Fuse Failure Machanism ...??

If you were here in the US midwest, I would say it was a nearby lightning strike and the fuses kept arcing after the first one opened. Anything like that possible in your area?

I can only say: good fuses!

A massive overcurrent, I'm sure, will blow all. I've seen two blow like this, I suspect one fails, but the failure arc is enough to cause the other (compromised at this point) to blow as well.

The lightning strikes sounds plausible.

I think there's another possibility...

Suppose the overload were enough to severely _bend_ the fuses, rather than blow them outright. When one finally popped, the others flexed back and cracked. So... Are they clear glass? Do you seen vaporized metal?

Another possibility is that the overload was _so_ severe that they really _did_ all blow at the same time. The fuse wire, however small it is, has thermal inertia. It can still break, even after the power has been removed, if it was hot enough a millisecond or two before.

Regardless, it's an interesting question.

Are those slo-blow fuses? If the fuse case is clear, it will show a spring and a resitor inside.

If it is a slow blow, these take a while to open after seeing an overload. It depends on the amount of overload and resistor heating. There's a small delay during which time the other fuses are also receiving the full benefit of the overload. Even if the load is removed during this time, thermal inertia (the time it take for heat to make it from the resistor to the fuse melt area) will cause the other fuses to blow.

However, if it's NOT a slo-blow fuse, then there's insufficient mass in the fuse wire to produce any thermal inertia effects, so this mechanism is unlikely.

--
Jeff Liebermann     jeffl@cruzio.com
150 Felker St #D    http://www.LearnByDestroying.com
Santa Cruz CA 95060 http://802.11junk.com
Skype: JeffLiebermann     AE6KS    831-336-2558

Ok. Few points from various people to answer here. The lightning strike scenario is a possibility. There have been a couple of storms in the area in the last few weeks, but in general, it's been a pretty quiet season here in the UK this year. The O/C main filter cap might just be a red herring here. I must admit that I replaced it at the same time as the fuses, as it was obviously bulging both top and bottom, and read open on the ESR meter. It could be that it was on its last legs ESR-wise, but still had enough 'guts' to perform its basic function, until the surge that took the fuses out, finished it off. That might add support to the lightning strike theory.

As to the fuse types. I take it the term "Wickman fuse" doesn't have any meaning in the U.S.? If not, allow me to explain. Wickmans are a type of wire-ended fuse for direct PCB mounting. They are a plastic cylinder about

10mm diameter and maybe 10 -12mm high. They have two leads coming out of the bottom pitched at probably 5mm. Imagine a small stubby radial leaded electrolytic, if you like. The plastic is usually either black or brown. They have always been popular in Philips gear, although other manufacturers do use them. They come in both fast and "T" rated types. The ones that were fitted in this particular supply, were "T" type, so yes, slo-blo's. However, with them being plastic, you get no indications of how they have failed. I am of course very familiar with the three normal failure modes of vapourised, melted, and broken, and if you could see how these had failed, I'm sure it would have given a clue as to what was going on. To be honest, I'm not even sure of how these fuses are constructed internally. I might try and break into one tomorrow to get a look.

As to the plugtop fuse, these are ceramic bodied, so again, you can't see the failure mode. I guess you might not have ever even seen one in the U.S. I've no idea what the fusing characteristic of them is. They come in 3A, 5A,

10A and 13A flavours, but most people would consider them to be little more than a calibrated nail. I have seen them stand up to overloads where you could almost hear the substation transformer jumping off its mountings, and yet they will also readily fail when a light bulb pops its clogs.

I quite like the thermal inertia idea as an explanation, but even though the Wickmans are "T" rated, I'm not sure that there is sufficient wire in that little case (assuming that it is in some way a 'conventional' wire fuse inside) to actually have that much inertia ...

Thanks all for your thoughts (including bz who responded (accidentally ?) directly to me). It's an interesting little puzzle.

Arfa

On Tue, 8 Sep 2009 01:53:05 +0100, "Arfa Daily" wrote:

: :"Jeff Liebermann" wrote in message :news: snipped-for-privacy@4ax.com... :> On Mon, 7 Sep 2009 13:11:51 +0100, "Arfa Daily" :> wrote: :>

:>>It had two 2.5A Wickman fuses just south of the :>>line input connector, so that's one each in the live and neutral feeds. :>

:> Are those slo-blow fuses? If the fuse case is clear, it will show a :> spring and a resitor inside. :>

:> If it is a slow blow, these take a while to open after seeing an :> overload. It depends on the amount of overload and resistor heating. :> There's a small delay during which time the other fuses are also :> receiving the full benefit of the overload. Even if the load is :> removed during this time, thermal inertia (the time it take for heat :> to make it from the resistor to the fuse melt area) will cause the :> other fuses to blow. :>

