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Hello everyone I'm Chris. I am taking the 2391 in February. This question is from the c&g main site under 2391 sample questions. Continuity of a ring final circuit has been tested as part of an initial verification of a new primary school. All the socket-outlets are connected directly to the ring. The circuit loop length is 58 m long, wired in 4 mm2 live and 1.5 mm2 cpc flat profile cable. The circuit is protected by a 32 A BS EN 60898 Type B circuit breaker. The installation has a measured Ze of 0.29 Ω. What is the expected value of Zs for this circuit? a) 0.24 Ω b) 0.28 Ω c) 0.53 Ω d) 0.58 Ω The answer is C.
I don't get that answer just wondered if anyone would like to help me as to what i am doing wrong. Many Thanks Chris.
 
approx
58 mts 4mm = 0.25375 ohms
58 mts 1.5mm = 0.67666 ohms
0.25375 + 0.67666 / 4 = 0.232604 then add ZE 0.29 = 0.522 taking 0.53 as the closest

edit just to add the max zs allowed - allowing for 80% and c-diff would calc to 1.09 - approx
Thank you Steve, feel like a total bell now. I was using the 4mm and 1.5mm combined value. 16.71 Makes total sense now. Thank you again.
 

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