Light fitting wiring - this can't be right, can it?

[trev77]

Hello again,

I have now had a closer look at the light fitting wiring, which is actually working,

Untitled

• trev77
• 15 Jul 2008
• 1

but I'm a bit confused. The switch contains two wires, a red and a black, with the black going to L1. I'm happy about that, although I thought the black wire should have a bit of red tape on it....has that gone out of fashion now?

Then there are 2 cables coming into the light fitting, which I guess is 1 from the switch and one from the circuit, assuming this is the last light fitting on the circuit.

The earths are all together, but then things start to go a bit loopy....the light fitting brown (live) flex seems to be teamed up with a red wire. I would have expected a (switched) black cable to go to the live flex???
And there is one black cable going in with the blue (neutral) flex, with the red wire from that cable joining up with the black wire from the other cable.

Does that make sense?

I thought the neutrals should be in one terminal, the earths in another, the lives in another, and the switched live going to a terminal with the live flex for the light.

So why have I got a red cable joined in a terminal with my brown flex from the light?

Cheers

Trev

 

[Spark123]

but I'm a bit confused. The switch contains two wires, a red and a black, with the black going to L1. I'm happy about that, although I thought the black wire should have a bit of red tape on it....has that gone out of fashion now?

The black should have a red sleeve.

The earths are all together, but then things start to go a bit loopy....the light fitting brown (live) flex seems to be teamed up with a red wire. I would have expected a (switched) black cable to go to the live flex???
And there is one black cable going in with the blue (neutral) flex, with the red wire from that cable joining up with the black wire from the other cable.

I'm lost!! I can see 2 reds and 2 blacks in the pic, the black to the switch should also have a red sleeve. Is it that they have used black as permanent live and red as switch live?
Can you take a clearer photo showing the two cables and all the wires?

So why have I got a red cable joined in a terminal with my brown flex from the light?

If another light was switched with this one this may be the case, however I can only see the 2 pairs of wires at the bottom

 

[trev77]

Untitled

• trev77
• 15 Jul 2008
• 1

I've undone the light connections (well, the earth just broke actually), but the brown flex from the light was going into the leftmost terminal with the red wire, and the neutral from the light was attached to the same terminal as the black wire 3rd from the left, next to the green

Hope that makes it clearer.

To recap, I have 2 questions:

1. Why is there a red cable going into the terminal with the brown flex from the light?

2. Why are there 1 red and 1 black wire from the different cables joined together in the 2nd terminal?

Cheers

Trev

 

[Spark123]

You would ideally have a red sleeve on the black wire from the switch. The 2 reds would join together but to nothing else (use the loop terminal). The black with the red sleeve to the L and the other black to the N.
Both earths together and to the earth terminal.
As it stands, they are either switching the neutral or using the black switch wire is the permanent live. Do you have a multimeter?
Turn off the power, split all 4 wires out and using ohms range test across red - black on each cable whilst someone operates the switch.

 

[trev77]

Well, yes I do have a multi-meter, but I have only ever used it for testing dead batteries. So you've lost me a bit after "split all 4 wires out" - if I were to use the ohms range test (haha, sounds like I know what I'm talking about, but I don't!) with the red and black cables and the light switch being switched, what results would I expect and what would they prove? And how does the test work, is it like checking for dead batteries? Do you put the black probe on the black wire and the red probe on the red wire? Or is that too simple?

Cheers

Trev

 

[Spark123]

It is to identify which wires are going to the switch. When the leads of the multimeter are touched together they will have a low resistance. When they are parted they have a high resistance.
When a switch is closed (on the end of the cable) the result is a low resistance.
When the switch is opened the result is a high resistance.
You can measure this resistance with a multimeter set to ohms range by connecting the meter to the correct red and black whilst someone operates the switch.
Testing batteries is testing volts, slightly different but related.

 

[trev77]

Ok, I think I understand that, thanks for your patience. Now if I could just clarify a few things....

I have identified a section on the multi-meter for measuring ohms, it's the green bit Three choices, X1, X10, X1K. That's like a strength setting, huh? Don't know how many ohms I'm expecting to get.