:> However, if it's NOT a slo-blow fuse, then there's insufficient mass :> in the fuse wire to produce any thermal inertia effects, so this :> mechanism is unlikely. :>

:>

:>

:> --

:> Jeff Liebermann snipped-for-privacy@cruzio.com : : :Ok. Few points from various people to answer here. The lightning strike :scenario is a possibility. There have been a couple of storms in the area in :the last few weeks, but in general, it's been a pretty quiet season here in :the UK this year. The O/C main filter cap might just be a red herring here. :I must admit that I replaced it at the same time as the fuses, as it was :obviously bulging both top and bottom, and read open on the ESR meter. It :could be that it was on its last legs ESR-wise, but still had enough 'guts' :to perform its basic function, until the surge that took the fuses out, :finished it off. That might add support to the lightning strike theory. : :As to the fuse types. I take it the term "Wickman fuse" doesn't have any :meaning in the U.S.? If not, allow me to explain. Wickmans are a type of :wire-ended fuse for direct PCB mounting. They are a plastic cylinder about :10mm diameter and maybe 10 -12mm high. They have two leads coming out of the :bottom pitched at probably 5mm. Imagine a small stubby radial leaded :electrolytic, if you like. The plastic is usually either black or brown. :They have always been popular in Philips gear, although other manufacturers :do use them. They come in both fast and "T" rated types. The ones that were :fitted in this particular supply, were "T" type, so yes, slo-blo's. However, :with them being plastic, you get no indications of how they have failed. I :am of course very familiar with the three normal failure modes of :vapourised, melted, and broken, and if you could see how these had failed, :I'm sure it would have given a clue as to what was going on. To be honest, :I'm not even sure of how these fuses are constructed internally. I might try :and break into one tomorrow to get a look. : :As to the plugtop fuse, these are ceramic bodied, so again, you can't see :the failure mode. I guess you might not have ever even seen one in the U.S. :I've no idea what the fusing characteristic of them is. They come in 3A, 5A, :10A and 13A flavours, but most people would consider them to be little more :than a calibrated nail. I have seen them stand up to overloads where you :could almost hear the substation transformer jumping off its mountings, and :yet they will also readily fail when a light bulb pops its clogs. : :I quite like the thermal inertia idea as an explanation, but even though the :Wickmans are "T" rated, I'm not sure that there is sufficient wire in that :little case (assuming that it is in some way a 'conventional' wire fuse :inside) to actually have that much inertia ... : :Thanks all for your thoughts (including bz who responded (accidentally ?) :directly to me). It's an interesting little puzzle. : :Arfa :

I would be more concerned with what caused the filter cap to bulge...

If it is a well known brand and correctly rated for the application then it should not have degenerated to that state as long as normal input voltage range was experienced. I also doubt that a possible short term lightning event would have caused the cap to bulge - this symptom is more likely due to a longer term overvoltage event.

If all the major input circuitry and components are ok I would suspect a leaky diode in the bridge rectifier.

Presumably, you don't see too many small high current switchers ? This is a small supply - probably around 6" x 3" x 1.5", and fully enclosed in a plastic case. The cap in question was a CapXon 120uF at 450v, and a 105 deg type, so a common enough make, more than adequately rated for the job. As is ever the case, it is the way that the designer has used it, that has led to its demise. It was layed over flat on the board, jammed in by the heatsink that carries the secondary side reccies etc. This is a very typical scenario, which anyone who works with them a lot, will tell you leads to drying out, bulging, and ultimate failure.

I'm not sure where you got it from that a lightning event was thought to be responsible for this bulging, Ross. I don't think I actually said that. The supply was dead as received. When I opened it up, the cap was obviously bulging, and very 'tired' looking, as I think I said. When checked, the two Wickman fuses in the front end, were both open. Without thinking too much about it, I associated the two 'problems', and went ahead and replaced all three items. When the supply still didn't work, I checked the input volts, and there wasn't any, because the plugtop fuse had also failed. It was only then that I started to question exactly what the failure mechanism had been, and how, or even *if* the cap had in some way been responsible. I came to the conclusion that the two sets of failures might be unrelated. Clearly, before the supply failed completely, the main filter cap must have been working to a degree, allbeit in its bulged condition, which I'm quite sure was a long term thing caused by poor ventillation and positioning. It really

*is* a very common occurence, which most often is due to poor mechanical design of a supply, and rarely anything to do with defective bridges in my day to day experience. I have of course had leaky diodes in bridges, but I normally find that they tend to produce more catastrophic failures than just a gently cooking cap. I must say that I didn't check the bridge specifically for this. I guess it will soon come bouncing back, if there is any such problem ... :-)

For all I know, the cap might still have been working enough to run the supply. As I had already replaced it, I guess we'll never know. When I checked it, it was only on the ESR meter, and only a quick check, as it was obviously in a bit of a state. It might not have been totally open, just outside the measurement range of the meter. The only place then, that the lightning came into it, was that I thought that as the cap might have been at *almost* the point of total failure from its slow heat degeneration, a large surge might have been enough to take it that last gasp to failure.

