RCD Keeps Tripping? Find the Real Cause Fast

How Residual Current Devices (RCDs) Work and Why They Trip

A residual current device (RCD), also known as a leakage circuit breaker, trips when it detects a current imbalance of 30mA or more between the live and neutral conductors. In older electrical circuits, or in installations where cables were not run through conduits, it can be extremely difficult to use RCDs effectively. Even if the system works initially, during damp or humid weather, the RCD may trip repeatedly. Identifying the exact cause and location of such leakage is often challenging.

Some suggest simply removing the RCD and replacing it with an air circuit breaker of the same rating—controlling only the live wire while connecting all neutral wires to a common busbar. While this may allow the circuit to function normally without tripping, this practice is extremely dangerous and strongly discouraged. It eliminates critical protection, putting lives and property at serious risk.

The Importance of Residual Current Devices (RCDs)

RCDs are essential safety components in residential electrical systems. They automatically disconnect the circuit when leakage current or ground faults are detected, preventing electric shocks, fires, and equipment damage. In daily use, circuits may occasionally develop faults, causing the RCD to trip. Before resetting the device, it's crucial to identify and resolve the underlying cause to ensure safety.

Below is a detailed explanation of common reasons for RCD tripping.

Why Do RCDs Trip?

RCDs are designed to prevent electrical accidents by cutting off power when dangerous leakage occurs. Tripping can be classified into two categories: normal tripping and abnormal tripping.

1. Normal Tripping

An RCD with a rated tripping current of 30mA will trip if the leakage current in the circuit exceeds approximately 25mA. This level of current is generally safe for humans (not causing fatal electric shock) and does not damage electrical equipment or cause abnormal operation. However, repeated tripping under these conditions indicates an underlying insulation issue that should be investigated.

2. Abnormal Tripping

This type of tripping is caused by defects in the RCD itself and falls into two subcategories: failure to close (reset) and nuisance tripping.

• Failure to Close:
  If the RCD cannot be reset when power is applied but no load is connected, the device itself is likely defective. Do not attempt to repair it yourself. Repaired RCDs must be tested with specialized equipment to ensure proper function. Using a repaired device without testing is unsafe.

• Nuisance Tripping:
  Random tripping—especially at night or when no one is home—indicates poor electromagnetic interference (EMI) immunity. RCDs exhibiting this behavior should be replaced immediately.

Sometimes, normal tripping due to minor leakage (around 25mA) can mimic nuisance tripping. This is often caused by insulation aging, where moisture causes leakage (tripping in damp conditions) but not in dry conditions. The most reliable way to distinguish between these cases is to measure the insulation resistance of the circuit and equipment.

• Standard requirement: Insulation resistance of each conductor must be ≥ 0.5 MΩ.

• If the total measured insulation resistance of the load circuit is less than 8.8 kΩ (calculated as 220V ÷ 25mA = 8.8 kΩ), normal tripping is expected.

Common Causes of RCD Tripping

• Poor Installation
  Loose terminal connections can overheat, oxidize, and damage wire insulation over time. This may cause arcing, burning smells, and voltage drops, leading to circuit breaker operation.

• Defective RCD
  Internal component failure or manufacturing defects can cause malfunction.

• Overloaded Circuit
  When the actual load exceeds the rated current of the circuit breaker—common after installing high-power appliances like air conditioners or water heaters—a replacement with a properly rated breaker is required.

• Leakage or Short Circuit in Appliances or Wiring
  If an appliance is leaking current, simply unplugging it and resetting the breaker may restore power.
  Troubleshooting method:

• Turn off all branch circuits.

• Energize them one by one.

• If the breaker trips when a specific circuit is energized, that branch contains the fault. Isolate and repair it before restoring power.

• Excessively High Supply Voltage
  This is dangerous and typically occurs in "three-phase four-wire" residential systems.
  Check:

• Are both incoming lines live?

• Are neighboring units also tripping?

• Use a multimeter to measure input voltage.
  Never force-reset the breaker. Doing so may destroy appliances or cause a fire.

How to Troubleshoot RCD Tripping

1. Circuit Isolation Method

Follow the sequence: main line → branches → endpoints.

• Disconnect all branch circuits.

• Energize the main line first. If it holds, the main line is fault-free.

• Re-energize branches one by one.

• The circuit that causes tripping upon energizing contains the fault. Focus your inspection there.

2. Visual Inspection Method

Inspect the protected area—including the RCD and connected wiring/equipment—for visible signs of damage. Pay special attention to:

• Corners and bends

• Junctions and splices

• Overhead line crossings

• Areas prone to moisture or mechanical damage

3. Numerical Comparison Method

Use test instruments (e.g., multimeter, insulation resistance tester) to measure voltage, current, or insulation resistance. Compare results with baseline or expected values to locate faults.

Note: If the neutral wire has degraded insulation or is improperly grounded (repeated grounding), it may cause the main RCD to trip frequently while downstream (secondary) RCDs remain unaffected.

4. Test Energizing Method

Used to determine if the RCD itself is faulty:

• Turn off power.

• Disconnect all load-side wires from the RCD’s zero-sequence current transformer.

• Attempt to reset the RCD.

• If it still won’t reset → RCD is faulty (repair or replace).

• If it resets successfully → RCD is functional; the fault lies in the distribution panel or downstream wiring.

Then:

• Disconnect all outgoing circuits.

• If the RCD still fails to hold → fault is in the panel (check wiring, meters, etc.).

• If it holds → fault is in the external circuit. Use the circuit isolation method to locate the exact point.

Safety Reminder:
Never bypass or remove an RCD for convenience. While it may stop nuisance tripping, it removes vital protection against electric shock and fire. Always diagnose and fix the root cause. When in doubt, consult a licensed electrician.