Earthing And Earth Bonding In An Electrical Installation.

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Earthing system parts.( Ref AS3000 )

1 Protective earthing conductors connecting exposed conductive parts as required.

2 Main earthing conductor.

3 Main earthing connection, terminal or bar.

4 MEN connection between the main earthing terminal,connection or bar and the supply

neutral bar.

5 Earth electrode.

6 Equipotential bonding of extraneous conductive and other parts as required.

The main earth conductor

Min 4 sq mm. conductor 12.7mm copper clad steel electrode.( Ref AS3000 )

The importance of the main earthing conductor should not be underestimated.

1 The main earthing conductor must never be disconnected.from the electrode.

2 With an MEN system the main earth conductor does not carry fault current due to

the relative high resistance of the soil (100 to 500 ohms) nominal, it returns to the neutral at

the transformer by way of the MEN link at the main switchboard.

The path is fault to PE conductor, MEN link, consumers main neutral,distribution neutral to

the transformer. The protective device trips, isolating the faulty circuit.

3 In the event of a broken supply neutral or the polarity of the consumers mains becomes

reversed the main earth will rise to the supply voltage. If earth bonding has been correctly

installed the touch voltage should not exceed 50Vac to any metal or conductive material

to earth.

4 One may receive a mild sensation of electric shock which is experienced as a tingling

sensation when ones hand is brought in contact with a conductive path to earth.

5 If the main earth was broken or had been disconnected one would receive supply voltage

through the body to earth, resulting in possible death, or a severe electric shock

requiring medical attention.

With the TT direct earthing system RCDs must be used on final subcircuits as the fault current in the main earth is insufficent to trip a MCB or fuse within the proscribed times.

Polarity testing must always be carried out before an installation is connected to supply.

Here are three examples that have occured in the past and shows why effective earthing is essential.

1 A person received a mild electric shock when he touched the metal of the main

switch board enclosure at his house. Investigation revealed that the neutral conductor

had burnt off a the pole.Touch voltage did not exceed 50Vac. Main earth was intact.

.

2 A person received a mild tingling sensation when he went to investigate why there was

loss of power to a motor in a 3 phase installation. Investigation revealed that

the supply authority had reversed a phase and neutral at a transition point earlier that day

while carrying out maintainance.

The main earth remained intact so the person received a mild sensation of electric shock.

(the touch voltage was less than 50Vac)

3 A person was killed when he touched a steel pole which supported a flood lighting fitting.

Investigation revealed that the PE conductor had failed due to corrosion within the fitting

and a earth fault had occured. the pole was at a potential of 230V.

The touch voltage was well above 50Vac.

An RCD would have prevented the death of this person.