CIRCUIT BREAKER SIZING

CIRCUIT BREAKER SIZING METHODS

   In this article reader will well understand about how to do the work with Circuit Breaker (MCB, MCCB, ACB) sizing with the professional methods and commonly used with the actual and so logical work daily like in the professional consultant company, most in Asia Country viz, in Singapore, Dubai, Malysia, Thailand, China, Australia, Cambodia, Vietnam And Others ...

   So, Below are the most common steps that readers have to pay attention to consider in this work :

1. THERMAL TRIP RATING CONSIDERATION :

Thermal trip rating consideration is the most common that always take account to CB and Cable sizing methods. But, the correct/professional methods is required the design engineer to well understand about few main factor as below:


     #1.1. Design Current or Full Load Current, Ib or Ifl

              i.e . Water Heater Load with P = 3.6kW consumption, V = 220V, Z = 50hz. 

              SOLUTION : Ib or fl = P/(V x Cosθ)  So, Ib = 19.25A

     #1.2. Trip rating of CB with varies type of loads starting methods ( Motor, Lighting....)

              There are many kinds of electrical loads also load starting methods, so below are the common methods to define the tripping current of CB (i.e. MCB-6A, 10A, 16A...) in the system to adopt with the current load starting : 

              
               Main rule to follow : Ib/Ifl <= In <= It/Iz

                  D.O.L : In = 2-2.5 times x Ifl

               A/T    : In = 1.5-2 times x Ifl

                  Star/Delta : In = 1.5-2 times x Ifl

     #1.3. Load Current Starting operation ( A/T, Star/Delta, Soft Starter, D.O.L ...etc.)

              This is refer to the second stage of starting current that will take time around 5- 15 sec (depending on kind of starting methods) After the first stage is Inrush current that usually take time around 0.04 sec (depending on kind of starting methods) and below are the simply ways to compute :

              A/T : Consider as reduce voltage starter which help to reduce starting current     quiet like to Star/ Delta Starter But, the different thing between two of  these are A/T is particular useful where progressive control of accelerating torque is required anyways, the price of A/T is relatively expensive than Star/Delta.  

                       Vstart (Star/Delta)= (1/1.732) x Vl , 

                       Istart (Star/Delta) = 1/3 x Istart (D.O.L Starter)

                            Vstart (A/T) = n x Vload, 

                       Istart (A/T) = n ^2 x Istart (D.O.L Starter)

              D.O.L : Does not help to reduce motor starting current, Starting current can be   seven to eight times greater than the running current. So, D.O.L is used only in small motors.

                          Istart = 7.5 times x I load

                 Note: A/T as Auto transformer
               D.O.L as Direct On Line
               I2 as CB thermal element trip operating current (Based on CB Curve)
               In as CB trip rating current
               Is as starting current (7.5 x Ifl) not inrush current (15 x Ifl)
               n as the tap ratio of auto transformer (normally n = 65% is used = 0.65)

      

              After you got the result of Starting current, then we can choose the appropriate CB size with the correct thermal operating time current that can adequately carry during starting time as illustrated above (5-15 sec, depending on load starting method). To serve in this case, below is CB Characteristic tabulate that will serve you in calculation :

        For MCB, Type B

        For MCB, Type C

              SOFT STARTER-Solid State reduce Voltage Starter : 

2. MAGNETIC TRIP OR INSTANTANEOUS TRIP

       This will be happened during initial/fist stage of starting up operation of motor (take time around 0.04 sec) that called Inrush current, Iinrush. This ratio will be computed to realise the CB type as the below :

    I inrush = 15 times x I load

    If Iinrush < 3 x In , MCB Type B will selected.
       Iinrush < 5 x In , MCB Type C will selected.


Below are the example of CB Selection and general methods in computation.

Hope you enjoy well understand with this article. Should you have any query.

Thanks & Regards,