333mV vs mA CTs

Elkor's meters can be ordered to work with 5A, mA, 333mV or Rogowski Coil CTs.  While we strive to give clients maximum choice and flexibility, we feel that 333mV CTs have specific deficiencies (especially against comparable mA CTs).  This includes:

  1. 333mV is not a defined standard
  2. There is an error in the definition alone.  Is it 333mV or 1/3V (ie: 0.333333333333)?  This alone constitutes a 0.1% error.
  3. Different meters have different input impedance ratings.  Ideally, the meter should have a very high input impedance to not distort the limited voltage output of the CT, but this is not standardized between manufacturers.  The CTs output must be calculated/calibrated with a particular input impedance, and if this is not standardized, the CT may perform differently across various meters.  
  4. CTs are current output devices at their core, and to generate the 333mV, manufacturers install a “burden” resistor internally.  It is extremely rare for manufacturers to precision trim these resistors, so errors due to selecting an exact value (resistors come in limited value ranges) are compounded by the fact that resistors typically come in a 1% tolerance.
  5. Due to the low output signal voltage and high loop impedance, 333mV CTs are prone to noise pickup, especially over long-distance runs.
  6. Depending on the turns ratio and required input-to-output rating, the burden resistor may need to be quite large.  CTs have limited output energy and the larger the burden resistor, the less linear the CT output.  This will also have an affect on CT phase shift, which manifests as errors in power factor and therefore power & energy readings.  This is especially true when the input current is low.
  7. Long wire runs create a voltage divider further degrading the measurement accuracy.

Elkor's mA output architecture has the following advantages:

  1. It allows the meter to have a very low input impedance.  This keeps the CTs in its linear range for wider, as well as alleviates the noise issue and allows for long wire runs.
  2. We negate the requirement for having multiple CT ratios/ratings within the same physical size.  Because of #1 above, it means that we can use the same CT for accurate measurements of (ex: 10A or 600A), without having multiple models.
  3. Since our burden is inside the meter, it is calibrated as part of the calibration process and removes the resistor as a source of error.

While Elkor does sell meters that can accept 333mV CTs, we feel that the mA architecture is superior to the 333mV “pseudo-standard” for the aforementioned reasons.