Product Overview
The LJZ-φ65 / φ75 / φ110 / φ150 zero sequence current transformer series is an indoor cast-resin insulated current transformer series designed for cable earth fault detection and grounding protection. The products are used in small-current grounding systems with rated frequency of 50Hz or 60Hz and rated voltage up to 35kV. The main difference between the models is the cable window diameter, which is selected according to the outside diameter of the three-phase cable group.
This series uses an annular iron core and cast-resin insulation. During operation, the three-phase cable passes through the central window of the transformer. Under normal conditions, the vector sum of the three-phase currents is close to zero. When a single-phase ground fault occurs, zero sequence current appears and the transformer outputs a signal to the matching earth fault relay. The catalogue reference relay is DD-11/60.
Product Type
| Item | Specification |
|---|---|
| Product name | Cast-Resin Insulated Zero Sequence Current Transformer |
| Model series | LJZ-φ65 / LJZ-φ75 / LJZ-φ110 / LJZ-φ150 |
| Product structure | Indoor annular core, cast-resin insulated zero sequence current transformer |
| Window diameter options | φ65 mm, φ75 mm, φ110 mm, φ150 mm |
| Rated voltage application | Indoor cable grounding protection systems up to 35kV |
| Rated frequency | 50Hz / 60Hz |
| Relay matching reference | DD-11/60 earth fault relay |
| Installation location | Indoor |
| Typical applications | Earth fault detection, grounding protection, residual current monitoring, motor feeder protection, cable line protection |
Product Display
Main Applications
- Indoor cable feeder earth fault protection
- Small-current grounding systems up to 35kV
- Non-effectively grounded neutral systems
- Motor feeder and generator feeder grounding protection
- Single-phase ground fault detection and alarm circuits
- Residual current monitoring for three-phase cables
- Medium-voltage switchgear, protection cabinets, and cable distribution systems
Key Technical Features
- One product series with multiple apertures: φ65, φ75, φ110, and φ150 models are selected according to cable outside diameter.
- Cast-resin insulated structure: The annular core is encapsulated with cast resin to support insulation, mechanical strength, and indoor service reliability.
- Zero sequence current detection: Detects residual current generated by phase current imbalance during cable grounding faults.
- Relay protection output: Designed for earth fault relay input, with DD-11/60 relay matching reference.
- Optional compensation capacitor connection: Some specifications support connection with or without compensation capacitor according to protection sensitivity requirements.
- Indoor fixed installation: Compact base mounting structure for installation inside switchgear, cable rooms, or protection panels.
Working Principle
The LJZ zero sequence current transformer measures the vector sum of the three-phase cable currents. When the three-phase system operates normally, the sum of the phase currents is close to zero and the secondary output is very small. When insulation failure or a single-phase ground fault occurs, zero sequence current flows through the cable system and the transformer secondary winding outputs a signal to the earth fault relay.
The primary insulation is provided by the cable itself. The transformer mainly checks the insulation performance of the secondary winding. According to the catalogue description, the secondary winding can withstand a 3kV power-frequency test voltage to ground.
Model Designation
The model code can be interpreted as follows:
| Code | Meaning |
|---|---|
| L | Current transformer |
| J | For earth protection / grounding protection |
| Z | Cast-resin insulated structure |
| φ65 / φ75 / φ110 / φ150 | Nominal inner hole diameter / maximum cable opening size |
Technical Data
| Item | Specification |
|---|---|
| Model series | LJZ-φ65, LJZ-φ75, LJZ-φ110, LJZ-φ150 |
| Application voltage | Indoor cable systems up to 35kV |
| Rated frequency | 50Hz / 60Hz |
| Insulation structure | Cast-resin insulated annular core |
| Secondary winding withstand voltage | 3kV power-frequency test voltage to ground |
| Relay matching reference | DD-11/60 earth fault relay |
| Installation location | Indoor |
| Application | Cable earth fault detection and grounding protection |
Model Aperture and Application Reference
| Model | Nominal
Window Diameter |
Typical Cable
Selection Purpose |
Structure
Description |
Installation
Note |
|---|---|---|---|---|
| LJZ-φ65 | φ65 mm | Small cable feeder or motor feeder grounding protection | Compact cast-resin annular structure | Select when the three-phase cable group can pass through a 65 mm window |
| LJZ-φ75 | φ75 mm | Medium cable feeder grounding protection | Cast-resin annular structure with compact base | Select when cable group diameter requires a 75 mm window |
| LJZ-φ110 | φ110 mm | Larger cable feeder grounding protection | Cast-resin annular structure with larger cable opening | Select when cable group diameter requires a 110 mm window |
| LJZ-φ150 | φ150 mm | Large cable feeder or higher cross-section cable application | Large-aperture cast-resin annular structure | Select when cable group diameter requires a 150 mm window |
Relay Matching and Sensitivity Reference
| Model | Relay
Type |
Relay Coil
Connection |
Relay Scale
Setting |
Primary
Zero Sequence Current |
Rated
Current Ratio |
|---|---|---|---|---|---|
| LJZ-φ65 | DD-11/60 | Series / Parallel, with or without compensation capacitor according to terminal connection | 15×1, 30×1, 15×2, 30×2 according to wiring mode | 1–3A, 2–4A, or 3–6A according to wiring mode | Subject to relay and wiring configuration |
| LJZ-φ75 | DD-11/60 | Series / Parallel | 15×1 / 15×2 | 2–5A | 50/1A |
| LJZ-φ110 | DD-11/60 | Series / Parallel | 30×1 / 15×2 | 1–3A | 20/1A |
| LJZ-φ150 | DD-11/60 | Series / Parallel, according to project protection scheme | Confirmed by relay setting and protection scheme | Confirmed by technical agreement | Confirmed by technical agreement |
Note: Protection sensitivity depends on relay type, coil connection method, compensation capacitor connection, wiring resistance, and commissioning settings. Final values should be confirmed according to the relay manual and project protection scheme.
