A faulty fibre patch lead can cause a red LOS light or intermittent signal drops. Check for sharp bends, loose connectors, visible damage, or contamination. If replacing the patch lead restores PON and clears LOS, the issue was internal — not an FNO line fault.
What the Problem Means
The fibre patch lead is the short optical cable connecting your wall fibre box to the ONT.
If this cable is:
- Bent too tightly
- Partially unplugged
- Physically damaged
- Contaminated at the connector
It can interrupt the optical signal and trigger a red LOS light.
This is an internal access-layer issue — not a street-level fibre break.
Correctly identifying a patch lead fault prevents unnecessary FNO escalation.
Why Patch Leads Fail
Common causes in South African homes:
- Tight bending behind furniture
- Pets chewing cable
- Cable pinched in doors or cupboards
- Accidental pulling during cleaning
- Repeated unplugging causing connector wear
Fibre cables are sensitive to bend radius. Excessive bending reduces light transmission and can cause intermittent optical signal loss.
Fault Symptoms Table
| Symptom | Likely Cause | Fault Layer | Action |
|---|---|---|---|
| Red LOS immediately after cable moved | Patch lead disturbed | Internal | Reseat cable |
| Intermittent connection when cable touched | Micro-fracture | Internal | Replace patch lead |
| Persistent LOS despite intact cable | Possible line fault | Access Layer | Escalate |
| PON solid, no LOS | Patch lead OK | Not signal-related | Check router |
Step-by-Step Patch Lead Inspection Checklist
Check for Sharp Bends
- Fibre should not be bent tighter than a gentle curve. Avoid 90-degree folds.
Inspect Visible Damage
- Look for cracks, flattened sections, or exposed fibre.
Confirm Secure Connection
- Ensure both ends are firmly inserted into:
- Wall fibre outlet.
- ONT port.
Do Not Look Into Connector Tip
- Fibre transmits invisible laser light.
Test Without Moving the Cable
- If connection stabilizes when cable is untouched, movement sensitivity suggests internal fracture.
Swap Test (If Available)
- Replace with a known working fibre patch lead.
- If LOS clears and PON turns solid → original cable was faulty.
Definition: Bend Radius and Optical Signal
The minimum safe curve allowed for a fibre cable without degrading light transmission.
Excessive bending disrupts the optical signal, reducing light intensity reaching the ONT.
When signal strength drops below threshold, the ONT triggers a red LOS condition.
When to Escalate
Escalate only after:
- Patch lead is confirmed intact
- Connections are secure
- Proper ONT restart performed
- LOS remains red
If replacing the patch lead does not restore signal, the issue is likely at the external access layer.
Typical South African FNO repair windows:
- 24–48 hours for isolated signal faults
- Up to 72 hours for fibre break investigation
When This Is NOT the Issue
If all of the following are true:
- PON is solid green
- LOS is off
- Internet is slow
Then the patch lead is not the cause.
See our fibre speed & performance guides