Removed a Light Fitting Today

[endecotp]

Thought experiment:

You've got four wires coming out of the ceiling and a four-way terminal strip on a new light fitting. Q1: How many ways are there of connecting them up? Q2: How many of those "seem to work", i.e. the light goes on and off when you operate the switch, the MCB / RCD doesn't trip, there is no smoke? Q3: What is the probability of randomly fiddling with things until it works resulting in something dangerous that seems to work, before you actually get it right?

Answers after lunch maybe!

 

[JohnW2]

Thought experiment: You've got four wires coming out of the ceiling and a four-way terminal strip on a new light fitting. Q1: How many ways are there of connecting them up?

The answer to that actual question (assuming only one wire in each terminal) is 24, but that's not the situation which the OP observed. In his case, although there was a 4-way terminal strip, only 3 of the ways were used, necessitating putting at least two conductors into one of the terminals.

Q2: How many of those "seem to work", i.e. the light goes on and off when you operate the switch, the MCB / RCD doesn't trip, there is no smoke? Q3: What is the probability of randomly fiddling with things until it works resulting in something dangerous that seems to work, before you actually get it right?

In your 4-way terminal strip (with all 4 ways used), the answers to those questions would depend on what the four terminals were connected to on the fitting side.

In a simpler situation, with three incoming conductors (L, N & E) and three connections to the fitting (again, L, N & E), there would be 6 possible ways of connecting and, of these 6 ways:
2 (with light connected to L&N, one way or the other) would work and would be 'safe' (other than wrong polarity to lamp in one case)
2 (with light connected to L&E, one way or the other) would work and be relatively safe in the absence of an RCD, but should trip an RCD if present.
2 (with 'earth' of light connected to L) would not work, would be dangerous if there were exposed-conductive parts but might possibly result in an RCD trip (N-E 'fault' through lamp)
In no cases would the light 'work' but be dangerous.

Kind Regards, John

 

[ban-all-sheds]

Thought experiment:

You've got four wires coming out of the ceiling and a four-way terminal strip on a new light fitting. Q1: How many ways are there of connecting them up? Q2: How many of those "seem to work", i.e. the light goes on and off when you operate the switch, the MCB / RCD doesn't trip, there is no smoke? Q3: What is the probability of randomly fiddling with things until it works resulting in something dangerous that seems to work, before you actually get it right?

Q4: How wise is the general approach of trying to do electrical work on the basis of trial-and-error/guessing, hoping to get it right by chance because you don't actually have any knowledge, understanding or competence to bring to bear on the problem?

 

[JohnW2]

Q4: How wise is the general approach of trying to do electrical work on the basis of trial-and-error/guessing, hoping to get it right by chance because you don't actually have any knowledge, understanding or competence to bring to bear on the problem?

I think that was probably the very point endecotp was hoping to illustrate mathematically with the answers to his questions.

As I said to him, his Q2 and Q3 cannot really be answered without more information but, in the simpler case of random guesswork in connecting L, N & E to three terminals, in no case would the light work but be particularly dangerous.

Kind Regards, John

 

[endecotp]

Q1:

Say you first bunch the wires together, and then put the bunches into terminals.
You can have (a) four bunches of one wire,
Or (b) two bunches of one wire and one bunch of two wires,
Or (c) two bunches of two wires,
Or (d) one bunch of one wire and one bunch of three wires,
Or (e) one bunch of four wires.

The number of permutations for a-e respectively are 1, 6, 3, 4 and 1. But cases c and d can be considered together as they both result in two bunches, so we have:

1 way of having 4 bunches.
6 ways of having 3 bunches.
7 ways of having 2 bunches.
1 way of having 1 bunch.

The number of ways that 1 bunch can be put into 4 terminals is 4.
The number of ways that 2 bunches can be put into 4 terminals is 4 x 3 = 12.
The number of ways that 3 bunches can be put into 4 terminals is 4 x 3 x 2 = 24.
The number of ways that 4 bunches can be put into 4 terminals is 4 x 3 x 2 x 1 = 24.

So the total number of ways of wiring the fitting to the wires from the ceiling is
1x4 + 7x12 + 6x24 + 1x24
= 4 + 84 + 144 + 24
= 256

 

[endecotp]

Q2 is harder.

