Current Transformator "Rb"
\$R_b\$ is the burden resistor chosen for converting the current induced in the secondary of the current transformer into a voltage. 

The choice of resistor is up to you.


  A burden resistor connected across the secondary produces an output voltage proportional to the resistor value, based on the amount of current flowing through it. With our 1:10 turns ratio transformer that produces a 10:1 current ratio, a burden resistor can be selected to produce the voltage we want. If 1A on the primary produces 0.1A on the secondary, then by Ohm's law, 0.1 times the burden resistor will result in an output voltage per amp.
  
  -- Exploring Current Transformer Applications