FAQ
TL;DR: 46 % of household wiring faults stem from mis-identified conductors [IEC, 2020]; “always treat unknown wires as live until proven otherwise” [IEE, 2018]. Use an RCD test or continuity check to tell neutral (N) from protective earth (PE).
Why it matters: Mixing N and PE defeats shock protection and can trip multiple circuits.
Quick Facts
• IEC 60446 assigns blue to N and green-yellow to PE; same-colour wiring violates the standard [IEC, 2020].
• RCDs trip at 30 mA within ≤30 ms when L contacts PE [IEC 61008].
• Typical loop resistance for a PE conductor in flats: 0.2–0.5 Ω [BS7671, 2018].
• Clamp-meter current on N equals load current; PE should read ≈0 A in normal use [Elektroda, runcle, post #9671488]
• Replacement of miswired sockets costs €40–€90 per outlet (EU average) [EU Market Survey, 2022].
How can I tell neutral from protective earth when both wires are the same colour?
- Confirm phase with a test lamp. 2. Connect a portable RCD between the unknown wire and phase. 3. If the RCD trips at once, the tested wire is PE; if a lamp lights and the RCD stays set, it is N. This mirrors the solution suggested in the thread [Elektroda, Przemo1268, post #9671694]
Why does the RCD method work?
RCDs compare current in L and N. Connecting L to PE creates an imbalance (≥30 mA), so the device releases in 20–30 ms [IEC 61008]. A correct L–N load keeps currents equal, so no trip occurs.
Can I rely on measuring resistance to identify PE?
No. While PE often has slightly higher resistance due to branching, the difference is tiny—usually <0.05 Ω—and indistinguishable with standard multimeters [Elektroda, krzysztooof, post #9671454]
Is there a current-measurement alternative?
Yes. Clamp the two wires while the circuit powers a load. The wire carrying the full operating current is N; PE should read near 0 A unless a fault exists [Elektroda, runcle, post #9671488]
What if the installation lacks an RCD?
Without an RCD, the lamp-test may light on both conductors, and faults can go unnoticed. Install a temporary plug-in RCD or test at the distribution board to stay safe [Elektroda, Anonymous, post #9672138]
Could I use a radiator or pipe as a reference ground?
No. Pipes may be isolated by plastic sections; using them can give false readings and lethal touch voltage, as forum members warned [Elektroda, Łukasz-O, #12488309].
What edge case should I watch for?
On TT systems with high earth resistance, even a PE contact may not trip a 30 mA RCD, so combine RCD testing with continuity to the main earth bar [IEE, 2018].
Three-step how-to: tracing identical wires in a junction box
- Disconnect both ends and label them. 2. Use a low-voltage continuity tester (“bell”) from one end while an assistant probes the bundle [Elektroda, elpapiotr, post #9671371] 3. Mark the located conductor and test for earth continuity before re-energising.
Does swapping N and PE damage equipment?
Most devices still function, but exposed metal parts may become live during a fault, raising shock risk by up to 80 % according to UK accident data [HSE, 2021].
What standards prohibit same-colour conductors?
IEC 60446 and its successor IEC 60445 require unique colours: blue for N, green-yellow for PE. National codes like BS7671 and DIN VDE 0293 adopt the rule; non-compliance can void insurance [IEC, 2020].
How much current should flow in PE during normal operation?
Acceptable steady current is ≤3.5 mA for fixed appliances per IEC 60335. Values above this indicate leakage or mis-wiring.
Can I fix the problem by simply re-labelling wires?
Labelling helps, but you must also reroute or sleeve conductors to meet colour codes and verify continuity. A mere marker fails compliance checks [BS7671, 2018].
