(3) For Short Circuit Protection (Magnetic Setting):

Example for Setting of MCCB  for Motor Circuit

  •  When we select an MCCB for motor application, it is necessary that the instantaneous release setting in the MCCB is set to a value higher than the highest anticipated Motor magnetizing inrush current during switching-on the motor.
  • The values for magnetizing inrush current (sub transient current) are higher in case of high efficiency motors as compared to standard efficiency motors.
  • By theoretical and empirical means it is established that the maximum ratio between peak and LRC can go up to 2.5 for high efficiency motors.

Motor Starting Current:

  • Motor starting current is one of the most important electrical parameter of motor to understand its electrical characteristics.
  • It is important to note the distinction between inrush current and starting current
  • The current drawn by the motor in different phases are.
  1. Inrush Current (Sub transient phase)
  2. Starting or Lock Rotor Current (Transient phase)
  3. Steady state operation.

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Inrush Current (Sub transient phase)

  • During the initial phase of motor starting Current drawn by motor is known as inrush current or peak current.
  • Inrush current is the current drawn between switch on and when the magnetic fields are established in the motor this current is due to magnetizing inrush component of the motor starting current.
  • Inrush Current: It is generally 13 to 17 x FLA for older motors to New Motor.
  • The duration of inrush current: It is in milliseconds.
  • Motor circuits are highly inductive. Motor can be started at any point on voltage wave of the circuit. Depending on the initiation of the circuit i.e. point on the voltage wave.
  • The magnitude of the
    asymmetry is directly related to X/R ratio of the circuit.

Starting Current (Transient phase)

  • Motor starting current or Lock Rotor Current is the current drawn while Motor is accelerating to full speed.
  • Starting Current: It is depend upon Starting method of Motor
  • For DOL Starter : 6 to 8 x FLA
  • Star- Delta: 2 to 3 x FLA
  • Auto Transformer: 2 to 3 x FLA
  • Soft Starter: 3 to 5 x FLA
  • VFD: 1.5 x FLA
  • The duration of Starting current: Depend upon Load and Application (10 To 40 Sec)

The magnetic settings for Motor should be as follows:

  •  Motor Starting Current < Magnetic Setting of MCCB < Short Circuit Current
  • The MCCB should not trip during starting of the motor. Therefore setting should be 1.3 times the starting current. Normally the starting current will be 6 times the full load current of the motor. Therefore it will be 7.8 times the full load current.
  • However you have to calculate the short circuit current at the motor terminal. This depends on source fault level at the bus feeding the motor and cable impedance between the MCCB and motor. The setting on the MCCB should be less than the calculated short circuit current.
  • Motor Acceleration Time < Magnetic Time Setting of MCCB
  • Another important consideration is the acceleration time should be less than the time set on the MCCB for the magnetic setting.
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About Jignesh.Parmar
Jignesh Parmar has completed M.Tech (Power System Control), B.E(Electrical) from Gujarat University. He has more than 14 years experience in Power Transmission-Power Distribution-Electrical energy theft detection-Electrical Maintenance-Electrical Projects(Planning-Designing-coordination-Execution). He is Presently associate with one of the leading business group as a Deputy Manager at Ahmedabad,India. He is Freelancer Programmer of Advance Excel and design useful Excel Sheets of Electrical Engineering as per IS,NEC,IEC,IEEE codes. He is technical Author for "Electrical Mirror" and "Electrical India" Magazines. He is Technical Blogger and Familiar with English, Hindi, Gujarati, French languages. He wants to Share his experience & knowledge and help technical enthusiasts to find suitable solutions and updating themselves on various Engineering Topics.

10 Responses to (3) For Short Circuit Protection (Magnetic Setting):

  1. Devendra Sehgal says:

    Dear sir,
    I am Devendra Sehgal and my qualifications B -tech (ECE) at this time my job profile purchase department plz send me Cable size calculation with depends on the loads of motor mcb select.
    With regards:
    Devendra Sehgal

  2. Birbal says:

    Very knowledgeable topic for daily use in life, thanks.

  3. Arabinda Chakraborty says:

    Thank you for a useful note. Regards Arabinda Chakraborty

    On Sun, Jul 2, 2017 at 10:28 AM, Electrical Notes & Articles wrote:

    > Jignesh.Parmar posted: “Example for Setting of MCCB for Motor Circuit > When we select an MCCB for motor application, it is necessary that the > instantaneous release setting in the MCCB is set to a value higher than the > highest anticipated Motor magnetizing inrush current durin” >

  4. Sivakumar says:

    Would the inrush restraint feature be of help here?

  5. Ashraf M. Mortada Darwish says:

    Thanks a lot for the valuble information…

    Regards, Ashraf Mortada

    ________________________________

  6. vijay says:

    for short circuit MCCb settings should be less then short circuit level but how much ?

  7. Vitthal Chavan says:

    Dear sir,

    Can you send the calculation of wire size as per ampere rating.

    On Sun, Jul 2, 2017 at 10:28 AM, Electrical Notes & Articles wrote:

    > Jignesh.Parmar posted: “Example for Setting of MCCB for Motor Circuit > When we select an MCCB for motor application, it is necessary that the > instantaneous release setting in the MCCB is set to a value higher than the > highest anticipated Motor magnetizing inrush current durin” >

  8. Naseebullah says:

    Dear sir,
    I appreciate you and your work sir.
    I am Naseebullah and my graduation is B.E Electrical engineering. Please share the method to explain and trace the single line diagrame. How to trace the oneline diagrame during faulty conditions for removing fault.
    Regards: Naseebullah

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