Earthing - Electrical Notebook

Canberra
Australia

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Earthing

Earthing . . . .  referred to as grounding in some parts of the world.


Earthing is the underestimated but necessary electrical element of any installation.  Underestimated as it is unseen, provides no interface to the user and is often overshadowed by the return path neutral.  However, the earthing system is the essential return path of an earth fault short circuit, ensuring that the protective device opens or ruptures.  The earth system, essential as it is deserves its own set of drawings for all installations.  This, however, is often neglected for the wording within the wiring regulations . . . . and may be justified for small installations.

Electrical circuits generally comprise of phase (live) conductors and neutral conductors.  These serve as supply and return paths of the circuit to electrical equipment. The purpose of the earth path is to provide an alternative current flow path should the live phase or neutral conductors become damaged.  This return path would meet a required maximum impedance such that the circuit breaker would open within the required time, rendering a safe system to the user by opening the circuit quicker than an electrical shock would cause harm to a person.

The following details are typically found within the local wiring regulations:
  • Earth conductor size
  • Maximum circuit impedance

The following are typical fault types
  • Phase to Earth
  • Phase to Phase
  • Neutral to Earth


Earthing Standards

The following are commonly referred standards for earthing:
  • IEEE80 - Earthing for substations
  • BS7671 - IET Wiring Regulations (UK)
  • AS3000 - Wiring Regulations (Australia)
  • AS2067 - Earthing for substations (Australia)

Earthing Systems

The wiring regulations stipulate various arrangements of earthing systems. Referred to in code within IEC60364, for example, as TN, TT & IT.  Various combinations of earthing render the TN-C-S system the most apparent within building services. These separate, classify and restrict the types of earthing arrangements, based on:
  • Earth connection at the source.
  • Earth connections at electrical equipment.
  • Earth connection with the neutral conductor.
  • Locality of an outbuilding or electrical distribution point.

The lettring decodes as follows:
  • T - Earth (Terre)
  • N - Neutral connection at the source
  • I - No earth connection or high impedance connection to earth
  • C - Combined earth-neutral conductor
  • S - Separate earth and neutral conductors


Other system arrangements typically include:
  • TN-C
  • TN-S

Apart from the benefits to outbuildings and swimming pools, TT systems are preferred by telecommunications applications as it eliminates noise throgh the neutral conductor.

Earthing arrangements may also be referred to as:
  • PE    - Protective Earth
  • PEN  - Protective Earth-Neutral
  • MEN - Multiple Earth-Neutral (Australia)

For the best part, the earth reference voltage is tied with the neutral reference voltage. Naturally this works readily for single phase, however, for unbalanced three phase systems, the neutral current will fluctuate away from zero, resulting in a neutral voltage relative to earth. Without suitable earthing, this differing voltage may present itself between conductive components resulting in electric shock.

Earth Practice

Earthing for protection extends to interfacing with lightning protection, equipotential bonding, swimming pool earth mesh, touch and step potential mitigation.  All essentially to reduce the risk of electrical shock through a common plane.




Location
Canberra,
Australia.
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