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.

Charles Middleton wrote:


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


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.

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

3. It's a faulty batch.


--
Grunff

"Charles Middleton" wrote in message
oups.com...
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.


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'.

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'.

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!)

Charles Middleton wrote:
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.

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
--
Email address is valid, but is unlikely to be read.

"Stefek Zaba" wrote in message
...


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'.

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!)


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.

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.

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.

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.

BigWallop wrote:

Configuring them all in parallel across the mains supply reduces the total
resistance in the whole circuit, remember.

Ok so far...


So it wouldn't surprise me if the
transformers that popped where the ones furthest from the incoming supply cable.

Because??


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.

How do you figure that?

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

--
Grunff

On Sat, 08 Jan 2005 15:50:09 +0000, Stefek Zaba
wrote:



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'.

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!)

Are they real transformers or solid state switch mode convertors?

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

DG

"Charles Middleton" wrote in message
oups.com...
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.


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?

BigWallop wrote:

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?

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.

BigWallop wrote:


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.

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...

BigWallop wrote:

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.


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


--
Grunff