Lesson Notes By Weeks and Term v5 - Grade 12

Lesson Video

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Performance objectives

• Identify key regulations like SANS 10142-1 and the OHS Act.
• Describe requirements for industrial cable sizing and protection.
• Explain the principles of earthing and bonding.
• Interpret single-line diagrams for industrial systems.
• Understand requirements for hazardous area installations.

Lesson summary

Industrial installations are a vital part of South Africa's economy, powering factories, mines, and other large-scale operations. Understanding the regulations governing these installations is crucial for ensuring safety, efficiency, and compliance with the law. As future electrical technicians and engineers, you will likely be involved in the design, installation, maintenance, or inspection of such systems. This week's focus will delve into the essential aspects of industrial installations and the relevant regulations, preparing you for a successful career in the electrical field.

Lesson notes

2.1 Regulatory Framework: SANS 10142-1 and the OHS Act SANS 10142-1: The Wiring Code: This is the South African National Standard for the wiring of premises. It dictates the rules and requirements for electrical installations to ensure safety and proper functioning. It covers aspects like cable selection, installation methods, protection devices, earthing, and testing. It is legally binding and must be followed in all electrical installations. Think of it as the "bible" for electrical installations in South Africa. Occupational Health and Safety Act (OHS Act): This Act is a broader piece of legislation aimed at ensuring the health and safety of employees in the workplace. It places a duty on employers to provide a safe working environment, including safe electrical installations. The OHS Act refers to SANS 10142-1 as the standard to which electrical installations must comply. An example is the duty on the mine foreman to ensure all electrical installations in the mine follow the Act and SANS10142-

1. Key Requirements of SANS 10142-1 (Examples): Competent Person: Installations must be carried out by a registered electrician or a person under their direct supervision.

Certificate of Compliance (CoC): A CoC must be issued for all new installations and alterations to existing installations, confirming that the work complies with SANS 10142-

1. Protection against Electric Shock: Requires effective earthing, bonding, and the use of Residual Current Devices (RCDs).

Overcurrent Protection: Requires appropriate circuit breakers and fuses to protect against overloads and short circuits.

Wiring Methods: Specifies acceptable wiring methods, cable types, and installation practices. 2.2 Cable Sizing and Protection Cable Sizing: The cable size must be adequate to carry the design current of the circuit without overheating.

Factors to consider include: Current Carrying Capacity (CCC): The maximum current a cable can carry continuously under specific conditions without exceeding its temperature rating. Tables in SANS 10142-1 provide CCC values for different cable types and installation methods.

Voltage Drop: The voltage drop along the cable length should not exceed specified limits (typically 5% for lighting and 3% for power circuits). Excessive voltage drop can lead to poor performance of equipment.

Grouping and Ambient Temperature: The CCC of a cable is derated if it is grouped with other cables or if the ambient temperature is high. Correction factors must be applied.

Cable Protection: Cables must be protected against: Overcurrent: Circuit breakers and fuses are used to protect cables against overloads and short circuits. The breaking capacity of the protective device must be sufficient to interrupt the fault current.

Mechanical Damage: Cables should be installed in conduits, trunking, or other protective enclosures to prevent physical damage.

Environmental Factors: Cables must be suitable for the environment in which they are installed (e.g., UV resistance for outdoor applications, chemical resistance for corrosive environments).

Example: Cable Sizing Calculation A three-phase motor draws a full load current of 25A. The cable is to be installed in a trunking system with three other cables. The ambient temperature is 35°C. Determine the minimum cable size required using PVC insulated copper conductor cable. Allow a 10% margin for future expansion. The cable length is 30m and the supply voltage is 400

V. Design Current: Design current = 25A * 1.10 = 27.5A Derating Factor for Grouping: Refer to SANS 10142-

1. For four cables in trunking, the derating factor is typically around 0.

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0. Derating Factor for Ambient Temperature: Assume from SANS 10142-1, the derating factor for 35°C is 0.94 (this will vary based on the insulation material and cable specifics).

Required CCC: Required CCC = 27.5A / (0.80 * 0.94) = 36.5A Cable Selection: Refer to SANS 10142-1 cable tables. Select a cable size with a CCC greater than 36.5A after the derating factors have been considered. For example, a 4mm² PVC insulated copper conductor cable might be suitable, but always verify against SANS 10142-1 tables. We need to ensure that after derating, the CCC is greater than 27.5

A. Voltage Drop Check: Voltage drop = (√3 I L (R cosφ + X sinφ)) / (1000 V).

Where: I = Design Current (27.5A) L = Cable Length (30m) R = Resistance per meter (from cable tables) X = Reactance per meter (from cable tables) cosφ = Power factor (assume 0.8) sinφ = √(1-cos²φ) = √(1-0.8²) = 0.6 V = Line Voltage (400V) Assuming values for R and X from a typical table for 4mm² cable (R = 4.61 mΩ/m; X = 0.08 mΩ/m): Voltage Drop ≈ (√3 27.5 30 (0.00461 0.8 + 0.00008 0.6)) / (1000 400) ≈ 0.016 Volts. Percentage Voltage Drop = (0.016 / 400) * 100 = 0.004%. This is very low, so Voltage drop is unlikely to be an issue with 4mm² cable.

However, ALWAYS perform the calculation and compare against the maximum permissible Voltage Drop of 3%.