Fibre 37 in a 48-Fibre Joint With Mixed Cable Designs
A 48-fibre cable enters a joint chamber. It is a stranded loose-tube design with four tubes of 12 fibres, and the tube and fibre colours follow the 12-colour sequence: blue, orange, green, brown, slate, white, red, black, yellow, violet, rose, aqua. The print legend reads 48F SM G.652D, four tubes. You must splice fibre 37 through to fibre 37 of a new 48-fibre cable of a different make, which uses a central tube containing four 12-fibre ribbons numbered in the same colour sequence. Only fibres 37 to 44 are needed for a new customer, but fibres 45 to 48 of the old cable are carrying live traffic. A trainee proposes cutting all four tubes at mid-span to search for the brown tube, and argues that brown is the fourth colour, so fibre 37 must be the fourth fibre inside the brown tube.
1. Correct the trainee's counting and state the exact position of fibre 37 in both cables.
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The trainee has confused the tube number with the fibre position inside the tube.
Counting rule. Fibres are numbered tube by tube, 12 per tube, and within each tube the sequence restarts at blue. So:
- Tube 1, blue: fibres 1 to 12
- Tube 2, orange: fibres 13 to 24
- Tube 3, green: fibres 25 to 36
- Tube 4, brown: fibres 37 to 48
The position inside the tube is: ((37 minus 1) mod 12) + 1 = 1. So fibre 37 in the old cable is the brown tube, first fibre, which is blue.
New ribbon cable. Ribbon 4 is brown and contains fibres 37 to 48; fibre 37 is the first fibre of brown ribbon 4, which is blue. The mapping is blue-on-blue, brown-on-brown.
Cautions. Ribbon numbering depends on the manufacturer's marker and on which cable end you are counting from. Confirm against the print legend and the ribbon marker thread, and confirm that both cables are being counted from the same direction before any splice is made. A reversed count would put fibre 48 against fibre 37.
2. Explain why cutting all four tubes at mid-span is unacceptable here and describe the correct cable-opening method.
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Cutting all four tubes destroys 48 fibres to reach one tube. Forty of them are not part of this job, and four of the brown-tube fibres are live customer circuits, so a single careless cut causes an outage. It also creates up to 48 unnecessary splices at about 0.1 dB each, fills the closure with trays, and increases the failure rate for years.
Correct method:
- Confirm which tube is needed and record its colour and position from the sheath print and the tube count.
- Open a sheath window of the length the closure manufacturer specifies, using a ringing tool set to the sheath thickness so the tube wall is not scored.
- Leave the other three tubes uncut and undisturbed, with enough slack to sit clear of the splice trays.
- Slit and cut only the brown tube. Because fibres 45 to 48 are live, the whole brown tube is cut and all 12 of its fibres are spliced in the closure: 8 new fibres to the new cable and 4 straight-through joints for the live circuits.
- Store the through-tubes and the uncut tubes in the closure's dedicated storage, respecting the minimum bend radius, and label the tray map.
3. How would you prove the identification is right before splicing, given that four of the fibres are carrying live traffic?
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You cannot use a visual fault locator or a tone on a live fibre without a maintenance window, and you must not disconnect live connectors just to identify them.
Acceptable proof, in order:
- Physical consistency: tube colour, tube position, cable print legend, fibre colour, and the count repeated from the opposite end giving the same result.
- Compare with the as-built records: fibre 37 should have a known length and a known OTDR event pattern from earlier commissioning. An OTDR trace taken in a maintenance window from the far end at 1625 nm with a live filter, or with traffic temporarily moved, should match that pattern.
- Where dark fibres exist, use them as a control: identify a spare fibre by VFL and confirm that the counting method places it where expected. If the spare is right, the method is right.
- Before cutting anything, isolate the live fibres with the network operator, book the window, and have the operator confirm the circuits that will be interrupted by the brown-tube cut.
- Record the result in the splice schedule: cable ID, end from which counting started, tube colour and number, fibre colour and position, and the bi-directional OTDR result after splicing. A clear record prevents the next technician from repeating the doubt.