Translation:

“Failure to visually inspect fiber optic connectors is the most common error when reconnecting existing networks as well as when establishing new ones. Contaminants on even new pigtails and patch cables are often the source of errors—errors that later lead to troubleshooting and repair of network.

No components can be guaranteed to be 100% clean upon delivery. However, high-quality components should be easy to clean before connection if they are dirty. If components are connected without being clean, there is a risk that contaminants will deposit in the fiber connector or cause scratches or burrs. These are much harder to remove once a component has been connected.

It is therefore important to always check whether fiber components are clean before connecting them.

There is usually no reason to clean equipment without being able to see if it is dirty.

Inspect, and if necessary—also clean active components such as SFPs. Contaminants can burn onto the end faces.”