For many years, splicing and terminating have been the most skill intense tasks in a fibre optic
cable installation. But today, because of high performance cleavers, connectors and fully
automated fusion splicers, the task has now become as easy as splicing copper twisted pairs.
Terminating fiber involves the use of suitable patch panels with proper fibre management
incorporated into the tray system. Because of fibres high information carrying capacity you need
less terminations and splices for major trunk or backbone networks compared to copper twisted
pairs. Nevertheless, connectors and splices for fibre are much more expensive than for copper.
Patch Panels
Patch panels provide a termination point to rearrange fibre connections and circuits at will. A patch panel is installed at an Optical Distribution Frame (ODF) or an Internal Distribution Frame (IDF). The ODF is usually used at the point where the underground or overhead fibre cable enters the building. In the case of Toronto Hydro, the ODF is situated in the substation, main transformer room, or the main electrical room at a customers location. The IDF is also at the same point but it utilizes the internal fibre cable to the telecom room. Patch cords can then be crossconnected between both the ODF and IDF patch panels to establish complete circuits.
A patch panels design should be carefully considered when setting up a new fibre optic local or backbone network. The panel should provide a centralized termination, interconnection and cross-connection facility for optical cables in a 19-inch standard format rack (see figure 1). The frame should be designed around a fibre management system which combines high fibre density with ease of use, fibre safety, and simplified maintenance techniques.
In a utility environment, for example, substations and customer main electrical rooms, it is also
important that the material be made of a non-flammable substance. The tray system within the
patch panel should be of Low Smoke Zero Halogen (LSOH) design.
System Capacity
High termination capacity can be achieved by a tray system which should facilitate no more than eight fibres per tray in different sizes of sub-racks. These sub-racks are built into the patch panel and support each individual tray. Recent studies on fibre faults conducted by British Telecom have shown that high fibre counts on tray systems in patch panels have contributed to more than 45 per cent of the faults on networks.
Their conclusion was to use less fibre per tray. Toronto Hydro and North York Hydro are using an eight fibre-per-buffer-tube system which means if we talk about a 96 fibre cable then this would be an eight fibre/12 tubes cable makeup.
The advantage is that at any time while working on the patch panel we are only dealing with 8 fibres at any one time.
Splice and fibre management are carried out on the fibre management unit of the splice tray of the patch panel. The tray system should carry the splice protectors. These protectors are used to protect the splice between the pigtail and the incoming strand. It is extremely important that the tray system has two areas built into it, one for the pigtails and the other for the incoming strands which will be spliced to the pigtails. In a good patch panel you will find a mandrel system built into the tray system. The minimum fibre bend radius is constrained by these mandrels to 35mm thereby ensuring operation at 1550 nanometers (nm) for singlemode fibre. These mandrels are far superior to open fibre management systems as they reduce the risk of bending radius problems.
Tray systems on patch panels should have a double port entry system. This design of tray allows it to be used as a joint, with a dedicated fibre return path for the outgoing fibres. Because of the connectors high attenuation factor, we may want to splice straight through the incoming to the outgoing fibre on the tray to lower the attenuation.
Over the years buffer tubes can become brittle and if they are moved often they can bend or kink
thus breaking all the strands contained in the tube. The use of a tube or cable manifold is a great
help here. It not only protects the tubes but it also allows you to remove the tray system to do
maintenance. The manifolded fibres are fed to the tray where they are secured with strain relief
glands. The standard tube size is 5mm O.D. and up to two tubes should be accommodated on
each tray.
ST Connectors.
The ST connector (see figure 2) is probably the most popular style connector made today for
patch panels. It was designed for in-premises wiring of buildings and many other applications. It is
made of a 2.5 mm ceramic ferrule with a quick release bayonet coupling. The quick release
bayonet locking mechanism requires a quarter turn during mating or unmating, while built-in
keying ensures repeatable performance during mating. Because of its features this connector is
ideal for use by utilities.
Safety when Terminating
Concern 1: Multimode fibre systems operate usually at 850 or 1300 nm wavelengths, and singlemode systems operate at 1310 or 1550 nm. These wavelengths are all in the infrared spectrum and outside the visible spectrum. Although optical sources such as light emitting diodes (LEDs) in fibre optic networks have a low power level, overexposure to a working fibre may cause damage to the retina.
Concern 2: Handling bare fibre can be a real concern to the person terminating or splicing the fibre. While stripping the protective coating from the cladding and core, the bare fibre is exposed and can easily be broken. If broken the splinters or scraps can penetrate the skin, and they can be extremely difficult to find and remove. If the fibres enter the eye, they may have to be removed by a doctor.
Whenever fibres are being worked with, precautions should be taken.
Ñ Do not look into the end of a fibre in order to determine if it is in operation or not. Use a fibre optic detector to determine if the fibre is live.
Ñ When terminating fibre, you must wear safety glasses with side shields.
Ñ Avoid touching your eyes at any time while terminating.
Ñ Keep your work area clean.
Ñ When cleaving the bare fibre put the unwanted scraps of fibre in a container.
As stated in my previous article (March edition), Toronto Hydro has made major technological changes to its telecommunications and SCADA systems. Hydro utilizes about 90 per cent of its vast underground duct system for the deployment of fibre and ten per cent for its overhead system. Corning SMF-28 cable is used in various strand denominations which vary from 24, 48 and 96 strand singlemode.
Corning SMF-28 is ideal for utility use because it is fully dielectric and can be placed confidently
near high voltage power lines. Hammond 19-inch cabinets are used at substations and at customer
locations. These cabinets can be locked for security purposes and are rugged and strong for harsh
environments. Many utilities such as Toronto and North York, are using Egerton 3U, 6U and 9U
Patch panels for the termination of their fibre cables. The 3U is used to terminate a 24 strand
cable, 6U to terminate a 48 strand cable and the 9U to terminate a 96 fibre cable. These patch
Panels are all LSOH rated and have detailed fibre management compartments with mandrels for
bending control of incoming strands and pigtails. The standard connector used by Toronto Hydro
is ST.
Patrick Boshell, formerly the supervisor of communications field services for Toronto Hydro, is now the vice-president of e>connect. Previously, Boshell has worked at British Telecom and Telecom Eireann as a fibre optic specialist. He was involved from the early stages in the transition from copper cable. He is an expert in the construction and maintenance of fibre optic and copper installations and is certified to European and EEC telecom standards.