Earth Leakage Relays (ELRs) are electrical protection devices used to detect leakage current flowing from live conductors to earth and disconnect the power supply before the leakage can cause electric shock, fire, or equipment damage.

Below is a clear, practical overview.

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What an Earth Leakage Relay Does

An ELR continuously monitors the current balance between:

• Live (phase) and
• Neutral

Under normal conditions, current flowing out equals current returning.
If some current leaks to earth (through insulation failure, moisture, or a human body), the imbalance is detected.

👉 When the leakage exceeds a preset value, the relay trips a circuit breaker or contactor.

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Why ELRs Are Important

• Protect people from electric shock
• Prevent electrical fires caused by insulation breakdown
• Protect equipment from earth faults
• Often mandatory in industrial, commercial, and residential installations (including Singapore)

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Common Types of Earth Leakage Protection

1. RCCB / ELCB (for people protection)

• Also called Residual Current Circuit Breaker
• Trips directly without an external contactor
• Typical sensitivity:
  • 30 mA → personal shock protection
  • 100–300 mA → fire protection
• Used in homes and small buildings

2. Earth Leakage Relay (core-balance CT type)

• Uses an external core-balance current transformer (CBCT)
• Relay output trips:
  • MCCB, ACB, or contactor
• Adjustable settings:
  • Leakage current (e.g. 100 mA – 30 A)
  • Time delay
• Used in industrial and commercial systems

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Main Components in an ELR System

1. Core Balance CT (CBCT)
   • All phase and neutral conductors pass through it
2. Earth Leakage Relay
   • Processes the signal from the CT
3. Trip Device
   • Circuit breaker, shunt trip, or contactor
4. Reset/Test Circuit
   • For periodic testing

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Typical Applications

• Factories and plants
• Data centers
• Construction sites
• Large HVAC systems
• Marine and offshore installations

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Key Settings to Know

• IΔn (Leakage Current Setting):
  Example: 300 mA, 1 A, 5 A
• Time Delay:
  Instantaneous or delayed (to avoid nuisance tripping)
• Trip Mode:
  Latching or non-latching

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Common Causes of Earth Leakage

• Damaged cable insulation
• Moisture ingress
• Ageing motors or heaters
• Incorrect wiring
• Accumulated small leakages in large systems

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Testing & Maintenance

• Test using the built-in test button(monthly recommended)
• Primary injection testing for industrial ELRs
• Check:
  • CT orientation
  • Neutral, not bonded to earth downstream
  • No parallel earth paths

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ELR vs RCCB (Quick Comparison)

Feature	ELR	RCCB
Adjustable sensitivity	✅	❌
Time delay	✅	❌
External CT	✅	❌
Industrial use	✅	❌
Direct shock protection	⚠️ (depends on setting)	✅