current transformer correctly select & dimension
The choice of the suitable current transformer depends on primary current, connection values and purpose of use. This guide leads step by step through the most important selection criteria.
More information
In four steps to the right current transformer
- Determine the primary rated current based on the line to be protected or measured
- Check the secondary rated current and the connection value of the target device (measuring device, meter, protection relay)
- Select accuracy class suitable for the intended use
- Select design suitable for the mounting situation
Determine primary current correctly
The primary rated current of a current transformer should be based on the actual expected operating current, not simply on the next higher available value. Significant oversizing means that the transformer is operated in the partial load range – there the measuring accuracy is lower than specified in the range of the rated current.
Designs at a glance
Plug-on current transformer (ASK) are plugged onto an existing line without having to disconnect it – suitable for subsequent installation. Wound-type current transformer (WSK) have a permanently wired primary winding and are particularly suitable for smaller currents and confined installation situations. Split-core current transformer (KBR/KBU) have a divisible measuring core and can therefore also be mounted around already laid lines that cannot be disconnected.
Understanding accuracy class (brief overview)
The accuracy class indicates the maximum permissible measuring deviation at rated load. For general operational measurements, class 1 is usually sufficient; for calibratable billing measurements, higher classes such as 0.2s or 0.5s are provided. A detailed explanation can be found in our article on accuracy classes.
Typical pitfalls during commissioning
The secondary circuit must never be open during operation – this can result in dangerous voltages at the connection terminals. Also pay attention to the correct terminal designation: K and L mark the primary side, k and l the secondary side. An incorrect connection direction leads to an incorrect measuring direction.
Still unsure which type is right?
Describe your application to us – contact us, we will suggest the right current transformer.