You have Ze and you have R1+R2
Ipf at the end of the circuit (not normally considered in calculation) is Uo/Zs.
Zs = Ze+(R1+R2)
Zs at ambient temperature may corrected to Zs at Line maximum permitted operating temperature by dividing by 0.8.
Alternatively maximum Zs for a 20A circuit breaker to BSEN60898 as given in table 41.3 in BS7671 can be corrected to ambient temperature by multiplying by 0.8 (chapter 14 BS7671), this will give you the values of Zs as given in the on site guide.
So long as the Zs for the circuit is less than the tabulated values for a 20A BSEN60898 then the circuit breaker will disconnect in the required time.
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