The power should remain off for this test? But if there is no power, then switching the light on and off will make no difference to the ohms, am I right? Sounds dangerous to test live wires.

Am I testing each of the 2 cables by putting red probes on red wires and black probes on black wires, then switching the switch? One of the cables will have a result, the other won't. Which one means what?

Sorry, I'm not an electrician.

Cheers

Trev

 

[Spark123]

X1 will be fine, the primary use for these ranges on the multimeter is to measure resistance. If you were try measuring a resistor (a small electronic device) you will need to use it on the correct range which covers that resistance, i.e. you may not be able to see enough of the range to measure a 10K resistor on the X1 range. For this, the x1k range is better.
Anyhoo, back on track!
As the resistance of the wire and switch will be next to nothing the X1 range is fine.
The power must remain off for the test yes, the power to do the test is provided by the small battery in the multimeter.
The cable on which you see the resistance go from high to low etc when the switch is operated is the switch wire, you need to sleeve the black wire red on this cable. This will go towards ensuring you wire it up correctly when you reconnect it.

Just looking, I can't see a loop on the fitting you posted a pic on the other thread. You may need to use a piece of terminal block to join the 2 reds in this case.
Black with red sleeve to L
Black without sleeve to N
Both earths to the earth terminal.

 

[ban-all-sheds]

The earths are all together, but then things start to go a bit loopy....the light fitting brown (live) flex seems to be teamed up with a red wire. I would have expected a (switched) black cable to go to the live flex???

It will work just as well with red as the switched live.

And there is one black cable going in with the blue (neutral) flex, with the red wire from that cable joining up with the black wire from the other cable.

Does that make sense?

Yup.

I thought the neutrals should be in one terminal, the earths in another, the lives in another, and the switched live going to a terminal with the live flex for the light.

They are.

So why have I got a red cable joined in a terminal with my brown flex from the light?

Because the switch is wired with red as the switched live.

To recap, I have 2 questions:

1. Why is there a red cable going into the terminal with the brown flex from the light?

Because it's the switched live.

2. Why are there 1 red and 1 black wire from the different cables joined together in the 2nd terminal?

Because they are the permanent lives - the red is in the lighting loop, the black goes to the switch.

 

[securespark]

The left hand cable is the one going to the switch. In your case, the black is connected to the live from the other cable & the red is the switched return from the switch.

The right hand cable is live, neutral & cpc from the last light.

 

[trev77]

Hi

Well, I must be doing something wrong, because no matter what I do I don't get any readings on the meter. Touching the 2 probes together gives a resistance of about 7.5 to 8, but touching the red probe to the red wire and the black probe to the black wire simultaneously (of each cable in turn) gives no reading, and switching the switch makes no difference - still no reading for either cable.

And before you ask, yes, I am touching the probes against the exposed wire at the end, not the sheathed wires

Any idea what I'm doing wrong? It's a dimmer if that makes any difference.

Cheers, thanks for your time

Trev

Oh, and it's not legible but the block on the right says it's the loop, the metal block on the left is for earth, next to that is SW for the switch and the one between SW and Loop must be for the neutrals.

Untitled

• trev77
• 15 Jul 2008
• 2

 

[ban-all-sheds]

Any idea what I'm doing wrong? It's a dimmer if that makes any difference.

Yes it does - you won't see any continuity.

But why are you doing all this? The light works, and it's obvious from how the choc-block is wired which is the circuit cable and which is the switch cable....

 

[Spark123]

Because how do you know which one is the switch wire?
In the old setup it would work either way around, but could have been switched neutral. How would you know?
And yes, you can't measure it because it is a dimmer switch, you'll need to use a normal switch to be able to test continuity.

 

[ban-all-sheds]

Because how do you know which one is the switch wire?
In the old setup it would work either way around, but could have been switched neutral. How would you know?

From his description it was wired like this:

So yes - the cable on the left could be the circuit one, with either reversed polarity, or the neutral switched, but it all seems unlikely.

Anyway - surely the easiest thing to do rather than f*rt around trying to do continuity tests on the switch to find the switch cable would be to measure the voltages at the choc-block to identify the circuit one?

 

[Spark123]

Depends really, I prefer the continuity method as it doesn't involve asking people to poke around on a live circuit.