Lesson Notes By Weeks and Term v5 - Grade 12

Lesson Video

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Performance objectives

• Describe key regulations from SANS 10142-1.
• Explain the use of industrial protection devices.
• Calculate earth fault loop impedance (Zs).
• Analyze different earthing systems (TN-S, TN-C-S, TT).
• Design a basic earthing system for a small workshop.

Lesson summary

Industrial electrical installations are the backbone of South Africa's economy. From manufacturing plants in Gauteng to mining operations in the Northern Cape and food processing factories in the Western Cape, these installations power the machinery and processes that produce goods and services, creating jobs and contributing to our nation's prosperity. Understanding the regulations governing these installations is crucial for ensuring safety, preventing accidents, and maintaining efficient and reliable power supply. This week, we'll delve into critical aspects of industrial installations, focusing on regulations, protection, and earthing systems specific to South African standards.

Lesson notes

2.1 Introduction to Industrial Electrical Installations Industrial installations differ significantly from domestic installations due to their scale, complexity, and the high power demands of the equipment they serve. They typically involve three-phase power systems, heavy-duty machinery, and hazardous environments.

Therefore, stringent regulations and specialized equipment are essential. 2.2 SANS 10142-1 (Edition 3): The South African Wiring Code SANS 10142-1 is the South African National Standard for the wiring of premises. It provides the minimum safety requirements for electrical installations in buildings, including industrial facilities. Understanding this standard is paramount for any electrical technician or engineer working in South Africa. The latest edition, Edition 3, has significant updates regarding arc fault detection, surge protection, and selection of protective devices.

It covers aspects such as: General requirements: Covering safety, design, selection of equipment, and workmanship.

Protection: Overcurrent, earth fault, insulation, and overload protection.

Earthing: Methods of earthing, earth continuity conductors, and earth electrodes.

Wiring methods: Conduits, trunking, cable trays, and direct burial.

Special locations: Hazardous areas, damp locations, and high-temperature environments.

Inspection and Testing: Initial verification and periodic inspection. 2.3 Protection Devices in Industrial Installations Circuit Breakers (CBs): Automatically interrupt the flow of current in the event of an overcurrent (short circuit or overload). Industrial CBs are often molded-case circuit breakers (MCCBs) or air circuit breakers (ACBs) designed for high fault current levels.

Overload Relays (OLRs): Protect motors from overheating due to sustained overloads. They typically have thermal or electronic elements that trip the circuit breaker when the motor current exceeds a pre-set value.

Earth Leakage Protection (ELP): Detects small earth leakage currents that can be dangerous. They typically use residual current devices (RCDs) or earth leakage circuit breakers (ELCBs). In industrial settings, careful coordination of ELP is vital to avoid nuisance tripping.

Fuses: Overcurrent protection devices that melt and break the circuit when the current exceeds a predetermined value. Fuses are generally considered a backup to circuit breakers.

Surge Protection Devices (SPDs): Protect equipment from transient overvoltages caused by lightning strikes or switching surges. SPDs divert the surge current to earth. 2.4 Earthing Systems An earthing system provides a low-impedance path for fault currents to return to the source, ensuring that protective devices operate quickly to disconnect the faulty circuit.

The main types of earthing systems are: TN-S: The neutral and protective earth (PE) conductors are separate throughout the system. This provides a very safe and reliable earthing system.

TN-C-S: The neutral and PE conductors are combined in part of the system (typically from the transformer to the main distribution board) and then separated. TT: The installation has its own earth electrode, which is independent of the supply authority's earth. This is often used in rural areas where a reliable earth connection from the supply is not available. 2.5 Calculating Earth Fault Loop Impedance (Zs) The earth fault loop impedance (Zs) is the total impedance of the earth fault current path, from the transformer secondary, through the supply conductors, the fault itself, and back to the transformer via the earth conductors. It is crucial for determining whether the protective device will operate quickly enough to clear a fault. According to SANS 10142-1, the disconnection time should be fast enough to prevent dangerous touch voltages. Zs = Ze + (R1 + R2)

Where: Zs = Earth fault loop impedance Ze = External earth fault loop impedance (supplied by the municipality) R1 = Resistance of the phase conductor R2 = Resistance of the earth continuity conductor

Example: A factory has a TN-S system. The external earth fault loop impedance (Ze) is measured as 0.2 ohms. The length of the 2.5 mm² phase conductor (R1) is 50 meters, and the length of the 2.5 mm² earth continuity conductor (R2) is also 50 meters. Calculate the earth fault loop impedance (Zs). The resistance of 2.5mm² copper conductor is approximately 7.41 ohms/km at 20°C. R1 = (50 m / 1000 m) 7.41 ohms/km = 0.3705 ohms R2 = (50 m / 1000 m) 7.41 ohms/km = 0.3705 ohms Zs = 0.2 + 0.3705 + 0.3705 = 0.941 ohms 2.6 Determining Earth Continuity Conductor Size SANS 10142-1 specifies minimum sizes for earth continuity conductors based on the size of the associated phase conductors. The table in the standard should be consulted directly, but as a general rule, for phase conductors up to 16 mm², the earth continuity conductor should be the same size.