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Capacitance measurement
Disconnect the tested lines from the switch, and select the capacitance measurement gear of the digital
multimeter by connecting it with the external terminal, A and B lines, to measure the capacitance between Line A
and Line B, that is, the capacitance C a-b between Line A and Line B. In the same way, the capacitance of Line
A to the ground, C a-gnd and the capacitance of Line B to the ground, C b-gnd.
Resistance measurement
Disconnect the tested lines from the switch, and select the resistance gear of the digital multimeter by
connecting it with the external terminal, A and B lines, to measure the insulation resistance between Line A and
Line B, that is, the insulation resistance R a-b between Line A and Line B. In the same way, the insulation
resistance of Line A to the ground, R a-gnd and the insulation resistance of Line B to the ground, R b-gnd.
Similarly, the loop resistance of the subscriber line can also be measured, and the way is to loop at remote end,
and connect Line A with Line B shortly, and disconnect the local A and B lines from the switch; use the digital
multimeter to connect the local A and B terminals, and select the resistance gear of the multimeter to read the
measurement resistance, that is, the tested DC loop resistance. The table below gives the reference values of
the DC loop resistance for subscriber lines with different core diameters per kilometer:
Table 12-5 DC loop resistance for subscriber lines with different core diameters per
kilometer
Core diameter (mm)
Loop resistance (Ω/km)
In practice, it may be the subscriber line of mixed core diameters, in this case, we should give it a
comprehensive consideration, the DC loop resistance of subscriber line should not be more than 1.1 kΩ. For the
external measurement, some parameter index of the external lines can be tested by means of 112 test terminal.
Through the measurement of the actual subscriber line, a reference table will be given for guiding the
commissioning of the ADSL service, and the details are shown in the table below:
Table 12-6 Line test reference table
AC voltage
(VAC)
DC voltage
(VDC)
Resistance
(Ω)
diameter:
0.4mm
Capacitanc
e (uF) core
diameter:
0.4mm
Based on the above table, we can make an analysis on the line status:
Judge whether there is tap of power line by viewing the AC voltage.
Judge whether there is swinging cross by viewing the DC voltage, V a-gnd and V b-gnd.
Judge the insulation of Line A and B -ground by viewing the resistance values, R a-gnd and R
b-gnd, and the test value of the actual line may not reach more than 5MΩ as required by the
national standard. The reason is for this is mainly the impact of the line environment, for
example, regional climate difference leads to the difference in line status, or different humidity
conditions, poor water-releasing performance of the junction box, the resistance of R a-gnd
and R b-gnd is lower than 200kΩ which is the lowest standard for commissioning the ADSL
line, so the ADSL service is easy to be interrupted.
The open line test requires to inform users to remove all the devices on the subscriber line for
measurement. C a-b is 45~55nF per kilometer, and the normality of the C a-b value can be
3P@C Installation Manual
Page 12-8
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SAGEM SA
V a-b
0V
V a-b
0V
R
a-b
loop
core
resistance
280Ω/km
C
a-b
load
open-circuit
45-55nF/km
0.32
0.4
0.5
470
280
190
V a-gnd
V b-gnd
0V
0V
V a-gnd
V b-gnd
0V
0V
R a-gnd
R b-gnd
>5MΩ
>5MΩ
C a-gnd
C b-gnd
30-40nF/km
30-40nF/km
SAGEM 3P@C
R a-b insulation
resistance
>5MΩ
C a-b on-hook
>0.35uF
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