Introduction
Current transformers (CTs) are among the most critical — and most misunderstood — components in low voltage protection systems. Select the wrong type or accuracy class, and your protection relay will either fail to trip during a real fault or nuisance-trip during normal operation. Either outcome is costly.
This guide explains the three main types of current transformers used in industrial LV systems: the standard three-phase current transformer (CT), the zero-sequence current transformer (ZCT), and the residual current transformer (LZT).
What Is a Current Transformer?
A current transformer produces a reduced, proportional secondary current from a high primary current, allowing protection relays, energy meters, and monitoring equipment to safely measure currents that would otherwise be too large to measure directly. CTs serve two distinct functions: Metering CTs (accuracy classes 0.1–1.0, saturate during faults to protect meters) and Protection CTs (accuracy classes 5P, 10P, must remain linear during faults so the relay receives an accurate signal).
Type 1: Three-Phase Current Transformer (CT) — WLCT
A standard three-phase CT set consists of three individual CTs, one per phase (L1, L2, L3), feeding a protection relay or energy meter. Construction types include bar-type (most common in switchgear), wound-type (for low primary currents below 100A), and split-core (for retrofit installations without disconnecting cables).
Key Selection Parameters
- Primary rated current (Ipn): 50A–5000A. Must match or exceed maximum continuous circuit current.
- Secondary rated current (Isn): 1A for long cable runs (>10m); 5A for short runs and legacy relays.
- Accuracy class: 0.5 or 1.0 for metering; 5P or 10P for protection.
- Accuracy limit factor (ALF): A 5P10 CT maintains <5% composite error up to 10× rated current.
- Burden (VA): Must exceed total burden of connected relays, meters, and cable resistance.
- Thermal short-time current (Ith): Must exceed the prospective fault current at the installation point.
Type 2: Zero-Sequence Current Transformer (ZCT)
A ZCT is a single toroidal CT through which all three phase conductors (and neutral in 4-wire systems) pass together. In a balanced system the vector sum is zero, so secondary output is zero. When a ground fault occurs, the residual current creates a net flux that triggers the earth fault relay — detecting faults as low as 30mA while being insensitive to normal load currents.
ZCT vs Residual Connection of Standard CTs
Three standard CT secondaries in a Holmgreen connection can detect earth faults, but CT mismatch errors limit sensitivity to 10–20% of rated current, and saturation can cause nuisance tripping. A dedicated ZCT provides sensitivity down to 1–5% of rated current and eliminates both problems.
Key Selection Parameters for ZCT
- Window diameter: Must accommodate all phase and neutral conductors including insulation
- Secondary rated current: 1A or 5A, matched to the earth fault relay input
- Sensitivity: For 100mA detection, specify a high-ratio ZCT (e.g., 50/1A) with a sensitive relay
- Accuracy class: 5P or 10P for protection applications
Type 3: Residual Current Transformer (LZT)
The LZT uses the same core-balance principle as the ZCT but is designed specifically for RCDs and ELCBs per IEC 62020 and IEC 60755. It is optimized for very high sensitivity (30mA–500mA) and defined response to DC components (Type A: pulsating DC; Type B: smooth DC).
LZT vs ZCT Comparison
| Parameter | ZCT | LZT |
|---|---|---|
| Application | Industrial earth fault protection (relay-based) | RCD/ELCB (personnel & equipment protection) |
| Sensitivity | 1A–100A typical | 10mA–500mA typical |
| Standard | IEC 61869-2 | IEC 62020, IEC 60755 |
| DC response | Not specified | Type A / Type B defined |
When to Use LZT
- Personnel safety (30mA shock protection)
- Fire protection (300mA leakage detection)
- Medical locations per IEC 60364-7-710
- EV charging requiring Type B RCD
IEC Standards Reference
- IEC 61869-1/2: Current transformers (replaces IEC 60044-1) | GB/T 20840.1/2: Chinese equivalent
- IEC 62020 / IEC 60755: Residual current transformers for RCD applications
Accuracy Class Quick Reference
| Class | Application | Max error at In |
|---|---|---|
| 0.1, 0.2 | Precision metering | 0.1%, 0.2% |
| 0.5, 1.0 | General metering | 0.5%, 1.0% |
| 5P, 10P | Protection relays | 1% at In, linear to ALF×In |
| PX, PXR | Differential protection | Defined by knee-point voltage Vk |
Common Installation Mistakes to Avoid
- Exceeding CT burden: Always calculate total burden and verify it is within the CT rated burden.
- Open-circuiting a CT secondary: Never open-circuit while primary is energized — the resulting high voltage is lethal. Always short-circuit the secondary before disconnecting any device.
- Wrong polarity: Observe CT polarity markings (P1/P2, S1/S2). Reversed polarity causes incorrect relay operation.
- Using a metering CT for protection: Metering CTs saturate during faults. Always use 5P or 10P class for relay applications.
- ZCT window too small: Measure the cable bundle carefully. Forcing cables through an undersized window damages insulation.
Wanlong Electric Current Transformer Products
- WLCT Three-Phase CT — Bar-type, wound-type, split-core. Primary 50A–5000A, secondary 1A/5A. Classes 0.5, 1.0, 5P, 10P. IEC 61869-2 / GB/T 20840.2.
- ZCT Zero-Sequence CT — Toroidal core-balance. Window 30–200mm. High-sensitivity earth fault detection. Class 5P, 10P.
- LZT Residual CT — For RCD/ELCB applications. Sensitivity from 30mA. Type A and Type B compatible. IEC 62020, IEC 60755.
All products manufactured under ISO 9001. OEM/ODM available — custom ratios, window sizes, and enclosure ratings on request. For datasheets or a quotation, contact our engineering team. Response within 24 hours.
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