss can occur from the outside of the bend, resulting in a
loss of performance.
 The fibre can fracture due to microscopic imperfections in the surface.
This type of break is invisible externally. If such a break occurs, excessive
light loss would be permanent and light would be scattered and reflected
from the break in the fibre. Returning the fibre to straight will not help and
the whole assembly will require replacement.
For glass fibres (orange or green outer sheath) the long term minimum bend
radius is approximately 30 mm.
Figure 2-35 illustrates the minimum bend radius for fibre optic cables.
30 mm
FIBRE OPTIC CABLE
Figure 2-35 Minimum bend radius of 30 mm
If the fibre is under tension (being stretched), the bend radius should be much
larger, at least doubled. This is because the tension in the fibre can increase the
possibilities of damage.
If in doubt about the radius, the fibre should be allowed to form its natural bend
radius by allowing it to bend under gravity only. This natural bend radius is
normally greater than 30 mm.