Ring Final Circuit Test

What It Proves

The ring final test proves the ring is continuous (no breaks), that it is not bridged or spurred where it should not be, and that the line, neutral and protective conductors are connected correctly at every socket.

Test with the circuit isolated, locked off and proved dead.

Step 1: End to End

Measure the resistance of each conductor loop at the board. Label the readings:

• r1: line loop (both ends of the line conductor)
• rn: neutral loop
• r2: protective conductor (cpc) loop

Expect: r1 and rn read about the same. For 2.5 mm² line and neutral with a 1.5 mm² cpc, r2 reads about 1.67 times higher. That ratio only applies to that cable. For other cable sizes, work out the ratio from the cable resistances.

Step 2: Cross-connect Line and Neutral

Cross-connect the line of one leg to the neutral of the other leg at the board. Test between line and neutral at each socket.

Expect: the same reading at every socket, equal to (r1 + rn) ÷ 4.

Step 3: Cross-connect Line and Earth

Cross-connect the line of one leg to the cpc of the other leg at the board. Test between line and earth at each socket. Operate the switch on switched sockets to prove polarity.

Expect: the same reading at every socket, equal to (r1 + r2) ÷ 4.

Worked Example

2.5 mm² ring with 1.5 mm² cpc.

• Step 2 at each socket: (0.40 + 0.40) ÷ 4 = 0.20 Ω
• Step 3 at each socket: (0.40 + 0.67) ÷ 4 = 0.27 Ω

Reading the Results

• One socket reads higher than the rest: a spur, a poor connection at that socket, or a socket wired incorrectly.
• r1 and rn are not about the same: a cable or connection fault on one leg, or the ring is not what you think it is.
• Open circuit at one end: a break in the ring. The circuit may still work from the other end, which is why the test matters.
• Polarity wrong at a socket: found in step 3 when you operate the switch.

The step 3 reading, (r1 + r2) ÷ 4, is the R1+R2 for the ring. Use it when you calculate Zs. See Expected Readings.