Chapter 4: System Earthing

4.1 Introduction

In Chapter 2 we have briefly covered that the phase-to-ground faults in a system can limit the ground fault current depending on adding external impedance between neutral and the earth. This chapter briefly covers the various methods of grounding that are adopted in the electrical systems. In the following clauses, the star-connected transformer is shown which are widely used in power distribution. The grounding methods are also applicable in case of generators, whose windings are also invariably star connected.

The following table highlights the possible problems that can occur in a system due to the common faults and the solutions that can be achieved by adopting system grounding.

Problems
Phase faults: High fault currents Only limited by inherent impedance of power supply.
Earth faults: Solid earthing means high earth fault currents Only limited by inherent zero sequence impedance of power system.
Consequence Heavy currents damage equipment extensively - danger of fire hazard. This leads to long outage times - lost production, lost revenue. Heavy currents in earth bonding gives rise to high touch potentials - dangerous to human life. Large fault currents are more hazardous in igniting gases - explosion hazard.

Problems

Phase faults:

High fault currents
Only limited by inherent impedance of power supply.

Earth faults:

Solid earthing means high earth fault currents
Only limited by inherent zero sequence impedance of power system.

Consequence

Heavy currents damage equipment extensively - danger of fire hazard.
This leads to long outage times - lost production, lost revenue.
Heavy currents in earth bonding gives rise to high touch potentials - dangerous to human life.
Large fault currents are more hazardous in igniting gases - explosion hazard.

Solutions
Phase segregation: Eliminates phase-to-phase faults.
Resistance earthing: Means low earth fault currents - can be engineered to limit to any chosen value.
Benefits Fault damage now minimal - reduces fire hazard. Lower outage times - less lost production, less lost revenue. Touch potentials kept within safe limits - protects human life. Low fault currents reduce possibility...

Solutions

Phase segregation:

Eliminates phase-to-phase faults.

Resistance earthing:

Means low earth fault currents - can be engineered to limit to any chosen value.

Benefits

Fault damage now minimal - reduces fire hazard.
Lower outage times - less lost production, less lost revenue.
Touch potentials kept within safe limits - protects human life.
Low fault currents reduce possibility...

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Practical Power System Protection

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Chapter 4: System Earthing

4.1 Introduction

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