Operation and Selection of Low Voltage Circuit Breakers.pptx

Operation and Selection of Low Voltage Circuit Breakers.pptx

1. Operation and Selection of Low Voltage Circuit Breakers

2. Principle of Operation of Low Voltage CB

3. Principle of Operation A breaker protects a circuit using two mechanisms : Thermal Trip (for overloads) Inside the breaker is a bimetallic strip . When current is higher than the rated value ( Ir ) for a long time, the strip heats up and bends. This bending pushes a lever → breaker trips after a delay . Used to stop cables/equipment from overheating under overload. 👉 Think of it like a thermostat slowly reacting to heat. Magnetic Trip (for short circuits) The breaker also has an electromagnet coil . If a very high fault current ( Im ) flows suddenly, the magnetic force pulls a latch. This trips the breaker instantly (milliseconds) . Stops dangerous short-circuits before they can cause fire or damage. 👉 Think of it like a magnet snapping a switch the moment a surge comes. ✅ In short: Thermal = slow trip, overload protection . Magnetic = instant trip, short-circuit protection .

4. Operating Voltage of CB

5. How To Select Ir Value? EX 1

6. How To Select Ir Value? EX 1

7. How To Select Ir Value? EX 2

8. How To Select Ir Value? More Examples Fixed breakers (MCB, small MCCB): must pick the nearest standard size. Adjustable breakers (large MCCB, ACB): choose the next higher rating and adjust Ir to your load current.

9. How To Select Ir Value? More Examples

10. Instantaneous Short Circuit Current (Im) B-Curve From 3- 5 In, used in protection of static loads such as lighting, heaters and sockets

11. Instantaneous Short Circuit Current (Im) C- Curve From 5- 10 In, used in protection of dynamic loads such as motors with high starting current

12. Instantaneous Short Circuit Current (Im) D- Curve From 10- 20 In, used in protection of equipment with a very high starting currents such as transformers

13. There are single phase and three phase ELCB