Finding Shorts in Electrical Wiring Circuits: Earth Faults in Radial Circuits

Key Definitions

Short Circuit: Low resistance path between line and neutral, or between phases in a three-phase system.
Earth Fault: Low resistance path between line and Earth (or CPC) or between phase and Earth in a three-phase system.
CPC (Circuit Protective Conductor): The earth conductor in a twin and earth cable.

Circuit: Radial cooker circuit, 6mm² by 2.5mm² twin and earth cable, circuit length of 90m in one direction.
Challenge: Test meter probes have a lead length of 1m, insufficient for directly measuring conductors.

Use a Wago connector at the point of use (e.g., cooker) as a return path.
Measurements:
- Line to Neutral: 0.12 ohms
- Line to CPC: 0.2 ohms
- Neutral to CPC: 0.2 ohms
Tables for Verification: Use Table B1 in Guidance Note 3 or Table I1 in the On-Site Guide.

Cable Specifications: 6mm² (line) by 2.5mm² (CPC) twin and earth.
Resistance Calculation: Combined resistance for 6x2.5 copper cable: 10.499 milliohms per meter.
Formula:
- Total resistance = milliohms per meter x cable length.
- Calculate length if resistance is known.
Expected Results:
- 19m should return a low ohms reading of 0.2 ohms.
- Resistance for line to neutral: 6.16 milliohms per meter for both 6mm² line and neutral.
- Line and neutral calculated resistance for 19m: 0.117 ohms.

Scenario: RCD or MCB tripping, indicating potential faults.
Fault Indicators:
- Line to Earth faults may trip RCD.
- Line to Neutral faults may trip MCB.
Example Fault: Nail piercing the cable about halfway, creating an earth fault.

Method: Test with two conductors at a time.
Comparison:
- Line to Neutral and Neutral to CPC readings may resemble healthy circuit.
- Line to CPC shows abnormal reading indicating fault.

Method: Use resistance values and Table B1 or I1 to calculate distance to the fault.
Example Calculation: Fault located approximately 9.5m from the consumer unit.

Formula Variants: For resistance or length calculations based on known data.
Tables Usage: Familiarize with on-site guide and guidance note tables.
Testing Tips:
- Use connectors for return paths.
- Test between two conductors to isolate faults.
- Record test results; avoid relying solely on memory.

Understanding and practicing with tables and formulas is crucial for accurate fault diagnosis and circuit testing.

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