The primary coating is a soft acrylate pre-coating. It immediately surrounds the cladding glass and protects the glass fiber against heavy stresses during installation. The thickness of the primary coating depends on the application as indoor or outdoor cable and in which temperature range the FO cable is used. It can be between about 100 µm and 400 µm. This means that glass fibers with the primary coating have a diameter of approx. 245 µm to approx. 500 µm. The primary coating is directly surrounded by the secondary coating.
When assembling FO cables, both coatings, the primary and the secondary coating, are removed so that the optical fiber can be inserted into the ferrule of FO connectors.