Search FAQs

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.