CT / PT sizing — ratio, accuracy class, rated burden, and the checks behind them

Give the system voltage, the circuit maximum load current, the bus short-circuit current and the secondary circuit (relay burden, cable section and length, and the CT winding resistance from the data sheet). You get the recommended CT ratio, the accuracy class and the rated burden (VA) — each one chosen from the standard tiers by an engine-computed check, not from a table copied here — plus the item-by-item verification: ratio limits, secondary burden, accuracy limit factor (saturation boundary), short-time thermal duty and dynamic withstand. Where the same data also sizes a voltage transformer, the PT ratio, class and burden come out of the same run.

CT / PT sizing — ratio, accuracy class, rated burden and the checksEngine basis: IEC 61869-2 / IEC 61869-3. Steady-state r.m.s. simplification — no transient saturation waveform.

System voltage class (kV)ApplicationAccuracy requirement (error class)CT secondary rated current (A)Circuit maximum load current (A)CT primary rating (A) — blank = engine suggestionBus short-circuit current Ik (kA)

CT secondary winding resistance Rct (Ω, datasheet) — blank = burden not checkableRelay / meter secondary burden (VA)CT secondary cable section (mm²)CT secondary cable length (m)CT rated short-time thermal current Ith (kA) — blank = engine standard tierIdyn (kA)

Include a PT check

PT primary connectionPT secondary rated voltage (V)PT secondary burden (VA)PT lead section (mm²)PT lead length (m)PT voltage factor

Protection composite-error class 5P / 10P; the accuracy limit factor ALF is recommended separately from the engine result.

Standards: IEC 61869-2 (current transformers — rated primary current series §5.201, secondary values §5.202, priority outputs §5.5.201, accuracy limit factor §3.4.208, secondary limiting e.m.f. §3.4.209) · IEC 61869-3 (voltage transformers — primary/secondary voltages §5.301, voltage factors §5.302) · IEC 60038 system voltages · IEC 60228 conductor resistance. Deliberately not modelled, and stated as such on the page: transient saturation (remanence, TPX/TPY/TPZ, K_td, d.c. component), 0.2S/0.5S metering error limits and ratio/phase-error point-by-point calculation, secondary circuit insulation and screening, ferroresonance.