Why was a fluescent light wired this way

[ADarkDividedGem]

I am replacing my single 36W fluorescent light fitting with a slim line LED version. Now that I have removed the fitting I am left with the following wires:


The wire on the left is ground the two wires in the middle were both connected to N and the wire on the right (with masking added afterwards to help identify it) connected to the L. Then there are three red wires, one from each Twin and Earth cable getting connected together and not doing anything.

Any idea why this wiring was needed?

Since my new fitting doesn't connect in the exact same place I need to extend this wiring. Any idea how best to do that?

 

[EFLImpudence]

Any idea why this wiring was needed?

Because that's how it is done.

https://www.diynot.com/wiki/Electrics:Lighting-Circuit-layouts

https://www.diynot.com/wiki/Electrics:Lighting-Rose

 

[333rocky333]

That is known as 3 plate wiring
One cable is the live in
One cable is the live out to the next room
One cable goes to the switch

Each cable consists of a red a black and an earth

 

[bernardgreen]

cable getting connected together and not doing anything.

As mentioned, one red wire is bringing the Live to the joint. one is taking Live on to another lamp and the third red is taking Live to the switch. ( that is the red in the cable with teh balck that went by itself to one side of the lamp,

Any idea how best to do that?

Use a terminal block with 4 connections

1 The three reds and nothing else.
2 The two blacks that were together and a wire to the Neutral of the new fitting
3 The single black and a wire to the Live of the new fitting ( this Switched Live from the switch and as such should have a red sleeve )
4 The three earths and a wire to the Earth terminal of the new fitting.

 

[ericmark]

There are advantages in the wiring system shown.
1) There is a permanent line for charging batteries in emergency lights and triggering them when there is a power cut.
2) The loop impedance is reduced and so also the volt drop.
3) Likely uses less cable.
4) Means a ceiling fan can be run without having light on.
There are also disadvantages.
1) Often no neutral at the switch.
2) When converting to lamps not designed for UK system often a junction box needs adding.
3) Because the ceiling rose is used as a junction box, and the ceiling rose is only rated at 6 amp, the supply fuse/MCB/RCBO has to also be rated no bigger than 6 amp.

When the silly MR16 lamp came out there was a move to using the switch back box rather than the ceiling rose as a junction box, but with a fluorescent fitting the standard three terminal connection block inside the lamps was simply swapped for a four terminal one instead. In mothers house we have no emergency lights, there is a window at the top of stairs and bottom, so in a power cut there is reasonable light from outside. In my own house the stairs are central and so I do have an emergency light at the top so giving light down the stairs. Also have an emergency light in garage lighting consumer unit. With the British system these were easy to fit.

Before the RCD I would not have bothered with an emergency light, but the regulations state:- Every installation shall be divided into circuits, as necessary, to: (iii) take account of danger that may arise from the failure of a single circuit such as a lighting circuit. So adding a RCD one has to ensure the circuits it forms do not cause danger when something trips the RCD, we consider in the main having sockets and lights on different RCD's does this, however adding an emergency light means there is no problem with the risk assessment when adding RCD protection.

Although in an office, shop, or railways premises you may need to install emergency lights, in the home it is up to the owner what he does, however the wiring at least makes it easy if the owner wants them.

Does that answer why we wire that way?

 

[ADarkDividedGem]

Does that answer why we wire that way?

Thanks that was a very helpful reply

 

[JohnW2]

3) Because the ceiling rose is used as a junction box, and the ceiling rose is only rated at 6 amp, the supply fuse/MCB/RCBO has to also be rated no bigger than 6 amp.

As you are aware, that is contentious and I, for one, do not agree with it (unless one proposes to connect a load to the rose/JB which takes >6A !).

Kind Regards, John

 

[securespark]

Yup, I agree. It's like 20A switches on a 32A breaker, but the switch is supplying < 20A.

 

[EFLImpudence]

I agree too - and I think we're back to the lack of 32A sockets, especially on a 32A radial circuit.


As a matter of interest, what is it that is limited to 6A in a ceiling rose?
The joining parts of the connector blocks are considerable larger than 1mm².

 

[JohnW2]

As a matter of interest, what is it that is limited to 6A in a ceiling rose? The joining parts of the connector blocks are considerable larger than 1mm².

As with other similar situations, I suspect that they would be perfectly happy/safe carrying a lot more than 6A but, for some reason (see **) are only tested (type tested) at 6A, and therefore cannot claim any higher 'rating'. ... a bit like, say, "50V cable", perhaps.

** it might even be that the relevant Standard only requires testing at 6A and that their testing seeks to do no more than confirm that it satisfies the testing.

