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I have been going round in circles with this question. I just cant seem to get an answer remotely close to the options.

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 a ring.

The circuit is 58m 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.

What is the expected r1 value?

0.27Ω
0.32Ω
0.43Ω
0.52Ω

So I have got the R1+R2 value from the OSG which is 16.71 mΩ/metre
I have multiplied this by the 58 metres equaling 969.18 mΩ
I have times by 4 for the r1+r2 value 3876.72 mΩ
I have then multiplied this by the 4 to 1.5 ratio which is 0.273 equaling 1058.34 mΩ
I then divided by 1000 to get ohms
My answer is 1.05. No where near any of the answers.

What am I doing wrong?

I'd appreciate any help with this. I don't want the direct answer, just how I'm screwing it up.

Thanks in advance.
 
Read the question properly!!! What value is it that you're being asked for?
 
I am being asked for r1. Everything I have looked for takes me to calculating R1+R2. I have read the question multiple times over the last few hours. What am I missing?

Thanks.
 
I am being asked for r1. Everything I have looked for takes me to calculating R1+R2. I have read the question multiple times over the last few hours. What am I missing?

Thanks.
Don't have the OSG to hand but I'm assuming that it should contain the Ohm/m value for 4mm Cu. r1 is that value x length of circuit.
 
r1 is just your first continuity reading, lets say thats the line conductor, aka brown. Then you do your rn aka neutral and lastly your do your r2 aka earth. Its the first readings for continuity you do in the field when testing. So r1 is just the ring continuity brown to brown, which you hope is very close to design if you have remembered how much cable you have used, lol.
rN generally is the same conductor size (not always) but vast majority of the time so it will be similar hopefully within 0.02 ohms.
r2 is the earth continuity which is often, but not always, a smaller sized conductor so yields a higher resistance reading.
The reason we use r1+r2 as a design parameter is because we are trying to design a circuit that will meet the minimum breaker disconnection times which are the ZS values which you will come across. We get that value from r1+r2+ze or zdb . Thats why r1+r2 keeps popping up.
Its good to have a very good idea what your r1, rn and r2 figures should be just before you connect the tester so you can immediately have suspicions if they are way out.
When i am working with an apprentice or mate , to break the tedium we have a guess at the readings and closest does not make the tea :)
Anyway you need to find the resistance /metre for 4sqmm cable you are using , it does not vary that much tbh between all types of cable, its roughly 4.6mOhm per meter.
Good luck
 

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