Introduction
In modern power transmission systems, Optical Ground Wire (OPGW) is the backbone of protection, control, and communication. It carries critical data for SCADA, protection relays, and grid monitoring while simultaneously serving as a shield wire against lightning.
For any OPGW communication system to perform reliably, one calculation is absolutely essential:
The Link Loss Budget
A properly calculated link loss budget ensures that optical signals can travel across long transmission corridors without excessive attenuation, data loss, or communication failure.
This article explains:
What link loss budget is
Why it is critical in OPGW projects
How it is calculated
Why OTDR and LS–PM values differ
A detailed explanation of a real OPGW link loss budget table
What Is a Link Loss Budget?
A link loss budget is the total allowable optical signal loss between the transmitting equipment and the receiving equipment in a fiber optic link.
In simple terms, it answers one key question:
Will the transmitted optical signal reach the receiver with sufficient power to work reliably?
Link Loss Budget Includes:
- Fiber attenuation over distance
- Fusion splice losses
- Connector losses
- System margin (safety buffer)
Why Link Loss Budget Is Critical in OPGW
OPGW links are very different from normal telecom fiber links because they:
- Span long distances (50–100+ km)
- Have multiple joint boxes
- Are installed in harsh electrical and environmental conditions
- Support mission-critical power system communication
Without an accurate link loss budget:
- Communication may fail after commissioning
- Protection relays may malfunction
- Troubleshooting becomes difficult
- System upgrades become risky
Key Components of an OPGW Link Loss Budget
- Fiber Attenuation
Signal loss per kilometer of fiber, typically measured at:
1310 nm
1550 nm
Example values:
0.22 dB/km @ 1550 nm
0.24 dB/km @ 1310 nm
- Fusion Splice Loss
Each joint box contains fusion splices.
Typical design value:
0.05 dB per splice
Even small splice losses add up over long distances.
- Connector Loss
Loss introduced by connectors at equipment or ODFs.
Typical value:
0.45 dB per connector
- Measurement Method
Loss is measured using:
OTDR (Optical Time Domain Reflectometer)
Light Source (LS) & Power Meter (PM)
Each method gives different results, which must be interpreted correctly.
OTDR vs LS–PM in Link Loss Budget
OTDR (Reflective Measurement)
- Measures backscattered light
- Shows splice locations, distance, and events
- Typically measures one connector
- Loss is estimated
LS–PM (End-to-End Measurement)
- Measures actual transmitted power
- Includes both end connectors
- Gives true total link loss
- Used for final acceptance
👉 Engineering rule:
LS–PM value is the reference for link loss budget approval.
Understanding the Attached Link Loss Budget Table (Step-by-Step)
The attached table represents a real OPGW link loss budget calculation for a long transmission link.
Let’s break it down clearly.
- Project & Link Information
Link type: OPGW (sometimes it could be UGNMFOC if it is used inside the Substation for communication rooms.)
Link length: 75.66 km
Joint boxes (JB): 22
This means:
22 fusion splice locations
Long-distance optical transmission
- Test Equipment Used
Two test methods are shown:
A. OTDR
Measurement from one end
Used for diagnostics and verification
B. Light Source & Power Meter (LS–PM)
End-to-end measurement
Used for link budget compliance
- Wavelengths Used
Measurements are taken at:
1550 nm
1625 nm
This ensures:
Compliance across operating wavelengths
Better understanding of fiber performance
- Fiber Attenuation (dB/km)
Typical values shown in the table:
0.22 dB/km
0.24 dB/km
Total fiber loss is calculated as at 0.22 dB/km
Fiber Loss = Attenuation × Link Length
Example:
0.22 × 75.66 ≈ 16.64 dB
- Splice Loss Calculation (22 Joint Boxes)
Each JB introduces one fusion splice.
Assumed splice loss:
0.05 dB per splice
Total splice loss:
(22+1) × 0.05 = 1.15 dB
Total loss at 0.22 dB/km = 16.64+1.15+0.45(connector loss)=18.25
This value is clearly reflected in the table remarks.
- Connector Loss – Why OTDR and LS–PM Differ
This is the most important part of the table.
OTDR Case: 22 JB & 1 Connection
Why only 1 connection?
Because OTDR testing is performed from one end only.
One connector at OTDR launch end
Far end is open fiber (not measured accurately)
Connector loss:
1 × 0.45 = 0.45 dB
👉 This is why OTDR total loss is lower.
LS–PM Case: 22 JB & 2 Connections
Why 2 connections?
Because LS–PM measures the link from one end to the other.
One connector at light source
One connector at power meter
Connector loss:
2 × 0.45 = 0.90 dB
👉 This gives the true end-to-end loss.
- Total Loss Calculation
Total Loss includes:
Fiber attenuation
Splice loss
Connector loss
That is why:
LS–PM total loss > OTDR total loss
This is correct and expected
Why This Link Loss Budget Table Is Technically Correct
✔ Proper attenuation values
✔ Realistic splice loss assumption
✔ Correct connector count
✔ Correct use of OTDR and LS–PM
✔ Logical difference between results
This table follows industry-accepted engineering practice for OPGW projects.
Why Utilities and Consultants Insist on Link Loss Budget
Ensures reliable protection signaling
Prevents hidden communication failures
Confirms OPGW installation quality
Supports future network expansion
Reduces long-term maintenance risk
A poorly calculated link loss budget can lead to silent failures, which are the most dangerous in power systems.
Frequently Asked Questions(FAQs)
What is an acceptable link loss for OPGW?
It depends on:
Equipment power budget
Link length
Number of splices
Typically, total loss must be below the transceiver budget with margin.
Why OTDR loss is lower than LS–PM?
OTDR measures only one connector and estimates losses, while LS–PM measures true end-to-end loss including both connectors.
Is OTDR mandatory for OPGW?
Yes, for diagnostics, documentation, and fault localization.
Is LS–PM mandatory?
Yes, for final acceptance and link loss budget compliance.
ALSO READ: Complete Guide to OPGW Testing: Composite, Sample, Fiber, and Field Acceptance Tests Explained
