Note On The Parallel Connection Of Multiple Voltage Measuring Devices In A System - Phoenix Contact SCK-C-MODBUS User Manual

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Solarcheck string voltage measurement with the SCK-M-U-1500V
Calculation basis for Table 4-1 and Table 4-2:
If
(0 < U+ < 1500 V)
(0 < U- < -1500 V)
(0 < Difference (U+, U-) < 1500 V
Then
Analog Out:
Difference (U+, U-) x 8 V
1500 V
The maximum measured value deviation of 1% (of the measuring range final value) is based
on the 100 .... 1500 V DC system voltage range. The deviation may be greater below
100 V DC.
4.1.2 Note on the parallel connection of multiple voltage
measuring devices in a system
Due to the circuit architecture in the device, the voltage measuring input is connected to
ground via the internal impedances. Resistance Rx is 20 MΩ respectively. If multiple volt-
age measuring devices are connected to the same generator in parallel, these impedances
are also connected in parallel. This results in overall reduced impedance of the generator to
ground. This can affect the response of any ground fault detection in the system.
The impedances of the voltage measuring devices used within a generator (per inverter in-
put) therefore have to be taken into consideration when defining the threshold values for
ground fault detection.
The following impedances result depending on the number of voltage measurements con-
nected in parallel in a system.
Table 4-3 Impedances
Number of parallel
voltage measurements
1
2
4
8
16
32
64
128
and
and
+ 2 V
Individual impedance
(+  PE)
20 MΩ
20 MΩ
20 MΩ
20 MΩ
20 MΩ
20 MΩ
20 MΩ
20 MΩ
...
Total impedance
(+  PE)
20 MΩ
10 MΩ
5 MΩ
2.5 MΩ
1.25 MΩ
0.625 MΩ
0.313 MΩ
0.156 MΩ
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