A uni-directional test will be conducted on all pigtail splices with no greater than a. 8 dB after 5 repeated attempts results in the replacement and re-splicing of that pigtail. dB loss in fiber optics is the reduction in light signal strength as it travels through a fiber cable, measured in decibels. The connector end is polished and tested under factory conditions, ensuring low insertion loss and high return loss. 1 dB per 100 feet (30 m) for 850 nm, 0. For singlemode fiber, the loss is about 0. While some loss is expected, excessive or unexpected loss can lead to poor performance, network downtime, and signal failure. Recognizing what constitutes too much loss is essential. At TREND Networks, we are frequently asked how much loss is allowed when conducting testing on fibre optic cabling.