Mains wiring: Interpretation of LoZ measurements; two-way switches? - Page 1
Forum section: electronics / general

author: geralds
author_rank: Contributor
timestamp: July 01, 2025, 11:37:11 pm
content: Hi,Following some recent renovation works, we were looking to replace some light switches and mains sockets.The wiring is from 1979/1980 when the house was first renovated. The house is in Germany, so mains system is 230V / 50 Hz.In Room 1, the light switches are two-way switches (double two-way switches, so that two set of ceiling lamps can be switched on/off from two different locations (I will call it location “A” and “B”, and the light circuits “1” and “2”).There are 7 wires on each side (I guess 1 Live (L) and 2 On/Off per circuit? Plus PE). Only one circuit (ceiling lamps) is actually used.When the (first) electrician checked one of the old two-way switches he noticed that on the 2 connecting wires (not sure whether this is the right term, but I will refer to them as “C1” and “C2”, and "C3" and "C4" respectively) per circuit carried some higher voltage.Some weeks later another electrician came around to check and to re-connect some switches and sockets. Again, he just noticed the higher voltages in the two-way switches, but didn’t have a solution to fix it at the time.Neither electrician had a DMM with him, so they couldn’t check the actual voltages.I have now purchased a Brymen BM2257 in order to check the voltages myself, and get the following results (approx. values) at location A (tables also attached as PDF, as I am unsure how to insert a table in the post):Switch A.1 (used for cealing lights)Lights Off ACV (AH) LoZ AutoV (AH)L – PE 229.8 229.6L – C1 179.5 ----L – C2 162.8 ----C1 – PE 43.12 ----C2 – PE 60.7 ----C1 – C2 17.25 ----Lights on: 2.15 A on L and [C1/C2] – measured with Clamp MeterSwitch A.2 (unused / spare - no lights etc connected)ACV (AH) LoZ AutoV (AH)L – PE 229.5 230L – C3 182.4 ----L – C4 159.8 ----C3 – PE 41.24 ----C4 – PE 63.61 ----C3 – C4 22.01 ----Measurements at location B are similar.Against this backdrop, a couple of questions:1. How should I interpret the LoZ measurements? Will I need to do anything here, or just leave it as it is? Just in case, re-wiring will not be an option. Replacement of mains fuses / circuit breakers should be possible though, if that could be a solution.2. Are there any additional measurements that I should take in order to analyse the situation better? If so, what?3. The lamps (circuit 1) turn on only if the wires (L, C1, C2) are connected at both Switch A1. and B.1, i.e. if only Switch A.1 is connected, but not Switch B.1, the lights will not turn on. Is this correct in a two-way switched circuit?The electrician will return on Friday, mainly to fix the circuit in another (adjacent) room where he couldn’t find a (unswitched?) live wire in order to permanently connect power to the electric shutters (seems also a bit odd to me because this was working before they started the renovation work…).If you have any pointers what the electrician should check (and ideally fix), that will be much appreciated.Finally, pls accept my apologies if there is anything unclear or if terminology used is incorrect. I am not a professional, but keen to learn to the best of my abilities.Many thanks

author: themadhippy
author_rank: Super Contributor
timestamp: July 02, 2025, 12:14:40 am
content: QuoteThe connecting wires are often called strappers.Depending on how the circuit is wired you may require 3 wires between the 2 switches.First thing i'd do is trace were each and every wire at both switches go,preferable with a high voltage insulation tester (megger) .

author: floobydust
author_rank: Super Contributor
timestamp: July 02, 2025, 03:32:21 am
content: Without LowZ, a multimeter is too sensitive and normally will give ghost voltage readings as you are seeing. These are due to capacitive coupling between the cables.Some people even report their LED bulbs will mysteriously flash/strobe a few times a minute and this is due to the bulb getting a tiny bit of power from the cable capacitance charging up the bulb's power supply.This all looks like it works. You have over 2A when it's on. I would say trust the LowZ voltage readings to indicate if power is present or not, and do not go on a goose chase with the ghost voltage readings and waste time/money.

author: geralds
author_rank: Contributor
timestamp: July 02, 2025, 05:06:29 pm
content: Thank you both, @floobydust and @themadhippy! That is already helpful.I will ask the electrician whether he has an insulation tester and can do the testing (though he is not a specialist electrician, but works within a building/construction company, so will see).Otherwise he should just make sure the wires are properly connected to the switches.Separately, I forgot to mention that the electricity company mentioned last week that we ought to connect the ground from the mains inbox to the potential equalisation panel (currently ground is only connected to the water pipes). I understand that this is just an additional ground/earth connection, and assume this has nothing to do with the ghost voltage readings, but if I am mistaken let me know.