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Ring Continuity Calculator | r1 r2 & R1+R2 | TMUK Group

Ring Continuity Toolkit

Calculate r1, r2, R1+R2 and Zs for BS 7671 Testing

r1 (Line) 0.00
rn (Neutral) 0.00
r2 (CPC) 0.00
R1+R2 0.00
Expected Ring Length -- m
Calculated Zs -- Ω
Based on a CPC resistance multiplier of 1.67 (2.5/1.5mm²). Expected (R1+R2) = (r1 + r2) / 4.
Running ring circuits in containment? Use our Containment Fill Calculator to ensure your trunking meets the BS 7671 space factor requirements ➔

Understanding Ring Final Circuit Continuity Testing

Testing the continuity of a ring final circuit is one of the most fundamental and essential procedures carried out during an Electrical Installation Condition Report (EICR) or Initial Verification. A ring circuit relies entirely on the integrity of its continuous loops to share the electrical load. If there is a break in the ring, the cable can become severely overloaded, presenting a major fire risk.

The 1.67 Multiplier Rule (End-to-End Resistance)

During "Step 1" of testing, you measure the end-to-end resistance of the Line (r1), Neutral (rn), and CPC (r2). In the UK, the most common cable used for ring circuits is 2.5 mm² Line/Neutral with a 1.5 mm² CPC.

Because the Earth wire is smaller, its resistance is higher. Specifically, 2.5 divided by 1.5 equals 1.67. Therefore, the end-to-end reading of your earth (r2) should always be approximately 1.67 times higher than your line reading (r1).

Calculating R1+R2 (Cross-Connection)

During "Step 2", you cross-connect the incoming Line to the outgoing CPC, and vice versa (the "Figure 8"). Because the two loops of wire are in parallel, their combined resistance at the midpoint of the ring is exactly one-quarter of their combined end-to-end resistance.

Expected (R1+R2) = (r1 + r2) / 4

Extracting Continuity from Zs and Ze

If you are fault finding or verifying test results, you can calculate the expected R1+R2 continuity value directly from your live loop impedance readings. The total earth loop impedance (Zs) at the furthest socket is the sum of the supply loop impedance (Ze) and the resistance of the circuit conductors.

Zs = Ze + (R1+R2)

By rearranging this formula, we can isolate the continuity value. Simply subtract the Ze reading measured at the distribution board from the highest Zs value recorded on the ring.

(R1+R2) = Zs - Ze
Worked Example:
If your measured Zs at the furthest socket is 0.45 Ω and your distribution board Ze is 0.15 Ω:

0.45 Ω - 0.15 Ω = 0.30 Ω

The calculated continuity resistance (R1+R2) for this ring circuit is 0.30 Ω. This value should match the dead test reading.

Expert Electrical Testing with TMUK Group Ltd

Ensuring absolute accuracy during dead testing is critical for generating compliant EICRs and safeguarding properties. The NICEIC-accredited engineers at TMUK Group Ltd possess the expertise and calibrated equipment to accurately map, test, and fault-find complex commercial and industrial ring final circuits across the West Midlands. Contact us today to secure your statutory compliance.