Kind Regards, John

 

[ericmark]

I do see the argument that if we calculate on the largest lamp size which could reasonable be fitted and I seem to remember that is normally taken as being 60W so 23 lamps = 6A so even with a 10A MCB if the number of lamps is 23 or less the ceiling rose should not be overloaded.

However bitter experience now comes in, I fitted an outside lamp, there was a spare 16A B type MCB I used an 8W bulb from Ikea which should have a built in fuse in theory, this went short circuit and the pins of the lamp holder became welded to the bulb. I will admit a lamp holder BA22d is only rated at 2A which is why there should be a built in fuse in the bulb, so even with a 6A MCB it could still be overloaded and the bulb could still weld itself to the pins, but it is less likely. So there is good reason to limit the lighting supply to 6A even if there are no ceiling roses fitted.

 

[JohnW2]

I do see the argument that if we calculate on the largest lamp size which could reasonable be fitted and I seem to remember that is normally taken as being 60W so 23 lamps = 6A so even with a 10A MCB if the number of lamps is 23 or less the ceiling rose should not be overloaded.

That's true but, as I said, I strongly suspect that a ceiling rose can easily take a lot more than 6A (flowing through its 'terminal bars') without problem, but it is 'rated' at 6A simply because that is the current it is tested with.

... this went short circuit and the pins of the lamp holder became welded to the bulb. I will admit a lamp holder BA22d is only rated at 2A which is why there should be a built in fuse in the bulb, so even with a 6A MCB it could still be overloaded and the bulb could still weld itself to the pins, but it is less likely. So there is good reason to limit the lighting supply to 6A even if there are no ceiling roses fitted.

Yes, but that obviously goes beyond the concept that the circuit's OPD is there only to protect the cables of the fixed wiring of the circuit. If we chose OPDs to protect attached equipment that had not been appropriately internally fused, we would probably end up with sockets circuits that had to have an OPD <13A, since even a (possibly) 13A fuse in a plug might not be adequate to protect the equipment..

Kind Regards, John

 

[EFLImpudence]

I fitted an outside lamp, there was a spare 16A B type MCB I used an 8W bulb from Ikea which should have a built in fuse in theory, this went short circuit and the pins of the lamp holder became welded to the bulb.

Ok. Is that a disaster?

I will admit a lamp holder BA22d is only rated at 2A which is why there should be a built in fuse in the bulb,

I don't see why.
If you think there should be a fuse somewhere then surely there should be a fuse in the holder.

so even with a 6A MCB it could still be overloaded

That's not overload. You said short-circuit which it was.

and the bulb could still weld itself to the pins,

So what?

but it is less likely. So there is good reason to limit the lighting supply to 6A even if there are no ceiling roses fitted.

I think you are confusing different things.

What was the short-circuit current?
Was the short-circuit limited to 10A?

Would a 6A MCB have opened quicker than the 10A did.

 

[ericmark]

Although the short circuit current with a 6 A protective device is not 6 A but something higher time is also reduced so the likely hood of welding contacts with 6 A is less than with 10 A or 16 A so although it is permitted to have 16 A protective device a 6 A one is far better. As to speed that depends on protective device and the loop impedance and with a MCB if within the limits then it should not make any difference, however although short circuit when found do not know if it went full short circuit straight away?

With the old tungsten bulb under the British standard there should be a fuse built in, so if there was ionisation inside the bulb the fuse will rupture, however don't know if this extends to other types of bulb? However since the holder is rated 2A if not then we would need a lot of rewiring so I would think the fuse is still used, I think with LED bulbs the fuse is a resistor not sure with CFL.

I have learnt my lesson, no electrical goods from Ikea in the future. It was the same with Habitat their electrical goods were not really very good technically but they did look good, still have some lighting track somewhere.

 

[JohnW2]

Although the short circuit current with a 6 A protective device is not 6 A but something higher time is also reduced so the likely hood of welding contacts with 6 A is less than with 10 A or 16 A so although it is permitted to have 16 A protective device a 6 A one is far better.

As to speed that depends on protective device and the loop impedance and with a MCB if within the limits then it should not make any difference ... as you say, with an MCB it will not usually make any noticeable different. In the presence of a 'fault' (of 'negligible impedance'), with any fairly credible loop impedance the current is likely to be so high as to magnetically trip a B6, B10 or B16 (i.e. effectively with teh same speed.

... however although short circuit when found do not know if it went full short circuit straight away?

That's true, but in a very general sense. If you are going to start considering fault which are not of "negligible impedance", then you would probably end up concluding that virtually no circuit, of any type, is, in your opinion, "adequately protected"!

Kind Regards, John