12 v downlights in house - failure (1 Viewer)

[Riverbankannie]

This isn’t Motorhome related but both a warning and a question why?
We have 6 downlights in our bathroom running in 2 sets of 3 off a transformer for each set.
One by one these have been failing, and it’s not been a failure of the lightbulb (halogen type).

We have read that the transformers are not reliable so have decided to replace with 240v led ones.

On removal of all the canisters, we found that all we found that all of the junction boxes, which connect the wire to the lamp holder, have signs of heat damage. It’s a bit scarey to think that this has been going on in our loft. There was no insulation covering them. The transformers have failed on all but one output.

So the question is, did the failure of the transformer cause the heating at the junction block or was it the other way round?

Either way, it’s not nice to think that this has been happening! In one or two instances, both terminals in the block have melted like this.

These lights are wired together with the bathroom fan, to one trip on the main board but this has never been tripped out.

 

[sonny]

Maybe the fault lies with the connecting block

 

[Riverbankannie]

Maybe the fault lies with the connecting block

That’s possible of course. It comes riveted to the side of the canister and they were purchased as fire rated down lighters. Everyone has overheated in the same way.

 

[Kev and Carolyn]

You have me concerned now with those....

Where are they from Annie? I have similar ones from Wickes. I must admit...I have a flickering one ??

Kev

 

[Riverbankannie]

You have me concerned now with those....

Where are they from Annie? I have similar ones from Wickes. I must admit...I have a flickering one ??

Kev

They were purchased through a friend who has an electrical wholesale business to the trade. We don’t have the original boxes any more and there are no makers marks on them.
We are going to replace the lamp holders with these GU10 ones and obviously remove transformers are re wire the lighting.
https://www.screwfix.com/p/luceco-gu10-gz10-lampholder/3751h
We were going to replace with integrated led downlights but they measure a little larger in diameter and we don’t want the bother of enlarging the ceiling holes.

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[Kev and Carolyn]

Thanks for the response Annie. Will check ours out tomorrow and get back to the forum ..

Regards Kev

 

[Burgo]

Always nice to see the brown wire connected to the blue one.

 

[pappajohn]

Terminal blocks with screws like that are a maintenance item.... Though once fitted they get forgotten.
Even wall sockets and light switches suffer the same problem in time.
The screws, over time, work loose due to the copper wires expending and contracting.
Once they become a little loose they can cause resistance, arcing and heat.

 

[Riverbankannie]

Always nice to see the brown wire connected to the blue one.

I did notice it and didn’t like to ask himself about that one!

 

[Riverbankannie]

Terminal blocks with screws like that are a maintenance item.... Though once fitted they get forgotten.
The screws, over time, work loose due to the copper wires expending and contracting.
Once they become a little loose they can cause resistance, arcing and heat.

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[pappajohn]

Always nice to see the brown wire connected to the blue one.

Makes no difference in this instance.
Unless you're stupid enough to stick a metal object in the very small bulb pin hole you can't touch either terminal of a GU10 lamp holder
And if you are stupid enough then tough, you won't try again.

 

[pickeringar]

I have had similar problems with Halgon lamps running off a transformer and hence 12v.

As most lamps are 50w at 12volt that means a continuious 4A current. By changing to mains 240v lamps, you reduce the current to around 0.2A and hence the likelihood of this happening again. With LED rather than Halogen you reduce the likelihood even more.

 

[sdc77]

When we moved into the current house we changed 34 downlights for LED ones and found probably 60% had signs of arcing/heat. A real worry.

 

[autorouter]

Terminal blocks with screws like that are a maintenance item.... Though once fitted they get forgotten.
Even wall sockets and light switches suffer the same problem in time.
The screws, over time, work loose due to the copper wires expending and contracting.
Once they become a little loose they can cause resistance, arcing and heat.

Agreed. There's a reason the fuse/MCB doesn't cut the current. The MCB will probably be 6 amps on a lighting circuit. The contact resistance gets hot due to 'stealing' the power from the appliance. 6 amps will allow well over a kilowatt of heat to be dumped in a connection before it trips. That's more than enough to set fire to the plastic insulation. Because it's a 2-core wire, the RCD probably will not trip either.

How can this be prevented? Use connectors certified for 'maintenance-free' situations, such as Wago-type connectors. Or at least make sure they are screwed very tight to start with.

https://www.screwfix.com/p/3-way-lighting-connector-224-series-pack-of-100/33918
In industry you'd use 'bootlace ferrules' that wrap round the bare wire strands and keep them all together and give a better contact

 

[Robin McHood]

Most probable cause poor original installation, basically the screws in the connector block weren't properly tightened. Unlikely the screws backed off in use unless your house vibrates a lot! Contributory factors are also cheap quality connectors that only use a screw to connect to the cable. Good quality ones have a metal leaf inside which serve to capture and compress all the cable strands ensuring that all the strands carry current. The less strands in full contact with the connector the more current the others have to carry which equals heat and over time causes the damage you have here. If the heat was from the transformer you would see signs of overheating on the cable insulation starting close to the transformer connections. You don't have that here.

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[RioRob]

Were these installed by a qualified electrician? Whoever did it didn't connect the wires correctly (blue to brown) didn't fix the outer cover under the strain relief and at a guess didn't prepare the ends of the wires correctly and tighten up the screws. Apart from all that, looks ok!