Thanks for you input anyway. It's all interesting stuff.

Arfa

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As far as mains glass-barrel fuses , its annoying when the owner throws away the blown fuse. Inspection can tell a lot, from

2 remaining sagged pieces of wire with a small break in the middle, could be due to fair wear and tear in effect, sagging over time and current carrying capacity going down over time. via still extant wires at the ends and one molten ball , no blacking (x2 or so overload) via a few sputter balls in the tube , some blacking (x5 or so overload ?) to totally black and cracked glass (> x10 overload?)

Its so long since I came across a Wickman I don't remember if I ground into/cracked it to explore its construction/ failure character.

-- Diverse Devices, Southampton, England electronic hints and repair briefs , schematics/manuals list on

Genrad 1659 RLC tester (Quadtech ) use them, which are American I think. Now I remember it was very short duration overlaods, from static discharges, that kept blowing them ,showing as very neat breaks as though cut with endsnips. After active stuff had been destroyed of course

If anyone knows the USA term I will add to my translator

-- Diverse Devices, Southampton, England electronic hints and repair briefs , schematics/manuals list on

Arfa,

I always used to check the plug top fuse first even though the customer invariably said that they had tried the fuse. In my experience customers never throw a ceramic plug top fuse away even the blown ones. They usually end up in a drawer to be tried again just in case it wasnt faulty! Not to mention the people who fit 13 Amp fuses to everything, or worse, wrap the fuse in tinfoil.

Roy

I don't know of a standard term, but browsing through DigiKey datasheets, the terms 'subminiature', 'micro', 'cartridge', and 'IEC

60127-3' are commonly used.

Littlefuse/Wickmann:

Littlefuse: Belfuse: Cooper/Bussmann: Schurter:

I've seen similar fuses on PCI extender/debug cards.

TM

:>

:> I would be more concerned with what caused the filter cap to bulge... :>

:> If it is a well known brand and correctly rated for the application then :> it :> should not have degenerated to that state as long as normal input voltage :> range :> was experienced. I also doubt that a possible short term lightning event :> would :> have caused the cap to bulge - this symptom is more likely due to a longer :> term :> overvoltage event. :>

:> If all the major input circuitry and components are ok I would suspect a :> leaky :> diode in the bridge rectifier. : :Presumably, you don't see too many small high current switchers ?

Not in the last few months. No. However I was repairing small and large SMPS quite frequently in the past. The symptoms you described are exactly the same as I found in small 5" x 3" format SMPS I recently repaired, although there was only one series fuse and a current inrush limiter - both of which had died. The main filter cap was also bulging, as in your case, and the fault was traced to a dud diode in the bridge rectifier. The suggestion was made as a possibility since I can't actually see the unit myself and all particulars were not disclosed initially.

:This is a :small supply - probably around 6" x 3" x 1.5", and fully enclosed in a :plastic case. The cap in question was a CapXon 120uF at 450v, and a 105 deg :type, so a common enough make, more than adequately rated for the job. As is :ever the case, it is the way that the designer has used it, that has led to :its demise. It was layed over flat on the board, jammed in by the heatsink :that carries the secondary side reccies etc. This is a very typical :scenario, which anyone who works with them a lot, will tell you leads to :drying out, bulging, and ultimate failure.

The fully enclosed nature of your unit does help to explain a possible reason for long term heating as a possible cause for the cap bulging. Pity you didn't include those details before since it is well known that heat is a major killer of caps in SMPS.

: :I'm not sure where you got it from that a lightning event was thought to be :responsible for this bulging, Ross. I don't think I actually said that.

The lightning suggestion was offered by snipped-for-privacy@neo.rr.com in his response and that is what I was referring to.

