Brain teaser - what got my spider sense tingling?

[happysteve]

Eh up

Just for fun, get you thinking...

Came across this on a job yesterday, which was to replace an 8 foot fluorescent with 2x 4ft integral LED battens. The fluorescent, in the loft of a boarded-out garage, was switched from a 3G switch (that did 2 other lights on the ground floor of the garage).

After I'd isolated and taken the lid off the old fitting, this is what I found:

I know it's hard squinting at photos of crossed-over wires trying to work out what's going on (and it wasn't much easier for me at the time) but something about this immediately made my spider sense start tingling... any idea what it was?

The black terminal connectors belong to the light fitting, and the earth is connected to the metal casing (there's a metal tab that goes into the central terminal block, as usual).

To help you out a bit, here's a diagram of the above (I don't have a brown pen, use your imagination!):

So the questions are:

(1) What are the roles of the two T&E cables?
(2) On the face of it, what's wrong with it?
(3) Why is this not permitted in the Regs?
(4) What (dead) tests might I do to make sure my suspicions are correct, and there isn't something else going on here?
(5) How should it be wired?
(6) Once I've replaced the light fitting (and wired up the replacements correctly), why is it really important that I inspect the 3G light switch?

As a bonus...:

(7) Why do you think my spider sense tingled straight away, before I'd even started consciously assessing what was going on here?

Enjoy

 

[gazdkw82]

Great post. Here's my take on it.

(1) What are the roles of the two T&E cables?

Usually supply to fitting and then supply to the next fitting. However if the other lights are on a different switches this cannot be the case. Perhaps supply in then feed back down to the switch which is breaking the neutral?

(2) On the face of it, what's wrong with it?

Incorrect polarity? Incorrect identification of conductors? Also there are 2 different sized condcutors by the looks of it.

(3) Why is this not permitted in the Regs?

Incorrect identification of switch or live conductors?

(4) What (dead) tests might I do to make sure my suspicions are correct, and there isn't something else going on here?

Long lead test from the switch to light to confirm switch wire. r1+r2 from the C/U to confirm supply.

(5) How should it be wired?

3 core supply t+e to switch. 3 core switched supply t+e to fitting.

(6) Once I've replaced the light fitting (and wired up the replacements correctly), why is it really important that I inspect the 3G light switch?

To confirm polarity and identification of live conductors.

As a bonus...:

(7) Why do you think my spider sense tingled straight away, before I'd even started consciously assessing what was going on here?

Not to sure, maybe the differently sized conductors? The rough --- work generally? The fact that it appears to be no neutral going through the load?

Best I can do at 5.20am after being up an hour already with the 3 year old.

Enjoy [/QUOTE]

 

[The Ghost]

That was a good reply @gazdkw82 I am not going to say anything as this is one for the trainees, isn't it? All I will say is as soon as I looked at that I would be saying "Hello, what is this?"

 

[happysteve]

Yep, some good answers there gazdkw82, and yes, this is for the trainees.

I'll leave it until late this evening before I comment further, to give other trainees an opportunity to share your thoughts

 

[The Ghost]

C'mon you trainees! Scared? Don't be nothing is going to happen have a go. I have noticed trainees don't usually respond to this kind of thing maybe because they are too scared I don't know. Such a shame really.

 

[MFS Electrical]

@Vortigern is right come on guys Stick your neck out and reply with things you think the worst that could happen is you get it wrong. But you can learn something from it so give it a bash!

 

[GMES]

As above, please don't be scared to have a go, whether you get it right or totally wrong it matters not, you will never learn if you don't have a go.

 

[Nigel]

To be honest I am struggling hahahaha

 

[happysteve]

Time to provide a bit of feedback, and a bit more insight into my spider sense.

It was the blue wires going to a connector block, in a light fitting. You see this quite often in a switch, if you're "feeding the switch" i.e. running a permanent live and neutral to the switch, and a switched live and neutral from the switch to the light.

In a light fitting (for instance a traditional ceiling rose) I'd expect an extra terminal for the permanent live... this is the "third plate" of the three plate method (the other two are N and switched live). You have one T&E which is the feed (permanent live, and neutral), then usually you'd send the permanent live to the switch on one core, and receive the switched live on the other. (This answers part (5) of the questions in the original post. You could also feed the switch with a N and do it that way, if you like, but the three-plate method is a legitimate way of doing it).

So even before I could work out what was going on, the presence of the blue wires in an extra terminal block got my spider sense going.

So the issue is one of POLARITY. But there are two possibilities of what sort of polarity issue we have. Either:

(a) The incoming polarity is incorrect, in other words incorrect identification of conductors (the blue is actually L, and the brown is actually N)... or...:

(b) The installer has decided to switch the N, rather than the L. This would mean that this incoming polarity in the feed cable is correct (L is brown, N is blue).

So where are we..? From the original post, I think we've sorted out (1), (2), probably (5), and (7).

My initial guess, out of options (a) and (b) above, was (b), i.e. switched neutral... with tests, my guess turned out to be correct.

So, that still leaves the other parts of the riddle:

(1) What are the roles of the two T&E cables?
(2) On the face of it, what's wrong with it?
(3) Why is this not permitted in the Regs?
(4) What (dead) tests might I do to make sure my suspicions are correct, and there isn't something else going on here?
(5) How should it be wired?
(6) Once I've replaced the light fitting (and wired up the replacements correctly), why is it really important that I inspect the 3G light switch?​

Crack on