I just replaced an existing dimmer switch (with two dimmers). I noticed that the previous wiring used what I understood to be the wrong colour codes.
One dimmer had blue => C and brown => L1.
The other had black => C and red => L1.
The blue and black wires are not banded or otherwise marked.
My understanding was that brown (and previously red) were permanent live and should go into C, so for my new dimmer I swapped them around and the lights work the same.
I know that doesn't mean they're safe, so should I keep the original wiring or use the new one based on colour? Is there any way to know for sure which is the switched live?
Best Answer
You figure out the live by using a voltmeter or voltage detector. If a voltmeter you compare it to neutral (or earth will suffice for testing, if present.)
You figure out the switched-live by noting which of the live wires is off with the switch off and dead with the switch missing.
Generally you are stuck with the colors that twin-and-earth come in: blue and brown (or black and red, I assume you're going back to those colors after Brexit completes?) So using a different color wire is not a luxury you have. Thqt's why I own 10 colors of tape, but many installers do not bother especially if the usage is obvious.
If power (always-live and neutral) are coming through the switch, the wire past the switch to the light will need neutral, so its usual neutral wire color must be used; the live color gets used for switched-live. i.e. You will see two blues spliced together in the back of the box and two browns on the switch.
If it is what we Yanks call a "switch loop", power comes to the light and only a single twin-and-earth goes to the switch. Neutral is not in the cable even though a neutral-colored wire is present. One is always-live and one is switched-live, anyone's guess which. The switch either connects them to each other, or does not. Dimmers might care which is the always-live, but if it's wrong, they just wouldn't work.