SMY133-SMY134-SMP133 Operating Manual
lower. Due to this, they are generally less resistant against overcurrents than the standard current
inputs.
Therefore, check possible worst conditions in the network and calculate maximum possible residual
currents on the RCT output, both the permanent and the dynamic ones.Then compare them with
maximum allowable static and dynamic currents of the instrument inputs that can be found in technical
specifications.
Particular care must be taken during the RCT installation. When, for example, only two power
conductors of a three phase cable is installed into the RCT by mistake, the RCT can push from its
output false residual current of magnitude corresponding up to rated phase current of the network!
Example :
rated nominal primary phase current :
120 A
RCT-ratio :
600 / 1
false primary residual current (2 phase currents only measured):
120 A
false secondary residual current :
120 / 600 = 0.2 A
If the RCT is powerful enough to source such false secondary residual current for long time
and its value exceeds maximum allowable current of the instrument input, it is strongly
recommended to check the RCT output current before connecting it to the instrument!
Never use standard CTs with 5A nominal secondary current for the RCM! Eventual
overcurrent could damage the instrument residual current input!
5.1.3 Special RCTs with DC-Current Output
Besides standard RCTs, RCTs with output current loop of 4-20 mA DC can be used too. In such case,
instead the
CT
RCM
–ratio set primary residual current values corresponding to 20 mA in the
Installation
group of parameters (see below).
Any current overload of the instrument inputs cannot occur in such case.
5.2 RCM Inputs Connection
Although it is not mandatory, we recommend to connect the S1-signal (or “k”) from a standard type
RCT to the
IΔ11
/
IΔ21
terminal and the S2-signal (“l”) to the
IΔ12
/
IΔ22
when standard RCT is used.
When special RCT with DC current loop output used, connect positive (+) pole to the
IΔ11
/
IΔ21
terminal and negative pole (-) to the
IΔ12
/
IΔ22
.
recommended wire type :
H05V-U (CY)
recommended minimum cross-section :
0.5 mm
2
maximum cross-section :
1.5 mm
2
Check measured residual currents immediately after the RCT connection - if any is higher
than maximum allowable current of the input (defined in the technical specifications),
disconnect the RCT immediately, otherwise the instrument can be damaged!!!
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