Siemens SIMATIC ET 200SP Manual page 21

The maximum value of the resistance of the connection cable is obtained with the following
formula:
R
L
Z
n
Figure 3-3
Based on the maximum cable resistance in ohms, you then calculate the maximum length of
the connection cable. To do this, check the data sheet of the connection cable you are using.
Note
The length of the connection cable (outwards and return) must not exceed the value of
200 meters.
Example
Current transformer 500/5 A
You use a current transformer with a transmission ration of 500/5 A that has a rated burden
Z of 5 VA according to the data sheet.
n
The maximum primary current in the application is 400 A. This means that the maximum
secondary current I
resistance amounts to R
The maximum value of the resistance of the connection cable (outgoing and incoming line) is
obtained using the following formula:
The maximum cable resistance between the transformer and the terminals of the Energy
Meter may not exceed 287.5 mΩ in this case. The corresponding cable length (outgoing and
incoming line) depends on the cross-section used of the copper line and can be determined
by using the following table.
Analog input module AI Energy Meter 400VAC ST (6ES7134-6PA01-0BD0)
Manual, 10/2017, A5E36061895-AB
Cable resistance in ohms
Rated burden current transformer in VA
Maximum value for the resistance of the connection cable
is 4 A. The burden of the AI Energy Meter including connection
max.
= 25 mΩ.
Burden
3.3 Current transformer selection data
I Secondary current of the current trans-
max
former
R Resistance of the Energy Meter (25 mΩ)
burden
Wiring
21