Testing and verification
Ring continuity: equal line and neutral
- where
- reading at each point with ends cross-connected (Ω)
- line end-to-end resistance (Ω)
- neutral end-to-end resistance (Ω)
Explain this equation
Gives the reading you should get at each socket on a ring final circuit when the line and neutral ends are cross-connected. Use it to confirm the ring is complete and has no interconnections.
r1 is the end-to-end resistance of the line conductor and rn that of the neutral, each measured between the two ends of the ring. R_cross is the expected line-to-neutral reading at any point with the ends cross-connected.
Cross-connecting the ends turns the ring into a figure of eight. At any point the meter sees two equal paths in parallel, each (r1 + rn)/2. Two equal paths in parallel halve that again, giving a quarter of the sum, and it stays the same all the way round.
r1 and rn of 0.4 Ω each give 0.2 Ω at every socket. Readings should be substantially the same throughout; a higher reading at one point suggests a spur or a poor connection.
Watch out: this needs line and neutral of the same size, so r1 equals rn, and the calculator checks for that. Line-to-CPC readings with a smaller CPC use r2 instead. This is a dead test only.
For an intact uniform ring with equal line and neutral resistances, opposite-end cross-connections give two equal resistance paths in parallel at every point. Their combined resistance is one quarter of the sum of the end-to-end values.
This lesson concerns line-neutral readings with r1 = rn. CPC combinations with unequal conductor resistance do not generally give a perfectly constant reading around the ring.
Enter your values to see the result.
Common mistake
Claiming the same exact quarter-sum at every line-CPC point when conductor sizes differ.
Limits
Ideal ring without spurs or parallel paths; dead testing only. This calculation does not replace the complete ring test procedure and interpretation.
Sources
The tools calculate the displayed equation. They do not automatically choose cable sizes, approve installations, supply current regulatory tables, certify equipment or replace inspection and testing.