Woodward highprotec MRI4, User Manual

Page: 42 / 727

Installation and Connection
TI X- Standard Phase and Ground Current Measuring Input Card
This measuring card is provided with 4 current measuring inputs: three for measuring the phase currents and one
for measuring of the earth current. Each of the current measuring inputs has a measuring input for 1 A and 5 A.
The input for earth current measuring either can be connected to a cable-type current transformer or alternatively it
is possible to connect the summation current path of the phase current transformer to this input (Holmgreen
connection).
Current transformers have to be earthed on their secondary side.
Interrupting the secondary circuits of current transformers causes hazardous
voltages.
The secondary side of the current transformers have to be short circuited
before the current circuit to the device is opened.
The current measuring inputs may exclusively be connected to current
measuring transformers (with galvanic separation).
Do not interchange the inputs (1 A/5 A)
Make sure the transformation ratios and the power of the CTs are
correctly rated. If the rating of the CTs is not right (overrated), then the
normal operational conditions may not be recognized. The pickup value
of the measuring unit amounts approx. 3% of the rated current of the
device. Also the CTs need a current greater than approx 3% of the
rated current to ensure sufficient accuracy. Example: For a 600 A CT
(primary current) any currents below 18 A cannot be detected any more.
Overloading can result in destruction of the measuring inputs or faulty
signals. Overloading means that in case of a short-circuit the current-
carrying capacity of the measuring inputs could be exceeded.
Ensure the correct tightening torques.
42 MRI4 DOK-HB-MRI4-2E