Electrical Earthing and Safety

Questions and Answers

What is the primary purpose of earthing the neutral of a transformer in an earth star distribution system?

To provide a secondary voltage source

To prevent high voltage from being applied to low voltage installations in case of insulation failure (correct)

To increase the voltage stability of the system

To reduce the size of the main earthing conductor

What is the advantage of having an earthed star point in an earth star distribution system?

It increases the load carrying capacity of the system

It provides for voltage stability by nullifying the vector sum of three phase currents to zero (correct)

It eliminates the need for protection fuses

It allows for the use of smaller transformers

What is the minimum size of the main earthing conductor?

2.5mm2

4mm2 (correct)

8mm2

6mm2

What is the purpose of the main earthing terminal / connection or bar?

To connect the main earthing conductor to the earth electrode Signup and view all the answers

What is the consequence of a short-circuit fault in an earth star distribution system without an earthed neutral?

The neutral point will rise in potential through the transformer windings Signup and view all the answers

What is the advantage of having a common earth reference between the distribution transformer neutral and the consumer's earthed equipment?

It ensures safe and reliable operation of the system Signup and view all the answers

What is the purpose of measuring the resistance of the main earthing conductor?

To check the connection to the earth electrode Signup and view all the answers

What is an extraneous conductive part?

A conductive part that is not part of the electrical installation Signup and view all the answers

What is the purpose of equipotential bonding?

To ensure that the extraneous conductive parts are at the same potential as the electrical installation Signup and view all the answers

What is the consequence of a phase to earth fault in an earth star distribution system?

The protection fuses will blow Signup and view all the answers

Which of the following is a circumstance where dangerous potentials may be present on extraneous conductive parts?

Distribution system load current flowing in the soil Signup and view all the answers

Which of the following extraneous conductive parts needs to be bonded?

Conductive water pipes Signup and view all the answers

What stipulates the type of conductors and their size to be used for equipotential bonding?

The Wiring Rules Signup and view all the answers

Why is equipotential bonding necessary?

To prevent electrical shock from the extraneous conductive parts Signup and view all the answers

What is the result of equipotential bonding?

The extraneous conductive parts are at the same potential as the electrical installation Signup and view all the answers

What is the purpose of connecting the bonding conductor to the earthing system of the installation?

To ensure that the electrical installation is connected to the earth Signup and view all the answers

What is the primary purpose of earthing in an electrical installation?

To ensure personnel safety and equipment protection during fault conditions Signup and view all the answers

According to AS/NZS 3000:2018, where can the definitions of terms used in the standard be found?

Section 1.4 Definitions Signup and view all the answers

What is the term for a situation where someone can touch exposed conductive parts while being in contact with the earth?

Earthed situation Signup and view all the answers

Which of the following regulations deals comprehensively with earthing arrangements and earthing conductors?

Section 5 of the wiring rules Signup and view all the answers

What is the term for a conductor that is more than 2.0m from the ground?

Readily accessible conductor Signup and view all the answers

Why is the earth considered the most important conductor in an electrical installation?

Because it ensures personnel safety and equipment protection during fault conditions Signup and view all the answers

What happens when a hot joint occurs in the neutral connection of the service mains?

The current will look for the path of least resistance Signup and view all the answers

What is the consequence of the fault current flowing through the ground?

Different parts of the earth can be at different voltages Signup and view all the answers

What is the purpose of the equipotential bonding conductor?

To ensure that any potential on the extraneous part is at the same potential as the earth and exposed conductive parts of the electrical installation Signup and view all the answers

Why is it dangerous to touch the shower head or tap while standing in the shower?

You are touching different parts of the earth that might be at different voltages Signup and view all the answers

What happens when the fault current flows through the earth?

A voltage drop occurs due to the impedance of the earth Signup and view all the answers

What is the term for the metallic pipe that connects the shower head and tap throughout the house?

Extraneous conductive part Signup and view all the answers

What is the purpose of the earth electrode in an electrical installation?

To connect earthed parts of electrical installation to the general mass of earth Signup and view all the answers

What is the MEN system of earthing used for?

To carry any fault current as a result of an active conductor passing current to earth Signup and view all the answers

What is the USE of Table 5.1?

To determine all earth sizes Signup and view all the answers

What is the requirement for the connection of the earth electrode?

It must be accessible and have adequate conductivity Signup and view all the answers

What is the purpose of the P.V.C.pipe in the earth electrode installation?

To allow water to enter the electrode area Signup and view all the answers

What is the type of electrode that must be driven to a depth of 1.2m?

Copper clad rod type electrodes Signup and view all the answers

Which of the following is NOT connected to the protective earth conductor?

MEN Signup and view all the answers

What is the requirement for the location of the earth electrode?

It must be located in moist soil Signup and view all the answers

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.