Questions and Answers
What is the primary purpose of earthing the neutral of a transformer in an earth star distribution system?
What is the advantage of having an earthed star point in an earth star distribution system?
What is the minimum size of the main earthing conductor?
What is the purpose of the main earthing terminal / connection or bar?
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What is the consequence of a short-circuit fault in an earth star distribution system without an earthed neutral?
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What is the advantage of having a common earth reference between the distribution transformer neutral and the consumer's earthed equipment?
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What is the purpose of measuring the resistance of the main earthing conductor?
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What is an extraneous conductive part?
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What is the purpose of equipotential bonding?
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What is the consequence of a phase to earth fault in an earth star distribution system?
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Which of the following is a circumstance where dangerous potentials may be present on extraneous conductive parts?
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Which of the following extraneous conductive parts needs to be bonded?
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What stipulates the type of conductors and their size to be used for equipotential bonding?
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Why is equipotential bonding necessary?
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What is the result of equipotential bonding?
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What is the purpose of connecting the bonding conductor to the earthing system of the installation?
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What is the primary purpose of earthing in an electrical installation?
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According to AS/NZS 3000:2018, where can the definitions of terms used in the standard be found?
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What is the term for a situation where someone can touch exposed conductive parts while being in contact with the earth?
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Which of the following regulations deals comprehensively with earthing arrangements and earthing conductors?
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What is the term for a conductor that is more than 2.0m from the ground?
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Why is the earth considered the most important conductor in an electrical installation?
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What happens when a hot joint occurs in the neutral connection of the service mains?
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What is the consequence of the fault current flowing through the ground?
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What is the purpose of the equipotential bonding conductor?
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Why is it dangerous to touch the shower head or tap while standing in the shower?
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What happens when the fault current flows through the earth?
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What is the term for the metallic pipe that connects the shower head and tap throughout the house?
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What is the purpose of the earth electrode in an electrical installation?
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What is the MEN system of earthing used for?
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What is the USE of Table 5.1?
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What is the requirement for the connection of the earth electrode?
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What is the purpose of the P.V.C.pipe in the earth electrode installation?
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What is the type of electrode that must be driven to a depth of 1.2m?
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Which of the following is NOT connected to the protective earth conductor?
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What is the requirement for the location of the earth electrode?
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Study Notes
Earthing
- Earthing is essential for personnel safety and equipment protection, particularly during fault conditions.
- Section 5 of the wiring rules deals comprehensively with earthing arrangements and earthing conductors.
Earthing Terms
- Readily Accessible: >2.0m from ground.
- Earthed: equipment or conductor connected to the general mass of earth.
- Earthed situation: any situation where someone can touch exposed conductive parts while being in contact with the earth.
- Extraneous conductive parts: conductive parts that do not form part of the electrical installation, but may be at the electrical potential of a local earth.
Extraneous Conductive Parts
- Examples: metal waste, water, or gas pipes from outside, cooling or heating system parts, metal or reinforced concrete building components, steel-framed structures, etc.
- These parts may be at a different potential than the electrical installation, posing a safety risk.
Equipotential Bonding
- Connects an earth cable to an extraneous conductive part.
- Ensures that any potential on the extraneous part is at the same potential as the exposed conductive parts of the electrical installation.
- Required for conductive water pipes, steel building frames, fire sprinklers, etc.
Equipotential Bonding Risks
- Dangerous potentials may be present on extraneous parts due to distribution system load current, lightning discharges, faults external to the building, or telecommunication voltages on equipment.
Earth Star Distribution System
- A four-wire secondary distribution transformer creates a zero reference point by solidly connecting the neutral star point to the general mass of earth.
- Provides a common earth reference between distribution transformer neutral and consumer’s earthed equipment.
Earth Star Distribution System Advantages
- Provides voltage stability.
- Phase to earth faults cause protection fuses to blow.
- Reduces the risk of high voltage applications to low voltage installations.
Main Earthing Conductor
- Connects the main earth terminal/connection or bar to the earth electrode.
- Minimum size is 4mm2, determined by T5.1, relating to the size of the largest active conductor.
Main Earth Resistance
- The maximum value of resistance is specified in two clauses in the wiring rules (S5 and S8).
- Must include the connection to the earth electrode and terminal/connection or bar.
Measuring Resistance of Main Earthing Conductor
- A test is required to measure the resistance of the main earthing conductor.
Main Earth Terminal / Connection or Bar
- A main earthing terminal/connection or bar shall be provided at the main switchboard (Cl. 5.3.4).
- Connects the protective earth conductor, main earth, MEN, equipotential bonding conductors, and functional earth conductors.
Earth Sizes
- Table 5.1 is used to determine earth sizes, with some exceptions (MEN, unprotected consumer mains, exposed conductive parts, etc.).
Earth Electrode
- Connects earthed parts of the electrical installation to the general mass of earth.
- Allows multiple paths for fault current to clear a fault.
- Types of electrodes are described in Cl 5.3.6.2, and copper-clad rod type electrodes must be driven to a depth of 1.2m.
Earth Electrode Requirements
- Electrode must be located in moist soil (Cl. 5.3.6.4).
- Connection must be accessible, with adequate conductivity (Cl. 5.5.1.2).
- Connection needs protection from corrosion and mechanical damage (Cl. 5.5.1.2).
Multiple Earthed Neutral (MEN)
- The distribution neutral is used as a low-resistance path for earth fault currents.
- Carries fault current as a result of an active conductor passing current to earth.
MEN Scenario
- A hot joint in the service mains can cause fault current to flow through the ground, resulting in different parts of the earth being at different voltages.
- This can cause electrocution if not addressed through equipotential bonding.
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Description
Learn about electrical earthing, safety measures, and equipment protection during fault conditions. Understand key terms and concepts related to earthing and equipotential bonding.