WAGO-I/O-SYSTEM 750
Connect Devices
53
750-493 3-Phase Power Measurement Module
Manual
Version 1.2.0
7.3.2
Current Transformers
For measuring currents greater than 1 A resp. 5 A you have to use external current
transformers.
Normally, the selection of the current transformers is not a critical factor. The
internal resistance in the current path of the 3-phase power measurement module
is so low that it can be neglected when considering the overall resistance of the
current loop.
The transformers must provide a rated secondary current of 1 A resp. 5 A. The
rated primary current I
pn
shall be equal or higher than the currents to be measured.
The normal, permissible overload level of 1.2 x I
pn
presents no problem for the
3-phase power measurement module, but can, however, lead to minor
measurement inaccuracies.
Do not operate with open load!
Please note that current transformers from some manufacturers shall not be
operated with an open output circuit! Connect the I/O module to the current
transformers’ secondary windings before commissioning the current transformers!
7.3.2.1 Accuracy
Please note that the overall accuracy of the measuring setup consisting of the
3-phase power measurement modules and the current transformers essentially
depends on the transformers’ accuracy category.
Do not use transformers of class 0.5 for accounting purposes!
A measuring system with a class 0.5 current transformer cannot be approved or
certified for accounting purposes as the 3-phase power measurement module is
not an approved utility billing meter as defined in the standard for electricity
meters (DIN 43 586).
7.3.2.2 Types of Current
The 3-phase power measurement module can measure any types of current up to a
frequency of 2 kHz. As currents are frequently generated by power inverters and
can contain frequencies lower than 50 Hz or even a DC portion, an electronic
transformer should be used for these applications.
7.3.2.3 Overcurrent Limiting Factor FS
The overcurrent limiting factor (FS) for current transformers indicates at which
multiple of its rated primary current saturation occurs which protects the
