Current transformer for protection question

[bizuputyi]

Homework Statement

Determine R_R value if the relay current has not to exceed 20mA. Neglect relay resistance.

2. The attempt at a solution

I believe the solution is behind Merz-Price theorem, where the voltage system is balanced. CT1 and CT2 are identical current transformers, however, R_L2 (leads resistance) and R_S2 (secondary winding resistance) upset the Merz-Price balance, therefore the need for R_R I guess. The relay pulls in when current through it exceeds 20mA. R_R is there to prevent unwanted relay operation (relay is to remain off under threshold current).
Now, without voltage and other resistance values I don't see the trick.

Is there a formula for this?

Thank you for your help.

 

[bizuputyi]

I believe I sorted out. The thread may be closed. Thank you.