Cables and installation — Fibre Optics Practice

Sample questions

1. 01Working outwards from the glass, which order of layers matches a tight-buffered indoor fibre?

   • ACladding, sheath, 250 µm coating, 900 µm buffer
   • BCladding, 900 µm buffer, 250 µm coating, sheath
   • CCladding, 250 µm coating, 900 µm buffer, sheath
   • DCladding, 250 µm coating, loose tube, 900 µm buffer

   Why: The 250 µm primary coating goes straight onto the 125 µm cladding, and a tight-buffered fibre then has a 900 µm secondary coating applied directly over it, with strength members and the sheath outside. Putting the 900 µm buffer first is the tempting slip, but the buffer is applied over the primary coating, never under it. A loose tube is the other way of holding fibres, not an extra layer in a tight-buffered cable.

2. 02A cable has several simplex sub-cables under one sheath, so each fibre leaves the cable already protected. What is this construction called?

   • ADistribution, with buffered fibres under one sheath
   • BBreakout, with each fibre in its own sub-cable
   • CLoose tube, with the fibres lying free in a tube
   • DZipcord, with two simplex units joined side by side

   Why: Breakout cable puts several simplex sub-cables, each with its own strength members and sheath, under an outer sheath, so each fibre comes out already protected. Distribution cable is the tempting answer because it also has several buffered fibres under one sheath, but they are not individually sheathed and are terminated inside a panel or enclosure. Zipcord is only two simplex cables joined, and loose tube is not a tight-buffered cable at all.

3. 03You are setting up a pull of distribution cable through a long conduit run. Where should the pulling load be attached?

   • ATo the fibres, so that the glass leads the cable through
   • BTo the 900 µm buffers, bunched together and then taped
   • CTo the strength members, as the maker’s instructions show
   • DTo the outer sheath, taped firmly along a good length of it

   Why: The strength members are what you pull on, gripped the way the manufacturer’s instructions show, and kept within the rated pulling tension. Taping the sheath to a draw wire is the tempting habit from small power cables, but the sheath stretches and passes the load to the fibres, which can then test with high loss. The fibres and their buffers must never carry the pull.