Compensation Capacitor and Wiring Reference
| Connection Mode | Terminal
Reference |
Typical
Use |
Remark |
|---|---|---|---|
| Without compensation capacitor | S1 / S2 or a / x according to model terminal marking | Standard earth fault relay connection | Used where the protection scheme does not require compensation capacitor |
| With compensation capacitor | S1 / S3 or a / x according to model terminal marking | Improved sensitivity protection circuit | Used when the relay and protection scheme require compensation capacitor connection |
Service Conditions
- Installation location: indoor
- Applicable system: small-current grounding system up to 35kV
- Rated frequency: 50Hz / 60Hz
- The three-phase cable group should pass through the transformer window together.
- The cable shield, grounding conductor, and compensation capacitor should be connected according to the selected protection scheme.
- The installation environment should be free from severe vibration, corrosive gas, heavy moisture, conductive dust, and serious pollution.
- For special cable diameter, relay setting, compensation capacitor connection, or protection sensitivity requirements, technical confirmation is recommended before ordering.
Standards and Compliance
The LJZ zero sequence current transformer series is designed for indoor earth fault protection and residual current detection applications. The primary insulation is provided by the cable, and the secondary winding insulation can withstand 3kV power-frequency test voltage to ground according to the product description. Final test requirements, relay matching, protection sensitivity, and wiring method should be confirmed according to the project protection scheme and applicable electrical installation regulations.
Installation and Dimensions
The LJZ series uses a cast-resin annular core and fixed base structure. The cable passes through the central opening of the transformer. During installation, all three phase conductors should pass through the same window in the same direction. The selected aperture should provide enough clearance for the cable group without forcing or bending the cable excessively.
Overall Dimensions Reference
| Model | Window Diameter | Overall / Mounting Reference | Mounting Hole | Remark |
|---|---|---|---|---|
| LJZ-φ65 | φ65 mm | Compact cast-resin body; base mounting dimension subject to drawing | 4 mounting holes according to drawing | Confirm final outline drawing before cabinet layout |
| LJZ-φ75 | φ75 mm | Approx. 165 mm height, 110 mm base width, 48 mm body depth according to catalogue reference | Mounting slot / hole according to drawing | Final dimensions subject to approved drawing |
| LJZ-φ110 | φ110 mm | Approx. 265 mm height, 250 mm base length, 120 mm base width according to catalogue reference | 4-φ14 according to drawing | Final dimensions subject to approved drawing |
| LJZ-φ150 | φ150 mm | Large-aperture structure; dimensions subject to selected drawing | According to final drawing | Confirm before production or cabinet design |
Unit: mm. Final dimensions, mounting holes, terminal positions, and body depth are subject to the approved outline drawing.
Connection and Wiring Notes
- All three phase conductors should pass through the transformer window together in the same direction.
- Confirm whether the protection scheme uses compensation capacitor connection before wiring.
- Connect relay coil terminals according to the relay wiring diagram and the transformer terminal marks.
- Check the relay scale setting, wiring resistance, primary zero sequence current setting, and sensitivity before energizing.
- Keep secondary wiring reliable and avoid loose terminal contact during protection operation.
- Commissioning should include insulation check, wiring check, and protection function verification.
Installation and Safety Notes
- Confirm the model, aperture size, cable outside diameter, relay type, wiring mode, and protection sensitivity before installation.
- Select an aperture that allows the cable group to pass through the window without mechanical stress.
- Fix the transformer securely on the base or panel using the mounting holes.
- Keep the cable centered as far as possible in the annular window.
- Check secondary terminals, relay wiring, compensation capacitor connection, and grounding before commissioning.
- Installation and maintenance should be performed by qualified electrical personnel.
Ordering Information
Please provide the following information when ordering or requesting a quotation:
- Product model: LJZ-φ65, LJZ-φ75, LJZ-φ110, or LJZ-φ150
- Maximum outside diameter of the three-phase cable group
- Rated system voltage and rated frequency
- Relay type, such as DD-11/60, or relay input specification
- Required primary zero sequence current value and protection sensitivity
- Wiring mode: with or without compensation capacitor
- Installation quantity and switchgear / cable feeder application
- Special requirements for terminal marks, drawings, inspection documents, certificates, or packaging
Selection Guidelines
- Confirm cable outside diameter: Select φ65, φ75, φ110, or φ150 according to the maximum outside diameter of the three-phase cable group.
- Confirm grounding system: Use the LJZ series for small-current grounding systems requiring earth fault detection.
- Confirm relay matching: Select the relay coil connection and scale setting according to the DD-11/60 relay or the specified relay.
- Confirm compensation requirement: Choose with or without compensation capacitor connection according to the protection scheme.
- Confirm sensitivity: Check that the primary zero sequence current setting meets the required protection sensitivity.
- Confirm installation space: Check aperture size, body dimensions, mounting holes, and cable bending radius before cabinet layout.