Using a Micrologic Relay in an ACB for Restricted Earth Fault (REF) Protection

Modern ACB protection units such as Micrologic trip units from Schneider Electric allow REF-style protection using residual current measurement and external CT arrangements. While Micrologic isn’t a traditional standalone REF relay, it can be configured to provide zone-based earth fault protection when installed correctly.

🔧 What REF Protection Is Doing in This Application

Restricted Earth Fault protection is designed to:

✅ Detect internal earth faults inside a defined protection zone

✅ Ignore external earth faults downstream or upstream

✅ Provide fast trip times for safety and equipment protection

Typical zones include:

• Generator windings
• Transformer LV windings
• Busbar sections
• Incomer sections of LV boards

⚡ How Micrologic Achieves REF-Type Protection

Micrologic units typically use residual earth fault measurement, meaning:

The relay monitors:

Ia + Ib + Ic (+ In if present) = Residual Current

If the sum ≠ 0 → Earth leakage detected → Trip output operates.

To achieve REF-style protection (restricted zone protection), you normally:

1️⃣ Use External Core Balance CT (CBCT) or Residual CT Arrangement

Installed around:

• All phase conductors
• Neutral (if present)

This ensures only earth faults inside the protected zone are detected.

2️⃣ Wire CT Secondary Into Micrologic Earth Fault Input

Depending on Micrologic version (P / H / X):

You configure:

• Earth fault pickup current (Ig)
• Earth fault delay time (Tg)
• Trip curve characteristics

For REF-style protection you normally use:

✔ Low pickup setting

✔ Fast or instantaneous operation

3️⃣ Define the Protection Zone Physically

The CT location defines the zone:

If CT is installed:

• After breaker → Protects downstream equipment only
• Before breaker → Protects busbar + breaker + connections

Correct positioning is critical.

🧠 Example REF-Type Application Using Micrologic

LV Incomer Protecting Transformer Secondary

Setup:

Transformer → CT Zone → ACB → LV Busbar

Micrologic monitors earth leakage only inside this zone.

If earth fault occurs inside:

✔ Trips instantly

If fault occurs downstream:

✔ Other protection clears fault

✔ ACB stays closed

🔋 Generator Application Example

Micrologic can be used when protecting:

• Generator LV output
• Generator breaker incomer

Using:

• CBCT around generator phase conductors
• Earth fault element configured low sensitivity

This provides early detection of winding insulation faults.

📊 Advantages of Using Micrologic for REF-Type Protection

✔ Integration

No separate REF relay needed in many LV systems.

✔ Cost Effective

Less panel space

Less wiring

Lower install cost

✔ Communication & Monitoring

Modern Micrologic allows:

• Event logging
• Fault recording
• Network comms
• BMS / SCADA integration

⚠️ Important Engineering Considerations

CT Matching

Incorrect CT ratios can cause:

❌ Nuisance tripping

❌ Protection failure

Stability During Through Faults

Must ensure CT saturation doesn’t create false residual current.

Neutral Considerations

If system has neutral:

Neutral must pass through CT window.

Earthing System Type Matters

Different approach for:

• TN-S
• TN-C-S
• IT systems
• Generator island mode

🏗️ Where This Is Commonly Used

✔ LV Incomers feeding critical busbars

✔ Generator incomer breakers

✔ Transformer LV secondary protection

✔ Data centres

✔ Industrial process plants

🛠️ How NKD-Type Integrators Typically Implement This

In real projects this usually involves:

• Selecting correct Micrologic variant
• Designing CT placement
• Setting earth fault pickup + delay
• Functional trip testing
• Injection testing
• Coordination with upstream HV protection

👍 When to Use Dedicated REF Relay Instead

Still recommended when:

• HV transformer differential schemes
• Generator primary protection
• High stability protection required
• Multiple CT schemes required