1Standard & Application

Governing standard

The standard changes which classes exist, how the core is characterised (internal EMF vs terminal volts) and whether R_ct sits inside the rating or beside it. Protection function / application CT class

2System

Nominal voltage (kV) Fault / breaker capacity (kA)

System X/R at the fault point Frequency (Hz)

T_p = (X/R) / 2πf is derived from these, and T_p is what sets the transient dimensioning factor a. A placeholder X/R under-sizes the core.

High-speed auto-reclosing (C-O-C-O)Retained flux on the second shot

3CT Nameplate (Vendor Data)

Class designation - paste it as the catalogue prints it A catalogue prints one string, not separate numbers. This fills the class, the rated output, the ALF and the FS below from it. Everything stays editable afterwards.

Rated primary - adopted tap I_pr (A) Every nameplate figure on this card belongs to the ADOPTED TAP, not to the full winding. On a 3000-1500/1 A CT used at 1500 A, R_ct roughly halves with the turns. Have only the full-winding rating? Enter it below for a labelled estimate at this tap - it is never used by any criterion. Rated secondary I_sr (A)

Secondary winding resistance R_ct at 75 °C (Ω) From the vendor's routine test report or the approved drawing, corrected to 75 °C and for the adopted tap. Catalogues do not print it for class P or Class C cores - for PX / PXR / TPS a maximum R_ct is part of the class definition and always exists.

R_ct not available yetVerify against a bound instead of a value

Catalogue ratings (optional) - short-time thermal, dynamic, insulation

Real catalogues (ABB TPU/KOFA, GE) print these; the tool did not collect any of them until now. Ith and Idyn are checked against the system fault current already entered above - Um and BIL are selection data with no checkable counterpart in this tool and are carried through to the report only.

Short-time thermal current I_th, 1 s (kA) Dynamic (peak) current I_dyn (kA peak)

Highest voltage for equipment U_m (kV) Basic insulation level BIL (kV)

Multi-ratio tap derating (optional) - estimate this tap from the full winding

IEEE C57.13 publishes the relaying accuracy class for the FULL winding of a multi-ratio CT. On a lower tap only part of the winding is in use, so R_ct and the terminal-voltage capability are lower, roughly in proportion to the turns in use. Fill these in only when you have the full-winding rating and want a reference estimate for the adopted tap above - the estimate is never used by any criterion, and it assumes a fully distributed winding, which is an approximation a real winding may not meet.

Full-winding rated primary I_pr,full (A) Full-winding R_ct at 75 °C (Ω)

Rated accuracy limit factor (ALF) The number inside the class string: 5P30. Rated output (VA) IEC calls this the rated output; "burden" is the ANSI term and means ohms there.

Rated knee-point voltage V_kn / E_k (V)

4Secondary Circuit

Lead resistance basis

Conductor cross-section (mm²) Route length L (m)

Loop factor k_loop A calculation sheet quoting a "2-way distance" has already accounted for the return conductor in the length: enter that length with k_loop = 1. Entering the one-way run needs k_loop = 2. Getting this wrong doubles or halves every burden-dependent result.

Connected device burden (VA) Contact resistance (Ω)

Relay and meter burdens are quoted in VA and become ohms at the rated secondary current - R = P / I_sn². Set contact resistance to 0 to reproduce a sheet that omits it.

Overrides - reproduce an existing calculation sheet

Every field here is derived by default. Fill one in only to match a legacy sheet; anything overridden is marked as such on the report.

R_lead (Ω) R_bc (Ω)

Operating ALF′ T_p (ms)

5Distance Protection Data

Close-in fault I_SC_A (kA) Zone 1 reach (fraction of line)

Line length (km) Relay resistance (Ω)

Line R1 (Ω/km) Line X1 (Ω/km)

The Zone 1 reach current and the loop X/R both come out of Z_T = Z_s + reach × Z_L, so they can never disagree with each other. Enter the reach current directly below only if the line data is not available. Override I_SC_B at the reach limit (kA) - optional

5Transformer Data

Rating (MVA) Winding voltage (kV)

Leakage impedance Z_k (per unit) Internal fault I_SC_int (kA)

Relay resistance (Ω) Used by the K-factor method only.

5Busbar Data

Maximum through / bus fault I_SC_max (kA) The linearity check (I_SC_max / I_pr ≤ 100) is a limit of the relay's analog input, not of the CT - it applies to every class.

5Restricted Earth Fault Data

Through-fault current I_k,max,thr (kA)

Transformer rating (MVA) Winding voltage (kV)

Leakage impedance Z_k (per unit)

Stabilizing resistor R_stab (Ω) Relay resistance R_r (Ω)

Internal secondary fault current (A) Without these three the peak voltage cannot be evaluated, and the tool reports it as unevaluated rather than as "no Metrosil required".

5Voltage Transformer Data

Winding application

Rated core burden (VA) Secondary phase voltage (V)

Connected instrument burden (VA) Sum of every device on this winding. The cable burden R_lead × I_max² is added by the tool.