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A Zero Sequence Current Transformer (ZSCT) , also called a core balance CT or residual current CT, detects ground fault currents in three-phase systems. Unlike standard CTs on individual phases, a ZSCT encircles all three phase conductors (and neutral if present). Under normal balanced conditions, the vector sum of currents equals zero. When a ground fault occurs, the unbalanced current produces a secondary output proportional to the fault current. This signal goes to a protection relay (e.g., 50N/51N or 50G/51G) which trips the breaker. Typical applications include solidly grounded, low-resistance grounded, or high-resistance grounded systems from 120V to 35kV.
| Função | Descrição |
|---|---|
| Ground Fault Detection | Senses residual current from 50mA to 100% of rated primary current. Outputs analog signal (mA or A) to relay. |
| High-Impedance Fault Identification | Detects low-magnitude faults (e.g., 1–5A) that standard overcurrent relays may miss. |
| Directional Ground Fault Indication | When paired with voltage reference, helps locate fault direction on radial systems. |
| Harmonic Filtering (Built-in) | Some models include low-pass filtering to ignore high-frequency noise (e.g., from VFDs) above 300Hz. |
| Split-Core for Retrofit | Allows installation without disconnecting primary cables. Hinged or U-shape design. |
| Relay Interface | Provides 5A or 1A secondary output (standard) or 0–10V output for electronic relays. |
This table helps engineers and procurement teams quickly match the right zero sequence current transformer to their system requirements.
| Parâmetro | Typical Range / Value |
|---|---|
| Primary Current (Residual) | 1A to 1000A (continuous); 10× rated for 1 second |
| Secondary Output | 5A, 1A, 0.333V, 0–10V (selectable by model) |
| Inner Diameter | 30mm, 60mm, 100mm, 150mm, 200mm, 300mm |
| Core Material | Nanocrystalline (for low current) or silicon steel (for high current) |
| Classe de precisão | 1, 3, 5 (per IEC 61869-2 or IEEE C57.13) |
| Frequência | 50/60 Hz (wideband version up to 400Hz) |
| Max Voltage | 720V AC (low voltage) / 35kV (medium voltage with insulation) |
| Temperatura de funcionamento | -25°C to +70°C |
| Enclosure Rating | IP20 (indoor) / IP65 (outdoor with seal kit) |
| Dielectric Withstand | 3kV for 1 minute (primary to secondary) |
| Montagem | Panel mount, busbar mount, or cable clamp |
Where a Zero Sequence Current Transformer Solves Ground Fault Blind Spots
A standard phase CT won’t tell you if a motor winding is leaking 2A to ground. A Transformador de corrente de sequência zero catches exactly that. Below are typical installations where our ZSCT helps locate arcing ground faults before they cause a fire, works with existing relays (GE, SEL, ABB), and eliminates nuisance trips caused by leakage currents in cable shielding.
Uses nanocrystalline or high-permeability silicon steel. Detects residual currents as low as 10mA. This Zero Sequence Current Transformer maintains linearity from 1mA to 200A, covering both leakage and fault ranges.
Built-in electrostatic and magnetic shielding. Reduces false trips caused by nearby busbars or motors. A reliable Zero Sequence Current Transformer must reject common-mode noise — ours achieves >60dB rejection at 50Hz.
Available from 30mm to 300mm opening. Fits single cables, busbars, or multiple conductors. You don’t need to disconnect cables; the split-core version clamps around existing wiring in under 2 minutes.
Rated for -25°C to +70°C continuous. No phase shift drift over 20 years. This Zero Sequence Current Transformer uses low-loss material, so accuracy remains ±1% from -10°C to +60°C.
Same mounting holes as ABB, SEL, or GE models. Direct replacement without drilling new holes. Comes with standard 4-meter twisted pair leads; extension up to 100m without signal degradation.
Every unit is tested with a primary injection report. ZHIMING engineers help with relay matching. We also provide a free sizing tool — just enter cable diameter and fault current, and the correct Zero Sequence Current Transformer is selected automatically.
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Yes. For solidly grounded systems, the ZSCT detects low-level ground faults (e.g., 5–20A). Choose a lower ratio (e.g., 50:5) for better sensitivity.
A ZSCT has one core encircling all phases, giving better balance and higher accuracy for low currents (<5% of CT rating). Three-CT residual method has larger errors due to phase CT mismatch.
Yes, but select secondary output accordingly: 5A for electromechanical (e.g., GE IAC), 1A or 0.333V for electronic relays. Our model has dual taps.
Use a ZSCT with a higher pickup threshold (e.g., 500mA instead of 100mA) or add a time delay (0.5–1s) in the relay. Do not filter below 50Hz.
Minor eccentricity (<10% of ID) has negligible effect on accuracy for fault-level currents (>1A). For low-level leakage (<100mA), keep conductors roughly centered to avoid stray field errors.
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