I think you can argue as follows: for it to "seem to work", you need the supply and the switch and the lamp in series, but in any order and with any polarity. If you take the lamp as the fixed point, since it is the terminal block, and start from its live terminal, the possibilities are

- switch or supply first
- correct or reverse polarity
- returning to the spare or the earth terminal
- now connected to the other of the switch or supply
- correct or reverse polarity
- returning to the lamp's neutral terminal.

That gives a total of 2 x 2 x 2 x 2 = 16 ways in which the wires can be connected to the terminals and appear to work.

So if you were randomly fiddling until it seemed to work, you'd try on average 256/16 = 16 permutations before you found one that seemed to work.

Q3: but how many of those 16 that seem to work are dangerous?

4 connect live to the lamp's earth.
4 connect neutral to the lamp's earth.
8 switch the neutral.
8 have the colours reversed at the switch.
(many do more than one of those things.)

Some of those faults are more dangerous than others. If we consider only the case here where the metal of the fitting is live, that's only 4 of the 16 ways of wiring it. So, you would need to try 16 random permutations until something appeared to work, and then you would have a one in four chance of it making the fitting live.

(BTW, I'm completely overlooking the (lack of) earth conductors in the cables to the switch and supply!)

 

[JohnW2]

Q1: Say you first bunch the wires together, and then put the bunches into terminals.

Fair enough, but that's obviously not what I was talking about. As I said ...

The answer to that actual question (assuming only one wire in each terminal) is 24 ....

.... So the total number of ways of wiring the fitting to the wires from the ceiling is 1x4 + 7x12 + 6x24 + 1x24 = 4 + 84 + 144 + 24 = 256

Agreed (with 'bunching' allowed) - but, as I said, without knowing where those four terminals 'go' in the fitting (4 would be unusual), one cannot determine which of those 256 possibilities would 'work' and/or which of them would be dangerous (your Q2 and Q3).

Kind Regards, John

 

[endecotp]

John, we have a picture of the terminal at the start of the thread; you can see L, N & E going to the lamp and one spare.

 

[EFLImpudence]

In the second picture, there appears to be an extra wire.

What would usually be the neutral connection looks like two blacks and
what would usually be the switched live looks blue - but no browns.

Speculation is not going to help.

 

[JohnW2]

John, we have a picture of the terminal at the start of the thread; you can see L, N & E going to the lamp and one spare.

Yes, but I misunderstood. You wrote:

Thought experiment: You've got four wires coming out of the ceiling and a four-way terminal strip on a new light fitting. ....

... and I took that to be a hypothetical generalised 'thought experiment', not relating specifically to the situation described in this thread. My apologies for misunderstanding.

Your proposed answers to your Q2 and Q3 assume that the two incoming cables are an L/N supply and a pair going to a switch. However, for the person who hasn't got a clue as to what's going on and is attempting 'trial and error', that's only one possibility. Another, for example (which would really need to be factored into your calculations) would be that one was a S/L & N (maybe coming from the JB) and the other was a secondary feed (again S/L & N) going to some other light.

Kind Regards, John

 

[JohnW2]

In the second picture, there appears to be an extra wire.

There does indeed. However, that's a different fitting, hence might well have some totally different variant of 'imaginative wiring'!

Kind Regards, John

 

[EFLImpudence]

Yes, but the green and yellow is connected the same way.

 

[JohnW2]

Yes, but the green and yellow is connected the same way.

Yes, it appears to be. I also forgot to add that it is presumably not just "an extra wire" but, rather, "an extra cable"(with two conductors), since there are 6 incoming conductors.

Kind Regards, John

 

[JohnW2]

The thing I find most difficult to understand is that I somehow doubt that what we're seeing is the result of someone working their way through all 256 of endecop's 'possible arrangements' (with or without 'bunching') until they found one which 'worked'. I would think that it's more likely that what we're seeing is the 'first effort' of someone who believed that they "knew what they were doing" - and that is hard to understand! We see/hear some pretty dodgy things from time to time in this forum, but this one must be a strong contender for 'the prize'!

Kind Regards, John

 

[endecotp]

In reality, I suspect that the 256 combinations are reduced by some "common sense" (but wrong) rules in the mind of the person fitting it such as "everything must connect to something else". I.e. any combination in which the earth terminal is not connected would be ruled out as "obviously wrong". Combined with some other less-wrong rules such as "the two reds were joined together before so they should still be joined together now", and you might be down to just a handful of combinations.