To fuse equipment or not to?

[John Woodgate]

I read in sci.electronics.design that Rich Grise <richgrise@example.net>
wrote (in <pan.2005.03.26.00.05.52.308759@example.net&gt about 'To fuse
equipment or not to?', on Sat, 26 Mar 2005:

On Fri, 25 Mar 2005 17:47:56 +0000, Pooh Bear wrote:

I'd not advise using bare fuse wire though. It works by melting i.e. hot !
You could have a potential fire hazard.


Real fuse wire melts way before it gets hot enough to burn anything.
It melts even cooler than solder. I found this out the hard way - I
lost a lug in a solder pot. (think Wood's metal spoon)

This stuff can't be used for low currents because it's too fragile.

AFAIK, copper wire is used.
--
Regards, John Woodgate, OOO - Own Opinions Only.
There are two sides to every question, except
'What is a Moebius strip?'
http://www.jmwa.demon.co.uk Also see http://www.isce.org.uk

 

[Tony Williams]

In article <fMzTbUEEkRRCFwoA@jmwa.demon.co.uk>,
John Woodgate <jmw@jmwa.demon.contraspam.yuk> wrote:

That could indicated a failure to conform to IEC/EN 60065.

All the wall warts (and even some small transformers)
that I have looked at have had a Woods-metal fusible
link inside them, close to and in series with the
primary. Used as an overtemperature failsafe.

--
Tony Williams.

 

[John Woodgate]

I read in sci.electronics.design that Tony Williams
<tonyw@ledelec.demon.co.uk> wrote (in
<4d51a1bbcatonyw@ledelec.demon.co.uk&gt about 'To fuse equipment or not
to?', on Sat, 26 Mar 2005:

In article <fMzTbUEEkRRCFwoA@jmwa.demon.co.uk>,
John Woodgate <jmw@jmwa.demon.contraspam.yuk> wrote:

That could indicated a failure to conform to IEC/EN 60065.

All the wall warts (and even some small transformers)
that I have looked at have had a Woods-metal fusible
link inside them, close to and in series with the
primary. Used as an overtemperature failsafe.

You reply does not relate to the quoted text but to my remark about
'real (low-melting point) fuse wire'. Note 'wire' - pellets are a
different matter when it comes to fragility, obviously.
--
Regards, John Woodgate, OOO - Own Opinions Only.
There are two sides to every question, except
'What is a Moebius strip?'
http://www.jmwa.demon.co.uk Also see http://www.isce.org.uk

 

[Pooh Bear]

John Woodgate wrote:

I read in sci.electronics.design that Pooh Bear
rabbitsfriendsandrelations@hotmail.com> wrote (in
42448C19.C1FC919F@hotmail.com&gt about 'To fuse equipment or not to?',
on Fri, 25 Mar 2005:

Interesting case regarding fusing. We just had an email about one of
our amplifiers that had failed, where the PSU pcb was destroyed by some
internal fault elsewhere. The track got so hot in one place that the
board was charred rotten and the track wasn't there anymore. FR4
material so no fire of course.

That could indicated a failure to conform to IEC/EN 60065.

I was chewing that one over. Would that be a fail due to temp rise of the
pcb ?

I happen to have EN60065 at home at the moment so any guidance as to finding
the relevant clause would be helpful.

I would really like to put my colleague *right* about my preference for
fusing - he's been on about reducing this for over 10 yrs. That's what
they're there for - to stop fires.

Incidentally I've come across a similar scenario with a QSC amplifier range
( unfused DC rails ) - where a far eastern company made a copy ( omitting
the fuses ) and their test house insisted they be included. I know test
houses sometimes differ over interpretation but this seems a big one to
disagree about.


Graham

 

[John Woodgate]

I read in sci.electronics.design that Pooh Bear
<rabbitsfriendsandrelations@hotmail.com> wrote (in
<4245781A.8052ADC4@hotmail.com&gt about 'To fuse equipment or not to?',
on Sat, 26 Mar 2005:

I was chewing that one over. Would that be a fail due to temp rise of
the pcb ?

Yes.

I happen to have EN60065 at home at the moment so any guidance as to
finding the relevant clause would be helpful.

If you have the 2002 edition, it's 11.2.3 to 11.2.6 and Table 3 b) and
e), column 3.

I would really like to put my colleague *right* about my preference for
fusing - he's been on about reducing this for over 10 yrs. That's what
they're there for - to stop fires.

It's not only that. If the board burns up under fault conditions, what
should have been a simple replacement of a faulty component becomes a
major cost or even a write-off case. That leaves you with a very
disgruntled FORMER customer.
--
Regards, John Woodgate, OOO - Own Opinions Only.
There are two sides to every question, except
'What is a Moebius strip?'
http://www.jmwa.demon.co.uk Also see http://www.isce.org.uk

 

[Pooh Bear]

John Woodgate wrote:

I read in sci.electronics.design that Pooh Bear
rabbitsfriendsandrelations@hotmail.com> wrote (in
4245781A.8052ADC4@hotmail.com&gt about 'To fuse equipment or not to?',
on Sat, 26 Mar 2005:

I was chewing that one over. Would that be a fail due to temp rise of
the pcb ?

