Overview
The LJZ-φ65/80/100/120/165 is an indoor epoxy resin cast zero sequence current transformer designed for 0.72/3kV insulation level systems operating at 50Hz or 60Hz. It features a fully enclosed cast-resin structure that encapsulates the annular iron core and secondary winding, ensuring high dielectric strength, moisture resistance, and long-term reliability in indoor environments.
This series meets standard requirements for earth fault detection and residual current monitoring in medium-voltage distribution systems. It delivers accurate zero sequence current signals to protection relays under normal and fault conditions, supporting applications such as grounding fault alarms, trip circuits, and small-current grounding system protection in switchgear and cable feeders.
Key Ratings Snapshot
| Parameter | Rating |
|---|---|
| Rated insulation level | 0.72/3kV |
| Rated frequency | 50Hz / 60Hz |
| Rated secondary current | 5A or 1A |
| Protection class | 10P5, 10P10, 10P15, 10P20 |
| Installation location | Indoor |
| Secondary winding test voltage | 3kV AC, 1 min |
Model Designation
- L: Current transformer
- J: For earth protection / grounding protection
- Z: Cast-resin insulated structure
- 65 / 80 / 100 / 120 / 165: Inner hole diameter / cable window size code (in mm)
Construction
The LJZ series adopts a fully enclosed epoxy resin casting structure, with the annular iron core and secondary winding completely embedded in cast-resin material. This construction provides excellent electrical insulation, mechanical robustness, and resistance to humidity and contaminants. The transformer has no exposed conductive parts on the primary side—the three-phase cables pass through the central aperture, serving as the primary conductor. The secondary terminals are accessible via a terminal block mounted on the base, suitable for fixed installation in switchgear, protection panels, or cable rooms. The design ensures stable performance under continuous indoor operating conditions and complies with standard power-frequency withstand voltage requirements for secondary windings.
Technical Parameters
Service Conditions
- Installation: Indoor
- Insulation level: 0.72/3kV
- Frequency: 50Hz / 60Hz
- Ambient temperature: -25°C to +40°C
- Environment: Free from severe vibration, corrosive gas, conductive dust, explosive media, and heavy moisture.
- Cable installation: All three phase conductors should pass through the window together in the same direction without mechanical stress.
Technical Parameters
The LJZ series are indoor epoxy resin cast zero sequence current transformers rated for 0.72/3kV insulation level, designed for operation at 50Hz or 60Hz within an ambient temperature range of -25°C to +40°C. They comply with GB 20840.1-2010 and GB 20840.2-2014 standards and support secondary currents of 1A or 5A with primary current ratios ranging from 50/1 (or 50/5) up to 400/5 or 150/1 depending on model.
Rated Current Ratio and Output
| Model | Protection Class |
5 A Secondary | 1 A Secondary | ||
|---|---|---|---|---|---|
| Rated Current Ratio (A) |
Rated Output (VA) |
Rated Current Ratio (A) |
Rated Output (VA) |
||
| LJZ-65 | 10P5 / 10P10
10P15 / 10P20 |
50/5 to 400/5 | 1 to 40 | 50/1 to 150/1 | 1 to 15 |
| LJZ-80 | 50/5 to 400/5 | 1 to 30 | 50/1 to 150/1 | 1 to 10 | |
| LJZ-100 | 50/5 to 400/5 | 1 to 30 | 50/1 to 150/1 | 1 to 10 | |
| LJZ-120 | 50/5 to 400/5 | 1 to 40 | 50/1 to 150/1 | 1 to 15 | |
| LJZ-165 | 50/5 to 400/5 | 1 to 30 | 50/1 to 150/1 | 1 to 15 | |
Installation & Wiring
Installation Diagram
The LJZ series zero sequence current transformer features a fully enclosed annular cast-resin structure with a fixed base for secure mounting on panels or indoor structures. The three-phase cable group must pass together through the central window in the same direction, ensuring proper clearance and avoiding mechanical stress on the transformer body. Secondary terminals (S1 and S2) are connected to the relay or monitoring device as per the wiring diagram.
Selection & FAQs
Selection Guide
- Confirm the grounding system, maximum residual fault current, relay pickup, and required 10P protection class.
- Select a 65mm, 80mm, 100mm, 120mm, or 165mm window from the complete cable-bundle diameter and clearance.
- Choose the current ratio and 1A or 5A secondary that match the relay input and protection study.
- Calculate relay and cable-loop burden and confirm the available output for the selected model, ratio, and class.
- Verify fixed mounting, same-direction routing of all phase conductors, polarity, grounding, and indoor conditions.
Frequently Asked Questions
Q1: What is the LJZ-65/80/100/120/165 used for?
It supplies residual-current signals to earth-fault relays in indoor switchgear, cable rooms, and feeder protection panels.
Q2: How does it detect an earth fault?
All phase cables pass through one core; any non-zero vector sum produces a secondary signal for the protection relay.
Q3: Which aperture sizes are available?
The solid cast-resin series provides 65mm, 80mm, 100mm, 120mm, and 165mm cable windows.
Q4: What ratio and secondary options can be selected?
Model-specific ratios range from 50/1A or 50/5A to 400/5A, with 1A or 5A secondary output.
Q5: Which protection classes are available?
Options include 10P5, 10P10, 10P15, and 10P20, with burden depending on model and ratio.
Q6: What is the difference from a split-core zero-sequence CT?
The LJZ has a continuous solid core for stable magnetic performance but requires the cable group to be routed through the window during installation.
Q7: Can cable screens or grounding conductors pass through the core?
Their routing must follow the approved residual-current scheme; incorrect screen or earth-conductor routing can cancel or create measured current.