Spot light transformers keep blowing

[Charles Middleton]

All,

Ive just finished installing 9 spot lights in my parents kitchen. These are all LV each with a small transformer for each fixture. So far 3 of the transformers have blown and taken the lamps with them and I am starting to wonder if there is something wrong with the way I have done the wiring.

Ive taken advice from a previous thread and used junction boxes to go from fitting to fitting. Ive also continued the earths through the junction boxes although they are not used for each transformer.

Due to joist constraints etc I found it best to wire it up like this:

O----J2-O------O | | | O | 0 | J1 |

0 0 | | 0 0

J1 makes the position of the original ceiling roes which I have now wired into a junction box.

J2 is another junction box which I have used to split the circuit to feed the left and right sides of the room.

O is a junction box, transfomer and lamp installation point.

Does this look ok and is there any reason why 3 transformers plus lamps have blown to date?

Thanks in advance for constructive critisicm.

CM.

[Grunff]

Nothing wrong that I can think of. If transformers are blowing, I'd say it's one of 3 things:

1. The transformer is of insufficient rating for the lamps.

1. The transformer is getting too hot, for instance if it's immediately above the lamp, or wrapped in thermal insulation.

2. It's a faulty batch.

[BigWallop]

What type trannies are they? You might have to watch how many you have connected in parallel across the supply cable. I count ten points for lamps on your diagram, which might be a bit unsafe for a supply cable of only 1 mm csa'.

[Stefek Zaba]

Why would that be? The supply cable's carrying 240V, each lighting point will be 50W max, total load 500W or a gnat's whisker over 2A. Two gnats' whiskers if we allow for transformer losses. 1mmsq's good for 10-14A depending on installation method, and in any case there's a 6A MCB back at the board.

Most likely cause I can see is the transformers overheating - thermal insulation or being very close to the back of the bulbs (worse if they're dichroic - the heat you're not getting on your neck has to go somewhere, after all!)

[Lee]

Assuming these are electronic transformers then heat is probably the most likely cause. Or just poor quality units.

A bit of a long shot, but are the ones that have blown towards the "end" of the supply cable?

Lee

[BigWallop]

What's the resistance through each individual transformers primary winding? Configuring them all in parallel across the mains supply reduces the total resistance in the whole circuit, remember. So it wouldn't surprise me if the transformers that popped where the ones furthest from the incoming supply cable. The total spike created by all the primary coils collapsing at switch off will cause a huge spike across the last few transformers on the line.

[Charles Middleton]

I think your right although measures have been taken to space the transformer from the lamp. They could also be low quality units - I didnt pay much for them. The ones that have blown have been on random places on the cable not just at the end. CM.

[Charles Middleton]

1. The transformers where supplied as a package with the lamps but I'll check that when Im around there tomorrow.
2. Very possible - see other reply on this thread.
3. Again - very possible. Cheers, CM.

[Charles Middleton]

Hi, these are electronic transformers so Im not sure if they have windings. They are *supposed* to have "soft start" and electronic short circuit protection which should make them resistant to some abuse! Ive got three replacements now so Ill put those in tomorrow, take care to space them from the lamps and see how I get on. Cheers, CM.

[Grunff]

Ok so far...

Because??

How do you figure that?

(as an aside - there's a good chance these are electronic transformers)

[Derek *]

Are they real transformers or solid state switch mode convertors?

Ive come across *very* poor (ten-a-penny) specimens of some of these.

DG

[BigWallop]

Ah ha !!! They're switch mode converters !!! Well, check the same thing as I said before. There is a limit to how many you put on a single supply, because they also create lower resistance paths across the mains. But, saying that, they also come in all sorts of shapes and sizes, and they do make faulty batches from time to time, so they are more likely to have been faulty when you fitted them.

They also don't stand enclosed spaces to well either, so make sure they are in the open as much as you can to allow them to breath better. A distance of 100 mm around them is usually enough, and sometimes it helps if they are fixed to small pieces of thin wooden board to dissipate heat away from them more efficiently. Do the ones you have, have fixing points on them?

[Charles Middleton]

supply, because

saying that,

distance of 100

I knew I should have opted for a different solution - a main transfomer feeding several lamps positions away from the lamps rather than these small things. The thing is I wanted the installation to be accessible. They dont have any fixing points on them so Ill have another look at the positioning today.

Cheers,

CM.

[Stefek Zaba]

Oh well, if you say so.

Not sure Messrs Ohm, Kerchoff, Faraday, or Horowitz-n-Hill would worship the same deities as you; but faith-based electrical installation is all the rage these days, I hear...

[Grunff]

Is it much worse if the paths run along ley lines?