= Current-transformer burden
{title2=$Z_b$}
The <current-transformer burden> is the load impedance connected to a <current transformer>'s secondary, including the instrument and secondary connection leads. It determines the <electric potential difference> needed to drive the secondary <electric current>. Changing it can change both <current-transformer ratio error> and <current-transformer phase displacement>. ABB's application guide explicitly connects these errors to the secondary burden and distinguishes standard burdens from actual connected devices.<Instrument Transformers: Technical Information and Application Guide by ABB>[Technology Review, Burdens, printed p.13]{ref}
For sinusoidal RMS secondary current $I_s$ and external burden $Z_b=R_b+iX_b$, the secondary terminal relations are
$$
V_s=I_sZ_b,\qquad S_b=|I_s|^2Z_b,\qquad |S_b|=|I_s|^2|Z_b|.
$$
Here $S_b$ is <complex electrical power>; its real part is dissipated <active power>. A volt-ampere burden magnitude alone does not determine its <power factor> or its complex <complex impedance>.
For an original resistive example, an instrument contributes $0.10\,\Omega$ and the complete outgoing-plus-return leads contribute $0.20\,\Omega$. At $5\,\mathrm A$ RMS, total burden is $0.30\,\Omega$, terminal magnitude is $1.5\,\mathrm V$, and burden <apparent power> is $7.5\,\mathrm{VA}$. The instrument dissipates $2.5\,\mathrm W$ and the leads dissipate $5\,\mathrm W$. Ignoring the leads would understate the required terminal magnitude by a factor of three. A stipulated rating of 5 VA at 5 A corresponds to only $0.20\,\Omega$ magnitude, so this setup would not satisfy that stated burden limit.
Equal VA values also need not mean equal complex loading. At 5 A, $Z_b=0.20\,\Omega$ and $Z_b=0.12+0.16i\,\Omega$ both have 5 VA magnitude. The second instead absorbs $3\,\mathrm W$ and $4\,\mathrm{var}$, with <power factor>0.6. The resulting phase and ratio effects depend on the transformer's own response.
The secondary winding's internal impedance is not part of the external burden, but both enter the induced-voltage and excitation calculation. NIST's equivalent circuit separates these terms explicitly.<A Calibration Service for Current Transformers by John D. Ramboz and Oskars Petersons>[§1.2, printed p.4, Figure 2 and equation 2]{ref} A proposed calibration comparison records the complete connected impedance at the relevant <frequency>, including leads, and repeats ratio and phase measurements at selected burdens. It does not infer unchanged accuracy merely because the nominal turns remain the same.
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