4.6 Common Cable Faults
Key Takeaways
- Split pairs pass simple continuity but fail NEXT badly because pair twist is broken; usually caused by mixed T568A/T568B color codes.
- Opens and shorts are detected by wire-map; opens at terminations are most common, shorts from nicked jackets or punched-down conductors touching.
- Macrobends cause wavelength-dependent fiber loss (worse at 1550 nm than 1310 nm); water ingress increases attenuation over time and is detected by elevated loss at affected wavelengths.
- Connector contamination is the single most common cause of fiber certification failures; always clean before re-testing or replacing.
Copper Faults
Split Pairs
A split pair is the most deceptive copper fault. Continuity is correct on all 8 pins, so a basic continuity tester reports the link as good. But the two conductors of one pair have been split onto pins belonging to different pairs at one end, breaking the twist that controls crosstalk. NEXT fails badly on every pair.
Cause: a technician terminates one end T568A and the other T568B without noticing the color-code mismatch. The pin-out is identical (1-2, 3-6, 4-5, 7-8), but the pairs are different.
Detection: a wire-map tester that shows pair assignment (not just pin continuity) flags the split. A certifier's NEXT sweep confirms the failure. Fix: re-terminate the wrong end to match the correct color code.
Opens
An open is a broken conductor. Wire-map shows the open on the affected pin and reports the distance to it (by TDR). Common causes: a conductor not fully seated in the IDC block (insulation not pierced), a kinked or stretched cable that broke a conductor, a broken contact in a patch cord or jack, or a conductor cut too short at the termination. Most opens are at the termination, not in the cable run — always re-terminate the suspect end before pulling new cable.
Shorts
A short is two conductors in electrical contact. Wire-map shows the shorted pair. Common causes: a nicked jacket at the strip point, a conductor punched down across two IDC slots, a staple or nail through the cable, or a damaged patch cord. A short at the termination is fixed by re-stripping and re-punching; a short in the run (from a staple) requires locating the damage and replacing the affected section.
Kinks
A kink in a copper cable changes the impedance locally, causing return loss and (sometimes) NEXT. A severe kink can break a conductor. Kinks are common where a cable is pulled around a tight corner or pulled through a conduit with insufficient slack. The certifier may flag return loss at the kink location (a TDR trace shows the impedance bump). Fix: cut out the kinked section and re-terminate, or replace the run if the kink is in the middle of a long horizontal.
Fiber Faults
Macrobends
A macrobend is a fiber bent tighter than its minimum bend radius. Light escapes the core at the bend, producing loss. Macrobends are wavelength-dependent: loss is much higher at longer wavelengths. A 1310 nm test may pass while a 1550 nm test fails. This is why OTDR testing of single-mode is done at both wavelengths.
Detection: OTDR shows a non-reflective loss event at the bend location. Visual inspection confirms a sharp bend, pinch point, or cable pressed against a tray edge. Fix: re-route to restore the minimum bend radius. Severely kinked fiber must be cut out and re-spliced.
Connector Contamination
Connector contamination — dust, oil, skin, or residue on the end-face — is the single most common cause of fiber certification failures. A contaminated connector can add several dB of loss and high reflectance, and the contamination can be transferred to the mating connector.
Detection: the OLTS shows high loss; the OTDR shows high reflectance at the connector; an end-face inspection scope shows the contamination directly. Fix: clean the connector end-face with a cleaning tool (reel-type or click cleaner), inspect under the scope, and re-test. Never test a fiber connector without cleaning it first, even a new one — the dust cap can shed fibers onto the end-face. A connector that fails after cleaning has physical damage (scratched ferrule, chipped end-face) and must be re-polished or replaced.
Water Ingress
Water ingress into an outside plant or underground fiber run causes progressive attenuation. Water in the cable changes the refractive index around the fiber and, over time, hydrogen-induced attenuation at specific wavelengths (the 1383 nm water peak in older fiber).
Detection: OTDR shows elevated loss along an affected section, sometimes with multiple non-reflective events. The loss tends to grow with time and temperature changes. Fix: locate the water entry (usually at a damaged splice closure or a breach in the jacket), replace the affected cable section, and seal the closure properly. Water-damaged fiber is not cleanable; the section must be cut out.
Broken Fiber
A broken fiber (from a crushed cable, a staple, or impact) shows on the OTDR as a strong reflective event at the break distance and a total loss of the backscatter signal beyond. Multiple fibers in the same cable breaking at the same distance indicates a single physical event (a dig-up, a crush). Fix: locate the break, splice in a repair section, and re-test. For multi-fiber cables, all broken fibers must be repaired and re-tested individually.
Fault Signature Quick Reference
| Symptom | Likely Fault |
|---|---|
| Wire-map continuity but NEXT fails on every pair | Split pair (mixed T568A/T568B) |
| Wire-map open at one pin | Open conductor, usually at termination |
| Wire-map shorted pair | Nicked jacket or staple through cable |
| OLTS loss on fiber passes at 1310, fails at 1550 | Macrobend |
| OTDR high reflectance at a connector + high loss | Contaminated or damaged connector |
| Fiber link attenuation grows over months | Water ingress in the run |
| OTDR sharp reflective spike, no signal beyond | Broken fiber |
Prevention
Most of these faults are preventable: use the same color code (T568B is most common in the U.S.) throughout the project; maintain pair twist to the IDC slot; respect bend radii; clean every fiber connector before mating; and seal every outside plant splice closure before burying.
A copper link passes wire-map continuity on all 8 pins but fails NEXT on every pair. What is the most likely fault, and what is the corrective action?
A single-mode fiber link passes the OLTS test at 1310 nm but fails at 1550 nm, with a non-reflective loss event shown on the OTDR. What is the most likely fault?