:The :supply was dead as received. When I opened it up, the cap was obviously :bulging, and very 'tired' looking, as I think I said. When checked, the two :Wickman fuses in the front end, were both open. Without thinking too much :about it, I associated the two 'problems', and went ahead and replaced all :three items. When the supply still didn't work, I checked the input volts, :and there wasn't any, because the plugtop fuse had also failed. It was only :then that I started to question exactly what the failure mechanism had been, :and how, or even *if* the cap had in some way been responsible. I came to :the conclusion that the two sets of failures might be unrelated. Clearly, :before the supply failed completely, the main filter cap must have been :working to a degree, allbeit in its bulged condition, which I'm quite sure :was a long term thing caused by poor ventillation and positioning. It really :*is* a very common occurence, which most often is due to poor mechanical :design of a supply, and rarely anything to do with defective bridges in my :day to day experience. I have of course had leaky diodes in bridges, but I :normally find that they tend to produce more catastrophic failures than just :a gently cooking cap. I must say that I didn't check the bridge specifically :for this. I guess it will soon come bouncing back, if there is any such :problem ... :-) : :For all I know, the cap might still have been working enough to run the :supply. As I had already replaced it, I guess we'll never know. When I :checked it, it was only on the ESR meter, and only a quick check, as it was :obviously in a bit of a state. It might not have been totally open, just :outside the measurement range of the meter. The only place then, that the :lightning came into it, was that I thought that as the cap might have been :at *almost* the point of total failure from its slow heat degeneration, a :large surge might have been enough to take it that last gasp to failure. : :Thanks for you input anyway. It's all interesting stuff. : :Arfa :

I will go with IEC 60127-3, Littelfuse 369 series

incidently ( Littelfuse & Wickman spellings)

-- Diverse Devices, Southampton, England electronic hints and repair briefs , schematics/manuals list on

Sorry Ross - wasn't getting at you. When I said "presumably you don't see ...etc" I was being quite genuine in asking. Although I didn't specifically say that it was a fully enclosed type, I did say in the original post that it was "...a small external switcher from an LCD TV." I assumed that anyone who was regularly involved in the repair of such items, would immediately recognise that description as being a small, plastic, fully enclosed type. Interesting that you had identical symptoms on a similar one, and that you actually found a leaky diode in the bridge. Can you from memory, put a value on the amount of leakage ? I figure it must have been quite small, as any leaky bridges I've had, have had a much more catastrophic effect than just over-working the main filter cap for long enough to dry it out. I was originally thinking U.S. 110v input and whether the reduced voltage might allow such a leak to fester away without exploding anything, but thinking about it some more, you're in Aus-land aren't you ? Line voltage nominally

230 - ish same as here in the U.K. ??

Ah, OK. I misunderstood you. I thought that you thought that I thought ( !!! ) that the lightning had led to the bulging. I was actually not associating the bulging with any lightning event, but considering the possibility that a surge on the line voltage from a nearby strike, as suggested by the poster that you cite, might have been responsible for the failure of the fuses, and for possibly 'finishing off' the cap which had already suffered long term damage from overheating.

Again, sorry if you thought I was being arsey with you, me old mate. I really wasn't. :-)

Arfa

That'd be my guess too. A nearby ground-strike will arc over a zapped fuse with no trouble at all.

--
    W
  . | ,. w ,   "Some people are alive only because
   \\|/  \\|/     it is illegal to kill them."    Perna condita delenda est
---^----^---------------------------------------------------------------

They sound like the fuses that used to be common on SCSI cards. They look very much like small value electrolytics, except hard plastic cases, with the value on the top, rather than on the side.

--
    W
  . | ,. w ,   "Some people are alive only because
   \\|/  \\|/     it is illegal to kill them."    Perna condita delenda est
---^----^---------------------------------------------------------------

:> :>

:> :> I would be more concerned with what caused the filter cap to bulge... :> :>

:> :> If it is a well known brand and correctly rated for the application :> then :> :> it :> :> should not have degenerated to that state as long as normal input :> voltage :> :> range :> :> was experienced. I also doubt that a possible short term lightning :> event :> :> would :> :> have caused the cap to bulge - this symptom is more likely due to a :> longer :> :> term :> :> overvoltage event. :> :>

:> :> If all the major input circuitry and components are ok I would suspect :> a :> :> leaky :> :> diode in the bridge rectifier. :> : :> :Presumably, you don't see too many small high current switchers ? :>

:> Not in the last few months. No. However I was repairing small and large :> SMPS :> quite frequently in the past. The symptoms you described are exactly the :> same as :> I found in small 5" x 3" format SMPS I recently repaired, although there :> was :> only one series fuse and a current inrush limiter - both of which had :> died. The :> main filter cap was also bulging, as in your case, and the fault was :> traced to a :> dud diode in the bridge rectifier. The suggestion was made as a :> possibility :> since I can't actually see the unit myself and all particulars were not :> disclosed initially. :>

