Extending current setting lower limit

Most of the current based protection functions have a current setting lower limit of 10-20%. This limitation is necessary to ensure the operating accuracy declared in the actual function block description.

If the desired current setting is lower than the limitation, Protecta recommends the following method:

1) CT has 5A rated secondary and the CT module has 1A/5A scaling options: Set 1A as CT module rated secondary. Consequently, current settings in the various protection functions shall be multiplied by 5 in order to compensate the change and secure proper operation. It follows that the lowest current setting will remain 5 times lower, if we do not change it. In order to get correct primary current indication, the CT primary rating shall be divided by 5. 

2) CT has 1A rated secondary and the CT module has 0.2A/1A scaling options: Set 0.2A as CT module rated secondary. Consequently, current settings in the various protection functions shall be multiplied by 5 in order to compensate the change and secure proper operation. It follows that the lowest current setting will remain 5 times lower, if we do not change it. In order to get correct primary current indication, the CT primary rating shall be divided by 5. 

The above method only apply for functions, which receive current measurement from the CT input in question.  

Note: Changing the rated secondary of a CT module has in impact on the maximum measured current. This information can be found in the corresponding section of the hardware description and shall be taken into consideration. Protection functions shall operate accurately in the high current range (e.g. instantaneous overcurrent) as well.