Yes.

I happen to have EN60065 at home at the moment so any guidance as to
finding the relevant clause would be helpful.

If you have the 2002 edition, it's 11.2.3 to 11.2.6 and Table 3 b) and
e), column 3.

I don't have that edition but I'm sure that'll point me in the right
direction.

I would really like to put my colleague *right* about my preference for
fusing - he's been on about reducing this for over 10 yrs. That's what
they're there for - to stop fires.

It's not only that. If the board burns up under fault conditions, what
should have been a simple replacement of a faulty component becomes a
major cost or even a write-off case. That leaves you with a very
disgruntled FORMER customer.

Errr - quite ! I agree wholeheartedly.


Thanks, Graham

 

[Fred]

"John Woodgate" <jmw@jmwa.demon.contraspam.yuk> wrote in message
news:s1MPlbb8kvQCFwEl@jmwa.demon.co.uk...

I read in sci.electronics.design that John Larkin
jjSNIPlarkin@highTHISlandPLEASEtechnology.XXX> wrote (in
0dq54198d00h6l4h1m0ogjvf6hbbpfk664@4ax.com&gt about 'To fuse equipment or
not to?', on Thu, 24 Mar 2005:

Around here some transformers (like for warts or doorbells) are "class 2"
impedance limited. If you short them, they just get sort of hot.
Regulation is ghastly, of course.

It doesn't have to be. In a normal transformer, with just two windings,
you allocate half the winding area to the primary and half to the
secondary, so that the resistive loss is equal (well, nearly) in both.

For an impedance-limited design, you have to allocate much less area to
the primary so that its resistance is high enough to limit the
short-circuit current to a safe value. You then let the secondary take up
all the remaining winding area, so that its resistive loss is much smaller
than in a normal transformer.

You get approximately the same regulation as for a normal transformer, but
of course if it's a very small transformer, using laminations with
conventional geometry, it won't be all that good anyway. These small
transformers really need lams with a BIG window in proportion to their
size, because the insulation on the wire, and the bobbin if there is one,
take up more winding area in proportion, at the expense of the copper,
than they do in a larger transformer.
--
Regards, John Woodgate, OOO - Own Opinions Only.
There are two sides to every question, except
'What is a Moebius strip?'
http://www.jmwa.demon.co.uk Also see http://www.isce.org.uk

I'm not convinced by this argument. You might as well distribute resistive
losses equally between the two windings to distribute power loss rather that
just in one winding. You'd also need more copper with putting all the
resistive loss in one winding.

Another method of introducing impedance is inductive loose coupled windings
but this makes the transformer more difficult design and hence more
expensive. The resistive - lossy method is cheaper using less copper and
hence becomes the chosen solution. It's a doddle with small transformers..

 

[Pooh Bear]

Fred wrote:

"John Woodgate" <jmw@jmwa.demon.contraspam.yuk> wrote in message
news:s1MPlbb8kvQCFwEl@jmwa.demon.co.uk...
I read in sci.electronics.design that John Larkin
jjSNIPlarkin@highTHISlandPLEASEtechnology.XXX> wrote (in
0dq54198d00h6l4h1m0ogjvf6hbbpfk664@4ax.com&gt about 'To fuse equipment or
not to?', on Thu, 24 Mar 2005:

Around here some transformers (like for warts or doorbells) are "class 2"
impedance limited. If you short them, they just get sort of hot.
Regulation is ghastly, of course.

It doesn't have to be. In a normal transformer, with just two windings,
you allocate half the winding area to the primary and half to the
secondary, so that the resistive loss is equal (well, nearly) in both.

For an impedance-limited design, you have to allocate much less area to
the primary so that its resistance is high enough to limit the
short-circuit current to a safe value. You then let the secondary take up
all the remaining winding area, so that its resistive loss is much smaller
than in a normal transformer.

You get approximately the same regulation as for a normal transformer, but
of course if it's a very small transformer, using laminations with
conventional geometry, it won't be all that good anyway. These small
transformers really need lams with a BIG window in proportion to their
size, because the insulation on the wire, and the bobbin if there is one,
take up more winding area in proportion, at the expense of the copper,
than they do in a larger transformer.
--
Regards, John Woodgate, OOO - Own Opinions Only.
There are two sides to every question, except
'What is a Moebius strip?'
http://www.jmwa.demon.co.uk Also see http://www.isce.org.uk

I'm not convinced by this argument. You might as well distribute resistive
losses equally between the two windings to distribute power loss rather that
just in one winding. You'd also need more copper with putting all the
resistive loss in one winding.

