Lesson Notes By Weeks and Term v5 - Grade 11

Lesson Video

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Performance objectives

• Identify and describe the function of control circuit components.
• Explain the operating principles of protection devices.
• Design and analyze basic control circuits.
• Calculate appropriate fuse and circuit breaker ratings.
• Describe the importance of earth leakage protection.

Lesson summary

This week, we delve into the critical world of control circuits and protection devices. These components are fundamental to the safe and efficient operation of electrical systems, from the simple lighting circuits in our homes to the complex machinery in factories and mines. Understanding them is essential for any aspiring electrician or electrical engineer. In South Africa, where access to reliable electricity is a constant challenge and where safety is paramount, a strong grasp of control circuits and protection devices is absolutely vital.

Lesson notes

2.1 Control Circuits Control circuits are used to control the operation of electrical equipment. They typically consist of low-voltage components that control higher-voltage power circuits.

The main components include: Relays: Electromechanical switches that use an electromagnet to open or close contacts. They are used to isolate control circuits from power circuits and to amplify signals. A small current in the relay coil can control a large current through the relay contacts.

Contactors: Similar to relays but designed to handle much higher currents and voltages. They are commonly used to switch motors, lighting circuits, and other heavy loads. Contactors are essential for motor starters and other high-power switching applications.

Timers: Devices that provide time-delayed switching actions. They can be used to control the duration of operation of equipment or to sequence events. Different types of timers exist, including: On-delay timers: The output is energized after a set time delay after the input is energized.

Off-delay timers: The output remains energized for a set time delay after the input is de-energized.

Switches: Manually operated devices used to open or close a circuit. Various types exist, including toggle switches, push-button switches, and selector switches.

Example: Consider a simple motor starter circuit. A start button activates a relay, which in turn energizes a contactor. The contactor then supplies power to the motor. An overload relay protects the motor from overcurrent. A stop button de-energizes the relay and contactor, stopping the motor. 2.2 Protection Devices Protection devices are designed to protect electrical equipment and personnel from damage caused by overcurrent, short circuits, and earth leakage faults.

Fuses: Simple, inexpensive overcurrent protection devices. They contain a fusible link that melts and breaks the circuit when the current exceeds a specified value. Fuses are "one-time" devices and must be replaced after they blow.

Circuit Breakers: Reusable overcurrent protection devices that automatically trip and interrupt the circuit when an overcurrent or short circuit occurs. They can be reset after the fault is cleared. Different types of circuit breakers exist, including: Miniature Circuit Breakers (MCBs): Used in domestic and light commercial applications. They typically have current ratings up to 100

A. Moulded Case Circuit Breakers (MCCBs): Used in industrial and commercial applications. They have higher current ratings and interrupting capacities than MCBs. Earth Leakage Circuit Breakers (ELCBs/RCCBs): Protect against earth leakage faults. They detect imbalances in the current flowing through the live and neutral conductors and trip the circuit if a fault is detected. ELCBs are crucial for preventing electric shocks. RCCBs (Residual Current Circuit Breakers) are the modern, more sensitive and reliable equivalent to older ELCB designs.

Overload Relays: Protect motors from overheating due to overloads. They are typically thermal or magnetic devices that trip the circuit when the motor current exceeds its rated value for a prolonged period. Overload relays allow for short-term overloads, such as during motor starting, but trip if the overload persists.

Example Calculation: Fuse/MCB Rating A single-phase motor draws a full-load current of 10

A. Calculate the appropriate fuse/MCB rating for protecting the motor.

Step 1: Determine the starting current. Motor starting current can be 3 to 7 times the full-load current. Let's assume a starting current of 5 times the full-load current. Starting current = 5 10A = 50A Step 2: Select a fuse/MCB with a rating higher than the full-load current but lower than the maximum allowable starting current duration. A fuse or MCB that is too close to the full-load current will trip during starting. A too-high value could delay tripping under overload conditions. For motor circuits, it is common to select a rating approximately 1.25 to 2 times the full load current for fuses, depending on the fuse type, and closer to the full load for MCBs.

Step 3: Choose a suitable standard rating. Common fuse/MCB ratings include 10A, 16A, 20A, 25A, etc.

Solution: A 20A fuse or MCB would be a suitable choice. It is higher than the full-load current (10A) to prevent nuisance tripping but also low enough to provide adequate overcurrent protection. For MCBs a curve B device would likely be selected.

Example Calculation: Earth Leakage Protection An electrical appliance has a rated current of 5A. Calculate the maximum earth leakage current that can flow before an ELCB/RCCB with a sensitivity of 30mA trips.

Step 1: The ELCB/RCCB sensitivity is 30mA Step 2: The device will trip when earth leakage current exceeds 30m

A. Guided Practice (With Solutions)

Question 1: A motor needs to be started and stopped using two push buttons. Design a control circuit using a relay, a start button (normally open), and a stop button (normally closed) to achieve this.