Preparation Of Optical Fibre For Installation: Tools & Techniques

By Patrick Boshell

This is the latest in a continuing series of fibre optic articles by Patrick Boshell. The most recent one appeared in our March 1997 issue, and dealt with fibre classification. What follows is a brief primer of fibre handling and safety techniques.

Cable Laying

Many of the fibre optic cable installation procedures are similar to those used to lay conventional copper cables in a professional manner, especially if the fibre optic cables are relatively short (<500m).

However, there are some special precautions which do need to be taken. If cables are to be pulled through long duct routes then special equipment will also be needed.

When installing external cables, it is also important to follow relevant safety procedures with respect to gas testing when working in manholes or other confined spaces.

Special Precautions

The following points outline some special precautions which are specific to fibre optic cable installation and therefore worth highlighting:

Fibre Stress: The fibres in the cable should not be subject to any undue stress. This means that if the cable is to be pulled into a long duct route then the specialized equipment and procedures detailed in the next section should be used. As well, if the cable runs vertically for a significant length (more than 10 m) then loops should be inserted every 10m or so.

Bend Radius: The cable manufacturer's minimum bend radius should be observed. i.e. there should be no bends tighter than specified either during installation or once the cable has been fixed. A rule of thumb for MBR is 12x cable diameter.

Cable Ties: If cable ties are used (particularly on tight jacketed cables), then it is very important that they are not over-tightened, thereby causing localized bending and fibre stress.

Spare Cable: At least 5m of cable should be left at each end to allow testing, positioning of enclosures, spare fibre for repairs etc. Where appropriate, spare loops of cable should be included along the cable run to assist repair in case of accidental damage.

Labeling: All cables and cable ends should be labeled clearly in accordance with the specification.

Cable End Protection: Where cable ends are to be left exposed then they should be sealed with heat shrink caps to prevent ingress of dirt or moisture.

Earthing: In many circumstances completely non-metallic fibre optic cables can be used to eliminate all earthing problems. If metallic elements are present then they should be earthed in accordance with the installation specification. Generally this will involve bonding metallic strength members or moisture barriers to earth at one end only of the cable run.

Cable Preparation

Cable preparation is the most important skill to master in fibre optics. Proper cable preparation is essential for splicing and connection. A large proportion of installation time is invariably spent on cable preparation and so the more efficiently it can be carried out, the greater the savings in time spent on site.

Correct use of the right tool for the right task is of great importance.

Techniques and Tools

Fibre Coating: A variety of different coating materials and thickness are used depending upon cable construction. The most common configurations are: 250 micron acrylate; 250/500 micron dual acrylate; 250/900 micron acrylate/nylon; or 250/400/900 micron acrylate/silicone/nylon

In some cases (at AT&T, for instance) PVC is used instead of the nylon for the 900 µm micron buffer. Thankfully Ñ as it was very difficult to remove Ñ silicone primary coating is now rare.

Stripping Tools For Fibre Coatings

Finger Nails: May be used with some success, but will quite likely break the fibre.

Fishing Line: A length of fishing line may be tied to a firm object and used but a certain knack is required.

Fishing Line Tool: A special tool uses two interlocking loops of fishing line, and applies a set tension to cut into the coating. Usually fairly effective and with little danger of fibre damage. Expensive.

Fujikura Tool / PS-01 or PS-02: A special tool designed to remove primary coating without exerting additional stresses on fibre. Uses blades set to cut a precise depth. Has a gauge to measure fibre length to be stripped.

Miller Stripper: BT approved tool for stripping primary coating. Yellow handled stripping tool with laser drilled hole just greater than 125µm.

CK/Siemens Stripper: A fine wire stripper with various tolerances. Not very effective on primary coated fibres through a 0.2 mm or a 200 µm microns version is available. Larger sizes e.g. 0.4, 0.5, 0.6 mm work quite effectively for secondary coating. Has gauge to regulate stripped length.

Chemicals

Acetone: May be used to remove any remaining traces of coating, although this is frowned upon by COSHH regulations.

Methyl Chloride: Stubborn coatings may be softened up, or even removed completely using methyl chloride or paint stripper (subject to health and safety regulations).

Cleaning

IPA (Iso-Propyl Alcohol): Fibres should always be cleaned with a suitable degreasing solvent such as Iso-Propyl Alcohol (IPA). This may be applied on a lint free tissue, or alternatively (with less chance of breaking the fibre) using a pair of tweezers with cotton bud ends stuck on them. The fibre should be wiped twice turning through 90 degrees between wipes. The fibre should be shiny and "squeaky clean".

Cable preparation tools

Kevlar Cutters: If "ruggedized" cords are being dealt with (or other cables incorporating Kevlar strength members) then special Kevlar cutters will be required. These may be either ceramic bladed scissors or special scissors with a serrated edge. (Sharp scalpels may be used but pose a safety hazard).

Jacket Stripping Tool: Basic wire stripping type tool, has various sized notches for different sizes of ruggedized cord.

Tube Cutter: This tool is really designed for cutting into coaxial cables. It has two blades which may be set to cut into different depths when ringing around cables. Used for scoring around the loose tube in stranded loose tube constructions, or may be used on 5 mm loose tube. Can also be used to cut the jacked of ruggedized unit.

Sheath Removal Tool: Has an adjustable depth blade for scoring round cable sheaths. Press the button to rotate blade through 90 degrees and cut along the length of the sheath.

Hacksaw, Stanley Knife, Pipe Cutters, etc.: Other standard cable preparation tools may be used as required on armoured cable or outer cable sheath.

Patrick Boshell is Vice-president, Engineering and Outside Plant with e>connect in Toronto. ET