SPARKYSTUDY NOTES
SPARKY STUDY NOTES sparkystudynotes.co.uk/learn/ring-final-circuits-the-three-step-test

Free notes

Ring final circuits: the three-step testWhy divide by four?

Free from the notes: Sparky’s video on it, the section word for word as members see it, and a PDF to keep.

From Sparky’s full video lesson. Members get the whole lesson on every notes page.

The notes

Exactly as they are on the site.

Download these notesPDF · free to keep

Ring final circuits: why they get their own test

Guidance Note 3 · 2.6.6 · pp.63–68, and On-Site Guide · pp.127, 129. A ring is the one circuit with its own continuity procedure, in three steps.

StepWhat you doWhat it proves
1End to end on each conductor separatelyr1, rn and r2
2Cross-connect line and neutral — the figure of eightR1 + Rn at every outlet
3Cross-connect line and cpcR1 + R2 at every outlet
Why it is separate. Imagine having to do all of this for a bonding conductor or a radial. The ring needs it because the two legs are in parallel with each other, and nothing else is.
Ring length. A ring final circuit has a maximum length, and it depends on the supply and the protective device. Too long and you stop satisfying the loop impedance requirement.

Step 1: end to end

Three separate tests, moving your two leads onto each conductor in turn. Both leads on one conductor, so each is an r2-type test — the same method as a bonding conductor.

Worked example: a ring in 4 mm²/1.5 mm² twin and earth, 15 m

Measured  r1 (line)    = 0.07 Ω

rn (neutral)  = 0.07 Ω
   Why the same? Line and neutral are the same csa and the
   same length, in the same cable.

r2 (cpc)      = r1 × (line csa ÷ cpc csa)
                = 0.07 × (4 ÷ 1.5)
                = 0.07 × 2.67
                = 0.19 Ω
What the ratio is telling you. The cpc has a smaller area than the line, so its resistance is 2.67 times higher. Put the other way round, r1 is 2.67 times lower than r2.
The other ring size. For 2.5 mm²/1.5 mm² the ratio is 2.5 ÷ 1.5 = 1.67.

Step 2: the figure of eight, line to neutral

In the consumer unit, cross-connect the two legs: the line of leg 1 to the neutral of leg 2, and the line of leg 2 to the neutral of leg 1. Connector blocks do the job.

Expected R1 + Rn = (r1 + rn) ÷ 4

Using the step 1 values

(0.07 + 0.07) ÷ 4
  = 0.14 ÷ 4
  = 0.035
  = 0.04 Ω on a two-decimal meter
Why divide by four? With the figure of eight made up, the ring becomes one loop of resistance r1 + rn. Wherever you plug in, you are measuring two halves of that loop in parallel: each half is (r1 + rn) ÷ 2, and two equal halves in parallel give half of that again — (r1 + rn) ÷ 4. Guidance Note 3 · p.66 shows the parallel network behind it.
Every outlet on the ring should read the same

You take the reading at each socket with a plug-in test adapter, leads between line and neutral. On a correctly cross-connected ring they all come out the same.

Step 3: line to cpc

Same cross-connection, but now between line and cpc. This is the R1 + R2 test method.

Expected R1 + R2 = (r1 + r2) ÷ 4

Using the step 1 values

(0.07 + 0.19) ÷ 4
  = 0.26 ÷ 4
  = 0.065
  = 0.07 Ω   (a less precise meter may show 0.06)

Again, the same reading is expected at every outlet on the ring.

Note the difference from step 2. Step 2 adds r1 and rn, which are equal. Step 3 adds r1 and r2, and r2 is the bigger number — so step 3 always gives the higher reading.

Free section from Sparky Study Notes. The full Level 3 notes, 1,463 practice questions and the video lessons: sparkystudynotes.co.uk

Test yourself on it

The tests are in the members’ area. Get one wrong and it shows you why, and where to revise.

Get access

Get new notes and exam tips from Sparky

About one email a fortnight. Unsubscribe any time. Privacy notice

That was one section. There are 583.

Sparky Study Notes is the whole Level 3 syllabus written up like this: the explanation, the worked example, and where it is in your book. With a video lesson for every part and a practice paper for every exam.

  • 583sections of written notes
  • 35video lessons
  • 1,463practice questions
  • 146diagrams, redrawn
  • 18th Edition
  • Building Regs
  • PAT
  • Fundamental Inspection & Testing
  • Initial Verification
  • Periodic Inspection & Testing
Get access — £100 12 months. One payment. See what’s inside