 

[138go]

Polarity of wires does not make any difference to this type of fitting as the bulb can be fitted either way round. Perhaps the fitting was only designed for 10W bulbs rather than 50W so they have got very hot. Removing and changing to LED fittings is a better idea as they don’t produce anywhere near the amount of heat.

 

[Robin McHood]

Polarity of wires does not make any difference to this type of fitting as the bulb can be fitted either way round. Perhaps the fitting was only designed for 10W bulbs rather than 50W so they have got very hot.

Its not the fitting that has got hot. If it was, you would see heat damage along the cables from the lamp starting at the fitting.

The brown core on the cable to the transformer is showing clear signs of insulation embrittlement due to heat at the screw connector. The heat has been generated at the screw connector and is radiating from that along the cable cores away from the connector. The heat generated in the connector, most likely due to poor assembly, has been sufficient to melt the plastic insulation around the connector. The European standard temperature rating for terminals of this type is 100C, a temperature that should never be reached in normal operation. To state that the wires can be connected either way round is functionally correct, in this case, but IS illegal under the IEE wiring regulations. The colours are there for a reason and should NEVER be disregarded. If any of my electrician's had done a job like this the least they could have expected is a written warning.

 

[pappajohn]

If the heat was from the transformer you would see signs of overheating on the cable insulation starting close to the transformer connections. You don't have that here.

How do you know? The transformer isn't pictured.

 

[Robin McHood]

How do you know? The transformer isn't pictured.

Because the cable going from the fitting towards the transformer shows no sign of heat damage even a few mills from the screw connector. As the heat travels along the copper cores away from the fault it diminishes the further it gets from the heat source. Oh and 55 years experience as Chief Electrical Engineer with legal responsibility for very large complex offshore electrical installations. Offshore, due to vibration, these sort of faults can have very serious consequences if such connectors are not properly made up. Frankly domestic (house) wiring gives me the heebie-jeebies (thats a professional term ).

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[pappajohn]

It could be an old multi outlet transformer and 2 or 3 metres from the downlighter.
Any heat caused by a poor connection would have dissipated long before it got to the fitting.
The damage was caused by loose terminal screws... Nothing else.

 

[pappajohn]

I remember fitting this as a temp measure in an outbuilding to extend a light until I got a proper joint box some 12 years ago... As usual it became permanent.
It started tripping the RCD last year, but only in damp weather.

 

[pappajohn]

The brown core on the cable to the transformer is showing clear signs of insulation embrittlement due to heat at the screw connector

I still can't figure this out.
The brown wire on the left to the transformer (2 core flex) is normal, the blue wire is burnt and appears to have partly pulled out of the terminal block.. Or the insulation has burnt off.

Both wires on the right, covered in braided heat resist sleeving, leading to the fitting do appear shiny and brittle at the terminal block.

Are you confusing the fitting with the transformer

 

[Robin McHood]

I still can't figure this out.
The brown wire on the left to the transformer (2 core flex) is normal, the blue wire is burnt and appears to have partly pulled out of the terminal block.. Or the insulation has burnt off.

Both wires on the right, covered in braided heat resist sleeving, leading to the fitting do appear shiny and brittle at the terminal block.

Are you confusing the fitting with the transformer

The photos are of two different fittings (you can tell because on one the wires are correctly colour connected bn/bn bu/bu and on the other, one side is reversed.) In the first photo the greatest damage is in the connector nearest the camera and to the left side of the connector on the bu core with the bn wire on the right side of that terminal showing the shiny look of heat embrittlement. On that fitting it looks like cause was improperly tightened screw on the left side. The connector further from the camera appears undamaged. Although the lamp side blue core has a slight shininess it doesn't have the telltale discolouration of the lamp side brown core. In the second photo there are two fault locations. In the lower connector the brown core from the transformer has severly overheated at the terminal screw but the upper connector is showing most damage at the fitting side. This may be heat transferrence via the lamp leads or possibly a second loose connection, this time on the lamp side of the connector. First option most likely in my opinion. On this second fitting, from what little I can see from zooming in on the connectors the two exposed parts of the lamp leads as they enter the connector do not appear to show any 'shiny' surface. As in both cases the damage appears to diminsh as you move away from the connector blocks I would suggest that the blocks are the prime fault location not the transformers or the lamp sockets. The transformer failures I would suggest are most likely due to overloading caused by the high resistive load of the failing lamp connectors coupled to the actual lamp load increasing the transformer current and tripping their inbuilt short circuit protection.

 

[Robin McHood]

Are you confusing the fitting with the transforme

In my earlier post, yes you are quite right, that should say (referring to the ops first photo) brown wire to the fitting

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[Richard n Ann]

The transformer failures I would suggest are most likely due to overloading caused by the high resistive load of the failing lamp connectors

But high (or higher) resistive load = lower current. I=V/R
I would suggest the transformers fail because they are Chinese shite

 

[pappajohn]

My apologies, I didn't look closely and assumed both pics were the same fitting

 

[Robin McHood]

But high (or higher) resistive load = lower current. I=V/R
I would suggest the transformers fail because they are Chinese shite

Yep that's true, obviously too early in the morning when I wrote that. Your second premise is more than likely correct

 

[WillH]

I am currently replacing all connector blocks on my downlighting with WAGO connections to bring them up to current standards.