A Simple Trick to Modify the Ratio on a Current Transformer

A Simple Trick to Modify the Ratio on a Current Transformer 

What do you do when you have a CT with a primary current ratio that is too high for your current load? Here’s the simple answer.

The secondary side of a CT consists of many turns of smaller wire wound to achieve a desired primary to secondary current transformation. A 300:5 ratio CT, for example, will transform 300 amperes on the primary to 5 amperes on the secondary.

Large changes in the CT ratio can be achieved by modifying the primary turns through the CT window. One primary turn is equal to one pass of the primary conductor.   Adding additional loops through the CT window will result in the electrical ratio being modified.

For example, if a lower primary rating is required due to a reduced load density, this can be achieved by adding one loop to the primary cable.  This will reduce the ratio to 150:5. An additional primary loop will reduce the CT ratio to 100:5.

Looping the primary will save you the need for buying a new CT with a lower ratio. This savings can also be achieved by using high accuracy, extended range CTs. Read more about that here.

Article written by Jon Rennie

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