Protection of the MCB / MCCB

Protection of the MCB / MCCB

MCB	MCCB
1. Short circuit	1.Short circuit
2. Over load	2. Over load
	3. Other protection

Chart 03.04.01

MCB / MCCB has two mechanism to trip

1). Bimetallic part

2). Electromagnetic part

Thermal tripping

This action provides bimetallic part. It is used to protect the circuit from over load.

Electromagnetic tripping

This action provides electromagnetic field. It is used to protect the system from short circuit.

PUSH BUTTON

There are many three types of push buttons are available.

1). Single headed push button, spring return.

2). Double headed push buttons, spring return.

3). Emergency stop and mushroom head push button.

RELAY - Reset mode

Reset mode

We can select two paths to resetting the overload relay.

1. Auto reset mode
2. Manual reset mode.

How to set the setting value of the overload relay

Assumed rated current of the motor is equal to the Ina = 18A

This setting is depends on the rated current of the motor.

RELAY

Feature of the Thermal Overload Relay

1. It has current setting dial. We can adjust the rated current of the motor.
2. It has trip indicator.
3. We can test the TOR (Trip On Not).

When trip the overload relay auxiliary contact point change. Therefore, cut the control circuit.

Feature of the Thermal Overload Relay

1. It has current setting dial. We can adjust the rated current of the motor.
2. It has trip indicator.
3. We can test the TOR (Trip On Not).

When trip the overload relay auxiliary contact point change. Therefore, cut the control circuit.

Utilization category

1) Resistive load

2) Normal motor

3) Transformer

4) Capacitor bank

Types of utilization category.

MOTOR STARTING METHOD

The starting current of 3-phase induction motor is usually 5 to 8 times the full load current or the motor. Therefore, starting when large size induction motor which are used in industry. Their starting current will cause on unacceptable level of voltage drop in the supply lines as a result this sudden voltage drop nearly will also effect the other consumer. Therefore, the supply authorities that when starting large size 3 – phase induction motors the users must adopt some starting method to reduce the starting current. Four methods are used in there.

Starting method

1) Liquid Rheostat

2) Direct on line starting method

3) Star – delta starting method

4) Auto transformer starting method

5) Rotor resistor starting method