Current Transformer Sizing, Technology Trends & Procurement Intelligence for Global OEMs

The distinction lies in core saturation philosophy and operating magnetic flux range. Metering Current Transformers are designed with high-permeability, low-coercivity materials (such as nickel alloys or high-grade CRGO) that provide supreme accuracy (Class 0.2, 0.2S, 0.5, or 0.5S) between 5% and 120% of rated current. However, to safeguard connected instruments against downstream fault spikes, metering cores saturate early—typically at an Instrument Security Factor (ISF) less than 5 or 10.

Conversely, Protection Current Transformers (such as 5P10, 5P20, or Class PS/PX) are designed to maintain linear transformation during massive system short-circuit currents. A 5P20 CT guarantees that at 20 times the rated primary current, the composite error will not exceed 5%, ensuring that numerical protection relays accurately detect fault magnitudes without saturation-induced tripping delays or relay blindness.

Under normal closed-loop conditions, the secondary current creates a counter-magnetomotive force (MMF) that almost entirely counteracts the primary winding's MMF, leaving only a tiny net magnetizing flux in the core. If the secondary circuit is open-circuited while primary current flows, this opposing MMF drops to zero instantly.

The entire primary current then acts as pure magnetizing current, driving the magnetic core into extreme saturation. This causes steep, high-voltage peak spikes (frequently reaching several kilovolts) across the secondary terminals during zero-crossings. These lethal voltages can break down insulation, generate violent flashover arcs, destroy connected switchgear, and present severe electrical safety hazards to personnel.

Secondary burden (expressed in Ohms or VA at rated current) comprises the total connected external loop impedance, including relay internal coils, digital power meter inputs, terminal block resistance, and the two-way resistance of secondary copper cabling. If the actual burden exceeds the rated CT nameplate VA, the CT requires higher internal core excitation voltage, causing increased ratio error and phase angle displacement.

Interestingly, modern digital meters introduce the opposite challenge: because electronic meters have tiny burdens (often under 0.2 VA), connecting them to legacy 15 VA or 30 VA CTs means the CT operates in an under-burdened state. In precision metering classes (especially non-S classes), this can alter the calibration balance. Specifying a modern 2.5 VA or 5 VA CT matched to the calculated wiring loop preserves optimal accuracy.

To avoid iterative sizing clarifications and ensure competitive project pricing, procurement engineers should provide:

• Rated Primary and Secondary Current: e.g., 800/5A, 1200/1A, or dual ratios like 400-800/5A.
• Insulation Voltage & BIL: System voltage (e.g., 0.72kV for LV, or 12kV with 75kV peak impulse for MV).
• Accuracy Class & Duty: e.g., Class 0.2S (metering), 5P20 (protection), or Class PX (differential).
• Rated Burden: 2.5VA, 5VA, 10VA, or 15VA based on secondary loop calculations.
• Short-Time Thermal (Ith) and Dynamic (Idyn) Withstand: e.g., 25kA for 1s or 40kA for 3s.
• Physical Geometry: Ring/window busbar size, cast-resin dimensions, or split-core constraints.
• Standards: IEC 61869-2, IEEE C57.13, or BS 3938.

Yes. Prima Transformers designs, winds, casts, and tests current transformers in full alignment with international standards like IEC 61869-2 and CE directives. Our 5,500 m² facility features an in-house dedicated current transformer computerized testing panel, allowing comprehensive routine validation including polarity verification, ratio and phase displacement testing, excitation curve analysis, inter-turn overvoltage tests, and high-voltage power frequency dielectric withstand testing before dispatch.