Ring final circuit test calculator: r1, rn, r2 and (r1+r2)/4

A ring final circuit continuity test takes three end-to-end readings at the board (r1 for the line, rn for the neutral and r2 for the protective conductor) then cross-connects the ends and reads at every socket. Each socket should read about (r1+r2)/4, and that figure is the R1+R2 recorded for the circuit.

Type in the three end-to-end readings and it tells you whether they make sense together, what steps 2 and 3 should read at the sockets, and roughly how long the ring is.

The three-step ring final continuity test of Regulation 643.2.1, as Guidance Note 3 and the On-Site Guide set it out. Checked against BS 7671:2018+A4:2026, updated 3 September 2026. Free, and nothing to sign in to.

Cable (line / cpc, mm²)

Step 1 readings: each conductor measured end to end at the board with the ring disconnected.

Step 3 reading at every socket: the R1+R2 to record

0.29 Ω

The readings make sense together

Every socket should read about this with line and cpc cross-connected, highest at the midpoint. Record it as the circuit's R1+R2.

Step 2: line and neutral cross-connected (r1 + rn) ÷ 4 at every socket
0.22 Ω
r1 against rn Same conductor, same length.
0.00 Ω apart
r2 ÷ r1 1.5 mm² against 2.5 mm², by resistance.
1.64 (expect 1.63)
Length of the ring, roughly r1 ÷ 7.41 mΩ/m
59 m

What should the ring final circuit readings be?

A ring final circuit is tested in three steps. Step 1 measures each conductor end to end at the board: the line loop r1, the neutral loop rn and the protective conductor loop r2. Line and neutral are the same size over the same route, so r1 and rn should match; the protective conductor of twin and earth is smaller, so r2 should be about 1.6 to 1.7 times r1 because 12.1 ÷ 7.41 on the conductor standard's resistances for 1.5 and 2.5 mm².

Step 2 joins the line of one end to the neutral of the other and reads between line and neutral at every socket. Step 3 does the same with line and protective conductor. Because the two halves of the ring now sit in parallel and each is half a loop, every socket reads about (r1 + r2) ÷ 4, highest at the midpoint and a little lower toward the board. That step 3 reading is the circuit's R1+R2, recorded in column 18 of the schedule of test results.

What the readings tell you

  • Readings the same at every socket, or peaking gently in the middle: a healthy ring.
  • Readings rising steadily away from the board: it is not a ring. It is broken, and you are reading a radial.
  • A socket reading noticeably higher than its neighbours: a spur, higher by the resistance of its own leg.
  • r1 and rn apart: an interconnection, a socket wired into one conductor only, or a break in one of them.

Worked example

The IET's own example: a ring wired in 2.5/1.5 mm² reads r1 = 0.44 Ω, rn = 0.44 Ω, r2 = 0.72 Ω.

  1. r1 and rn match, so the line and neutral loops are the same conductor.
  2. r2 ÷ r1 = 1.64, against 1.63 expected for 1.5 mm² over 2.5 mm², so the cpc is the size it should be.
  3. Step 2 should read (0.44 + 0.44) ÷ 4 = 0.22 Ω at each socket.
  4. Step 3 should read (0.44 + 0.72) ÷ 4 = 0.29 Ω at each socket, and 0.29 Ω is the R1+R2 recorded.
  5. 0.44 Ω ÷ 7.41 mΩ/m puts the ring at about 60 m of cable.

Questions the trade asks

r1 and rn should be the same to within a few hundredths of an ohm, because they are the same size of conductor over the same route. r2 should be about 1.6 to 1.7 times r1 on a 2.5/1.5 mm² cable, because the protective conductor is smaller. A 2.5 mm² loop reads about 7.4 mΩ for every metre of ring, so r1 also tells you how long it is.

With the line of one end linked to the protective conductor of the other, the two halves of the ring sit in parallel, so every socket reads about a quarter of the two loop resistances added together, highest at the midpoint. That reading is the circuit's R1+R2, and it is what goes in column 18 of the schedule of test results.

Readings that are all about the same, or that peak gently in the middle, are what a healthy ring gives. Readings that rise steadily as you move away from the board mean it is not a ring at all: it is broken somewhere and you are reading a radial. A spur reads higher than the socket it is taken from by the resistance of its own leg.

Line and neutral are the same size on the same route, so different readings mean different lengths of copper: an interconnection somewhere, a socket wired into only one of them, or a break. Find it before going on. A ring with a broken neutral looks like a ring at every socket and will not carry the current it is rated for.

Yes. The expected r2 ÷ r1 becomes about 2.6, because 1.5 mm² is that much more resistive than 4 mm², and the step-3 readings vary more around the ring than they do on 2.5/1.5, by as much as a quarter, because the two conductors are so different.

The figures go straight onto the certificate

In Pascal the maximum Zs is filled in for every circuit as you pick the device, the schedule of test results carries all 32 columns, and the observation library suggests the wording, code and regulation as you type. EICs, EICRs and minor works, from £25 a month.

See the certificate software

A calculator applies a method to the numbers you give it. It does not know the installation in front of you, and it is no substitute for BS 7671, the guidance or the judgement of the person signing the certificate. Check anything you rely on.