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THD Registers

THD is a measurement of the harmonic distortion of a signal given it's fundamental frequency, and thus may be larger than 100%. If there is insufficient amperage on a CT or the circuit is not active, the noise may calculate to high percentages.

THD is typical on current waveforms depending on the load and is not always a cause for concern.

The eGauge can calculate Total Harmonic Distortion (THD) on a given CT or voltage reference. Each THD measurement will use one physical register.

The eGauge can measure up to the 10th harmonic. To measure THD,

  1. Navigate to Settings -> Installation
  2. Under Registers, click the "Add Registers" button
  3. Name the register as appropriate
  4. Using the dropdown menu, change the register type from P to =
  5. A second dropdown menu will appear.  Select the unit type to "total harm. dist" or "THD [%]"

In the formula field, enter the following:  THD(Hz, chan("XX")) where Hz is the system frequency in Hz and XX represents either a CT (CT1, CT2, CT3, etc) or a voltage reference (L1, L2, L3).

Click "Save" at the bottom of the page when finished (valid credentials required)

Sample THD Register for CT1 on a 60Hz system

Firmware 4.0 and later:

CTx channel names have been replaced with Sx channel names. Thus, instead of using chan("CT1"), use chan("S1"). Configuration is otherwise identical:


See also