Another method of introducing impedance is inductive loose coupled windings
but this makes the transformer more difficult design and hence more
expensive. The resistive - lossy method is cheaper using less copper and
hence becomes the chosen solution. It's a doddle with small transformers..

You're missing the point Fred. The idea is to make the primary 'impedance
protected' like those Boxer AC fans of old.


Graham

 

[John Larkin]

On Sun, 27 Mar 2005 18:04:09 +0100, "Fred" <Fred@nospam.com> wrote:

"John Woodgate" <jmw@jmwa.demon.contraspam.yuk> wrote in message
news:s1MPlbb8kvQCFwEl@jmwa.demon.co.uk...
I read in sci.electronics.design that John Larkin
jjSNIPlarkin@highTHISlandPLEASEtechnology.XXX> wrote (in
0dq54198d00h6l4h1m0ogjvf6hbbpfk664@4ax.com&gt about 'To fuse equipment or
not to?', on Thu, 24 Mar 2005:

Around here some transformers (like for warts or doorbells) are "class 2"
impedance limited. If you short them, they just get sort of hot.
Regulation is ghastly, of course.

It doesn't have to be. In a normal transformer, with just two windings,
you allocate half the winding area to the primary and half to the
secondary, so that the resistive loss is equal (well, nearly) in both.

For an impedance-limited design, you have to allocate much less area to
the primary so that its resistance is high enough to limit the
short-circuit current to a safe value. You then let the secondary take up
all the remaining winding area, so that its resistive loss is much smaller
than in a normal transformer.

You get approximately the same regulation as for a normal transformer, but
of course if it's a very small transformer, using laminations with
conventional geometry, it won't be all that good anyway. These small
transformers really need lams with a BIG window in proportion to their
size, because the insulation on the wire, and the bobbin if there is one,
take up more winding area in proportion, at the expense of the copper,
than they do in a larger transformer.
--
Regards, John Woodgate, OOO - Own Opinions Only.
There are two sides to every question, except
'What is a Moebius strip?'
http://www.jmwa.demon.co.uk Also see http://www.isce.org.uk

I'm not convinced by this argument. You might as well distribute resistive
losses equally between the two windings to distribute power loss rather that
just in one winding. You'd also need more copper with putting all the
resistive loss in one winding.

The only advantages I can imagine to lopsided winding losses would be

1. Thermal. Since cooling is critical to an impedance-limited
transformer, it may be better to have the heat where it's easiest to
conduct it out.

2. If you also want to protect against an internally shorted primary.
Skinny wire will just fuse.


But otherwise, lowering the short-circuit current trashes the
regulation in proportion, no matter how you distribute the losses. You
can always model a transformer as an ideal transformer with all the
losses lumped and reflected to a resistor on one side or the other.

John

 

[John Woodgate]

I read in sci.electronics.design that John Larkin
<jjSNIPlarkin@highTHISlandPLEASEtechnology.XXX> wrote (in
<nnud415eci3rvv73vd3gdc6hm0hnsv1chh@4ax.com&gt about 'To fuse equipment
or not to?', on Sun, 27 Mar 2005:

But otherwise, lowering the short-circuit current trashes the
regulation in proportion, no matter how you distribute the losses.

My comments were not about impedance-protection but the use of
safely-fusible wire for the primary winding. I have been looking for a
mathematical analysis, but the core geometry is involved, so no general
solution is possible.

You can't get as good regulation with any other winding area
distribution than that which makes winding resistances proportional to
turns-squared. But the loss need not be disastrous.
--
Regards, John Woodgate, OOO - Own Opinions Only.
There are two sides to every question, except
'What is a Moebius strip?'
http://www.jmwa.demon.co.uk Also see http://www.isce.org.uk

 

[Pooh Bear]

John Woodgate wrote:

I read in sci.electronics.design that Pooh Bear
rabbitsfriendsandrelations@hotmail.com> wrote (in
42444ECC.FBAF0ABB@hotmail.com&gt about 'To fuse equipment or not to?',
on Fri, 25 Mar 2005:

I'd not advise using bare fuse wire though. It works by melting i.e.
hot ! You could have a potential fire hazard.

Not enough energy involved, unless you use one of those Celluloid
printed boards. (;-)

At the current involved I guess so.

Interesting case regarding fusing. We just had an email about one of our
amplifiers that had failed, where the PSU pcb was destroyed by some
internal fault elsewhere. The track got so hot in one place that the board
was charred rotten and the track wasn't there anymore. FR4 material so no
fire of course.

For ages one of my colleagues has wanted to reduce internal fusing. Now he
can see the effects !

I'm just considering doing the maths for cost saving in manufacture vs
cost of replacing trashed product. Not to mention customer confidence.


Graham