Splice Closure

Fiber routes are spliced at many points in the field because of cable drum lengths, branches and subscriber drops. A splice closure holds these fusion splices and, where needed, PLC splitters. It is water- and dust-tight (typically IP68). On aerial routes it is mounted on a pole or messenger wire. Underground it sits in manholes and handholes or is buried directly.

There are two basic types. In a dome (vertical) closure all cables enter from one end. In an in-line (horizontal) closure cables enter from both ends. Inside are splice trays, each typically holding 12 or 24 splices secured with heat-shrink splice protectors. Cable entries are sealed with heat-shrink sleeves or mechanical gasket/gel systems, and mechanical sealing makes re-entry in the field much easier. Telcordia GR-771 is a common reference for closure performance requirements.

A good fusion splice typically adds 0.05–0.1 dB of loss. However, fiber coiled tighter than its minimum bend radius inside the closure causes permanent bending losses that become especially pronounced at 1550 nm and longer wavelengths. Tray capacity, slack management and correct use of cable retention hardware therefore need attention.

The location of every closure, its splice diagram (which tube and fiber colour is joined to which), splitter ratios and OTDR results should be documented. Without these records, repairs take long even once a fault has been located. Fiber planning software can generate splice diagrams and the bill of quantities (BOQ) at design stage.