Optional Fibre Optic Interface - Siemens 7SR21 Manual

2.1.3

Optional Fibre Optic Interface

When connecting via the optional fibre optic interface the selection of fibre-optic cable is important. Fibres must
TM
be terminated with ST (BFOC/2.5) connectors.
The recommended type is 62.5/125µm glass fibre. Communication distances over 1 km are achievable using this
type of fibre.
A budget loss calculation should be made for all installations. The following table gives the Launch power and
receiver sensitivity of each of the fibre optic communication ports on the 7SR21 & 7SR22 relays when used with
specific fibre optic types.
Fibre Type
62.5/125µm
1mm Polymer
200µm PCS
The main factors limiting transmission distances with fibre-optics are: -
•
Transmitter launch power
•
Attenuation, based on light frequency, fibre material and fibre diameter
•
Number of intermediate connectors and splices
•
Receiver sensitivity
•
The light power at the receiver must be above the sensitivity of the receiver in order that effective
communication can occur.
•
Consult fibre manufacturers' data for actual values of fibre attenuation.
•
Fibre cables are supplied on reels of finite length which may necessitate additional jointing.
•
Typical losses at connectors are 0.5-1.0dB each. This allows for normal age related deterioration. Consult
manufacturers' data for actual values.
•
Typical Splice losses are <0.3dB.
•
A 3dB safety margin is usually allowed after the budget calculation is performed.
•
Following installation the actual losses should be measured for each fibre using a calibrated light source
and meter and the measured values compared to the calculated estimate before the relay is applied.
•
The fibre optic data Comms link will be broken if the relay element is withdrawn from the case.
©2010 Siemens Protection Devices Limited
Tx Launch Power (dB)
Min Max
-11.7 -15.7
-6.4 -10.4
-2.8 -6.8
7SR21 & 7SR22 Data Communications
RX Receive Sensitivity (dB)
Min Max
-24 -9.2
-24 -9.2
-24 -9.2
Chapter 4 Page 7 of 15