Current Transformer (CT) types – what is the difference

We manufacture and stock high quality air cored Rogowski coil CTs and we also stock budget iron cored CTs – We are sometimes asked what the difference is between these types and why choose one over the other?

The Rogowski coil CT was originally developed by Walter Rogowski in the 1920s. Unlike iron cored CTs, they usually have a non-magnetic core (typically an air core) which gives them several clear advantages to iron cored CTs – for example Rogowski coil CTs:

  1. have a very wide dynamic range meaning that high current pulses can be measured without worrying about saturation effects since the air core cannot saturate.
  2. are immune to any DC current flowing in the system; it does not affect their operation – again because they cannot saturate.
  3. suffer no hysteresis loss nor eddy current loss.
  4. are light weight when compared to iron cored CTs.
  5. are highly linear over their entire operating range
  6. are inherently safe so there is no danger from an open circuit secondry. With conventional iron cored CTs, the user has to be sure never to allow the secondary of the device to become open circuit otherwise hazardous voltages can be generated and the CT can destroy itself. Modern iron cored CTs have an internal burden resistor so that an open circuit condition is very unlikely to happen unless the coil becomes physically damaged. Never EVER use a damaged iron cored CT – they can kill you!
  7. can be safely connected and disconnected while circuits are in use, they do not load the circuit.
  8. are smaller that an equivalent iron cored CT when large loads are involved.
  9. have a low inductance and can respond more rapidly than iron cored CTs.

Summary

Parameter Iron Cored CTs Rogowski (air core) CTs
Cost Low Medium to High
Linearity Fair Very Good
High current capability Good Very Good
DC and high current saturation problem Yes Not affected
Output affected by temperature Small effect Very small effect
Hysteresis and eddy loss Yes No
Weight Heavy Light
Size Bulky for large currents Often smaller than iron cored for large currents