How do we select the proper current transformer for a given application?

In order to select the proper indoor type current transformer for an application, there are four phases need to be addressed:

1- Define customer requirements based on the primary circuit and the metering and protection chains; i.e. define customer requirements in terms of current transformer specifications, key can be summarized as follows:

• Primary rated current(s) in Amp.
• Secondary rated current(s), i.e. 1A or 5A.
• Rated frequency (50 or 60Hz).
• Rated voltage or insulation level Ur/Ud/Ui in kV.
• Short-time withstand current and duration in kA/sec.
• Desired accuracy class in measurement chain (e.g. class 1, 0.5 …etc).
• Desired rated burden (in VA) and instrument security factor (FS) for measurement chain (e.g. 5VA Cl0.5 FS10).
• Desired accuracy class in protection chain (e.g. class 5P, 10P …etc).
• Desired rated burden (in VA) and accuracy limit factor (ALF) for protection chain (e.g. 2.5VA Cl5P10).
• Special classes such as class PX, defined by the knee point voltage (Ek or Vk), internal resistance (Rct) and the magnetizing current (Ie).
• Rated continuous thermal current in percent of rated primary current.
• Other special requirements such as out-of-standard temperature/environment/altitude; connections; curves; treatment; terminal covers.
• Installation type: either inside a SE cubicle (in this case the cubicle type and rated current need to be known) or in a non-SE cubicle.

​
2- Select a “referenced” product from the CT/VT catalog (please refer to the latest catalog published).

3- If no suitable CT has been found, you can still check the feasibility of the solution on “ELONET” using the “configured order” option for a given CT type or SE cubicle (note: in case a feasible solution has been found, the configured CT can then be ordered/procured via dedicated supply chain or intra-group purchase team).

4- If still no solution found, you need to fill-in a consultation form (per CT solution required) and send it back to technical support team for feasibility study; consultation form attached.

Released for:Schneider Electric South Africa