# calculating R1+R2

Discussion in 'Electrics UK' started by sime27, 11 Feb 2007.

1. ### sime27

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whats the method used to calculate R1+R2 from r1 rn r2 readings on a ring final?

3. ### davelx

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Easy to work out using basic circuit analysis principles - just draw out the circuit with lumped component values and apply the Ohm's Law to get the equivalent resistance.

Or look at page 70 in the on-site guide.

4. ### rlc

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end - 2 end resistances for the phase, neutral and cpc being r1, rn & r2 resp.

Taking some dummy values for say 2.5/1.5mm cable:

r1=0.54ohms
------|===|------

+
r2=0.54*1.67 = 0.90ohms
------|===|-----

note 1.67 being scaler based on 2.5/1.5 with no units.

R1 + R2 for the ring:

0.54 + 0.90
(r1+r2)/4 = ------|===|------------|===|----- = 1.44/4 = 0.36 ohms

alternatively you can calculate using Product /Sum or 1/Rt method but your measured highest value is the one you record:

Prod/Sum = (0.54 * 0.90)/(0.54+0.90) = 0.486/1.44 = 0.3375 ohms

Resistance for a 2.5mm^2 @70degC ~18 milli-ohms / metre and note the lower the temperature the lower the resistance will be due to the temp coefficient.

5. ### davelx

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But, does the OP understand why it's (r1+r2)/4? IMO he should. And I don't mean "because it's a ring", I mean does he understand how to calculate the equivalent value of resistances in series and parallel combinations. That level of understanding should be a requirement for all sparks, IMO.

6. ### plugwash

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and what about the understaning needed to show that the furthest point on the ring is the worst case?

7. ### ricicle

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Or a long spur

8. ### rlc

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sime27: does any of this information help you or do you require further detailed explanation - as pointed out it's a fundamental requirement to understand the basic principles, lumped circuit model, nodal analysis, measured / calculated results and what affect they have on the circuit.

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