2391-52 question

Bear · Oct 18, 2020

hi all, Got a question that i am not understanding correctly? Any body know how to work this out, I thought the answer was D, but apparently its A, According to the answer sheet. But i got my answer from page 63 bs7671.

An existing farmhouse and outbuildings are to be inspected and tested in order for the owner to comply with their insurer’s requirements

The lighting circuit in the hay barn is protected by a 5 A BS 3036 and a 300 mA RCD, the circuit has a measured Zs of 0.82 Ω.

The supply and installation forms part of a TN-C-S system.

What is the maximum measured earth fault loop impedance for the 5 A BS 3036 fuse?

A. 7.28
B. 9.10
C. 11.68
D. 16.80

Wilko · Oct 18, 2020

Hi - I think you’ve looked at the distribution circuit table with 5 seconds disconnect, but this is a final circuit with 400ms dis time. Check Table 41.2 and since its measured in the field we assume it’s at 20 deg C and multiply by 0.8 factor. Hope that helps

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loz2754 · Oct 18, 2020

1. You used the wrong table. You used the table for a distribution circuit. The correct table is on the previous page.
2. You have apply the 0.8 factor for temperature at the time of measurement being less than the normal operating temperature.

Wilko beat me to it....

Julie. · Oct 18, 2020

Well a whole host of wrongness...

1) It's a final circuit - so 5 secs disconnection time is wrong - must be 0.4 secs
2) even if you take this "correct" value - you have taken it from the DESIGN document - i.e whilst hot, the question states MEASURED - so either look the value up in an install document (OSG) or test document (GN3) - or correct for temperature - which is usually 0.8x the value from the design document.

So - I should have checked for other replies before posting ...........