Ratios look simple; the classes are not
Current transformers (CTs) and voltage transformers (VTs) let instruments and relays work with defined secondary quantities instead of MV currents and voltages. Their ratios look simple, 100/5 A or 20000/√3 : 100/√3 V, but the accuracy class, the connected burden and the intended use matter as much as the ratio.
1. Current transformer basics
For a 100/5 A CT, 100 A primary gives 5 A secondary and 50 A gives 2.5 A. This is the nominal ratio, not a statement of accuracy at every burden and fault current. The external load on the secondary, its leads, instruments, relays and terminals, is the burden, usually stated in VA at rated secondary current.
2. Burden, and why 1 A helps on long runs
20 m one way of 2.5 mm² copper at 20 °C (ρ = 0.0175 Ω·mm²/m) is 0.280 Ω. A 5 A CT then spends 25 × 0.280 = 7 VA in the leads; with a 1 VA relay the total is 8 VA, within a 10 VA rating as a simplified screening. A 1 A CT spends only 0.28 VA in the same leads: at equal resistance the 5 A wiring burden is 25 times the 1 A burden.
Warm leads have more resistance (the calculator corrects for temperature in Advanced: 7.0 VA at 20 °C becomes about 8.5 VA at 75 °C). The factor 2 is a dedicated two-wire loop. With three star-connected CTs and a common return, a phase fault sees one lead length and an earth fault sees two; the calculator lets you choose.
3. Measuring classes and the security factor
IEC measuring classes 0.1, 0.2, 0.5 and 1 limit the ratio error and phase displacement at specified current and burden points; class 0.2 does not mean exactly 0.2 % everywhere. The special classes 0.2S and 0.5S tighten the limits at low currents, which matters for energy metering at light load. The conditions come from IEC 61869-2 and the CT certificate. A measuring CT may have a security factor FS that limits the current passed to instruments during overcurrents; FS and the protection ALF serve different purposes.
4. Protection classes 5P and 10P
| Nameplate part | Meaning |
|---|---|
| 100/5 A | rated primary and secondary currents |
| 10 VA | rated burden at which the class claims apply |
| 5P | protection class: composite error at most 5 % at the accuracy limit |
| 10 | rated accuracy limit factor: 10 × rated primary current (1000 A here) at rated burden |
10P allows a 10 % composite error at the same limit. Neither says the CT measures correctly at higher multiples: saturation grows with excitation. The effective ALF changes with the actual burden; estimating it needs the secondary winding resistance Rct:
This is a resistive approximation. Without Rct the calculator says “ALF not evaluated”, whatever the VA comparison shows.
5. PX, transient classes and saturation
PX-type CTs are specified by the excitation characteristic and knee-point voltage, the winding resistance and the excitation current; they matter for differential and high-impedance schemes. Transient classes and remanence need still more data. During severe faults the secondary current may stop following the primary; CT matching, remanent flux, DC offset and relay inputs all play a part. None of this follows from a generic 5P calculation.
6. Never open an energized CT secondary
A CT whose primary carries current while its secondary is open can develop dangerous voltages. Use suitable shorting links and approved test procedures, qualified personnel only. This is the opposite of the VT rule below.
7. Voltage transformer basics
A matched notation gives the nominal ratio because the √3 terms cancel. A nameplate may also mix notations: for (20000/√3) : 100 V the ratio of the two winding voltages is 11,547 / 100 = 115.47. KiloVolt always calculates the ratio of this winding from its winding voltages and says what it means. Some VTs have separate measuring and residual (open-delta) windings for earth-fault protection, written for example 100/3 V; each winding has its own ratio, rating and burden, and they must not be added into one pool. The ratio of one …/3 winding is not the residual-voltage relationship of the complete broken-delta connection of three windings, which depends on the connection and the network earthing and is not calculated here.
8. VT measuring and protection classes
Measuring classes 0.1, 0.2, 0.5, 1 and 3 define voltage-error and phase-displacement limits over declared voltage and burden ranges, not a constant error. Protection classes 3P and 6P define accuracy under the rated voltage factor and burdens; the P means protection, not three-phase or a pole count. The voltage factor, insulation arrangement, earthed or isolated primary and the earth-fault voltage rise all matter; a nominal 100 V secondary does not show that a VT suits 59N or other zero-sequence functions.
9. VT burden: under-loading can matter too
A 30 VA VT with 5 VA connected is 16.7 % loaded. That is a useful number but not an accuracy pass: IEC 61869-3 defines accuracy within burden ranges (for example range I or range II), and an under-loaded VT can sit outside the range in which its class was proven. The calculator therefore says “accuracy NOT VERIFIED”. Fuses, leads, meters and relay inputs also cause a secondary voltage drop.
10. Never short-circuit a VT secondary
A VT secondary must not be short-circuited; use the rated secondary fuses or protection and the specified earthing arrangement.
11. Reading typical nameplates
- CT 100/5 A, 10 VA, 0.5: nominal measuring ratio and metering class.
- CT 100/1 A, 10 VA, 5P10: protection CT, 1 A secondary, accuracy-limit multiple 10 at rated burden.
- VT 20000/√3 : 100/√3 V, 30 VA, 0.5: phase-to-earth measuring VT.
- VT … 3P or 6P: protection class; the voltage factor and burdens are needed too.
The calculator shows ratios, lead resistance, burden and class explanations. It does not verify protection-CT saturation or metering accuracy. The MV Transformer Protection Settings tool uses a CT ratio for its relay reference pickups; that does not validate the CT either.
IEC 61869-2 (IEC webstore) · IEC 61869-3 (IEC webstore) · Schneider Electric: accuracy limit factor