Definition
Lowering the primary current of a current transformer (CT) means reducing the current flowing through the transformer’s primary conductor. This can be done physically by changing the number of cable turns in the CT opening.
Why would you want to lower the primary of a TC?
In some installations, the actual current may be lower than that for which the CT was designed. It is therefore necessary to reduce the primary current by winding the conductor several times around the CT opening. Up to four turns can be made without affecting measurement accuracy. This solution also allows a through-type CT to be used instead of a wound primary CT. The through-type CT is an economical, practical and quick alternative to implement (less wiring time).
Example of 2 passes
Example of 3 passes
Example
If you have a 40/5 type TAT022 CT, but your output is 20 A, simply run the cable through the CT twice. The CT will then see 20 A, allowing for better use of the measurement scale.
Another example: using a 60/5 TC type TAT022, and making 4 passes, this equates to a ratio of 15/5, perfect for a 16 A start.
Attention
When configuring the energy meter, it is essential to adjust the CT ratio value taking into account the operation performed upstream. Let us return to our previous example, with a CT of 40/5 and two cable passages. For our 20 A output, we will need to program a ratio of 4.
That is:
- 40/5 = 8
- 8/2 = 4