Understanding Earthing Systems (TN, TT, IT) for SANS 10142 - Part 2
Introduction
Earthing systems are fundamental to both Installation Rules Paper 1 and Paper 2. They feature in SANS 10142‑1 under supply systems, earthing requirements, and installation verification—so you must know the purpose of earthing, how each system is arranged, and how protection is verified.
1. TN Earthing System
In TN systems the source neutral is earthed and a protective conductor is provided by the supply. Variations: TN‑S (separate PE and N), TN‑C (combined PEN), and TN‑C‑S (combined then separated).
- Where you will see it: Most utility supplies to urban premises.
- Exam focus: Identify the conductor arrangement; know when PEN is allowed; bonding requirements.
- Protection concept: Faults create high earth‑fault currents that operate protective devices quickly.
2. TT Earthing System
In TT systems the installation has its own earth electrode. The supply provides neutral but not a protective earth conductor.
- Where you will see it: Rural sites or where supply earth is not provided.
- Exam focus: Earth electrode resistance and earth‑fault loop impedance considerations; Earth Leakage Device reliance.
- Protection concept: Disconnection typically via Earth Leakage Devices because fault currents are lower than TN systems.
3. IT Earthing System
IT systems have no direct connection between live conductors and earth, or are connected through a high impedance. First faults do not usually trip immediately; continuous monitoring is used.
- Where you will see it: Special installations (e.g., some medical or industrial processes).
- Exam focus: First‑fault vs second‑fault behaviour; insulation monitoring devices; equipotential bonding.
- Protection concept: Safety relies on insulation monitoring and controlled disconnection criteria.
Why Earthing Appears in Both Papers
- Paper 1: Definitions, system recognition, and regulatory intent.
- Paper 2: Calculations, verification, and application (you may be asked to draw and label TN/TT/IT).
To understand how earthing questions appear in exams, read our guide on the difference between Installation Rules Paper 1 and Paper 2 .
Exam Tips & Common Mistakes
- Drawings matter: Practise quick, labelled sketches of TN‑S, TN‑C, TN‑C‑S, TT, and IT with key conductors marked.
- Protective devices: Know which system commonly relies on ELDs (TT) and why TN often achieves high fault currents.
- Loop impedance basics: Be able to state that disconnection depends on the fault current and overall loop impedance (exam framing often checks this concept).
- Bonding vs earthing: Don’t confuse main bonding with the earthing conductor’s role—examiners test this.
- Special locations: For IT, remember insulation monitoring and the difference between first and second fault conditions.
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What You Will Learn
You will learn how to:- Interpret and apply the OHS Act and Electrical Installation Regulations
- Calculate Voltage Drop, PSCC (AC & DC), Touch Voltage correctly
- Perform inspection, testing, verification, and complete CoC tasks
- Master SANS 10142‑1 installation requirements
- Work under exam time pressure