:> :This is a :> :small supply - probably around 6" x 3" x 1.5", and fully enclosed in a :> :plastic case. The cap in question was a CapXon 120uF at 450v, and a 105 :> deg :> :type, so a common enough make, more than adequately rated for the job. As :> is :> :ever the case, it is the way that the designer has used it, that has led :> to :> :its demise. It was layed over flat on the board, jammed in by the :> heatsink :> :that carries the secondary side reccies etc. This is a very typical :> :scenario, which anyone who works with them a lot, will tell you leads to :> :drying out, bulging, and ultimate failure. :>

:> The fully enclosed nature of your unit does help to explain a possible :> reason :> for long term heating as a possible cause for the cap bulging. Pity you :> didn't :> include those details before since it is well known that heat is a major :> killer :> of caps in SMPS. : : :Sorry Ross - wasn't getting at you. When I said "presumably you don't see :...etc" I was being quite genuine in asking. Although I didn't specifically :say that it was a fully enclosed type, I did say in the original post that :it was "...a small external switcher from an LCD TV." I assumed that anyone :who was regularly involved in the repair of such items, would immediately :recognise that description as being a small, plastic, fully enclosed type. :Interesting that you had identical symptoms on a similar one, and that you :actually found a leaky diode in the bridge. Can you from memory, put a value :on the amount of leakage ? I figure it must have been quite small, as any :leaky bridges I've had, have had a much more catastrophic effect than just :over-working the main filter cap for long enough to dry it out. I was :originally thinking U.S. 110v input and whether the reduced voltage might :allow such a leak to fester away without exploding anything, but thinking :about it some more, you're in Aus-land aren't you ? Line voltage nominally :230 - ish same as here in the U.K. ??

That's OK Arfa.. words alone don't convey facial expressions unfortunately. I have certainly been guilty of unintentionally upsetting people with terse remarks on the NG's :-)

Actually, when the SMPS came to me the diode had actually gone fully short circuit. I guessed that if it had catastrophically failed in one hit the probable result would have been that the fuse and inrush limiter would have failed rapidly and the cap would not have been overheated long enough to cause bulging. I reasoned that the diode had started failing (become leaky) which gradually applied overvoltage and incorrect polarity pulses to the cap which caused it to overheat and begin bulging. Only when the diode had gone almost short did it cause the fuse and inrush limiter to rupture but by then the cap had also ad enough and it was not in good shape either.

Our mains voltage, while nominally quoted as 230V, actually sits around 255V in most cases so components in SMPS here are stressed quite a bit. : : :>

:> : :> :I'm not sure where you got it from that a lightning event was thought to :> be :> :responsible for this bulging, Ross. I don't think I actually said that. :>

:> The lightning suggestion was offered by snipped-for-privacy@neo.rr.com in his :> response and :> that is what I was referring to. : :Ah, OK. I misunderstood you. I thought that you thought that I thought ( :!!! ) that the lightning had led to the bulging. I was actually not :associating the bulging with any lightning event, but considering the :possibility that a surge on the line voltage from a nearby strike, as :suggested by the poster that you cite, might have been responsible for the :failure of the fuses, and for possibly 'finishing off' the cap which had :already suffered long term damage from overheating.

Yes, it is possible that this sequence of events could have occurred. I mistakenly assumed that given all operational conditions were normal, and based on my experience, the cap would be in good order and not already bulging. I reasoned that if an induced lightning splat did occur, it might blow the fuses but it is unlikely it would have caused the bulging cap at that time. I failed to take into account that since you hadn't seen the unit until after it had completely failed you couldn't have known what sequence of events had transpired. The cap may have been already bulging due to its normal operating environment before the fatal over-current (whatever the cause) blew the fuses. I sincerely apologise for this inexcusable oversight.

In most cases in my experience it is rare for failure of the main filter cap/s following the bridge in universal SMPS to occur, since they are usually well selected types for the application and physically large enough to dissipate any heat generated in their vicinity. I think I can truthfully say that I have had to change only about 2 such capacitors out of the several hundred small SMPS I have repaired. In most cases the Tan-Delta of the insitu caps have exhibited extremely low values even though they have already seen many years of service.

It is also not unreasonable that the cap in your unit is not as good as it should be and this has lead to excessive heating and subsequent bulging.

I am sure that you would now have checked the individual diodes in the bridge rectifier with your DVM and found them to be ok, hence your question on "how much leakage" did I find in my case. In my case there was a shorted diode and I reasoned it hadn't failed short circuit suddenly but had caused the bulging cap over a period of time before finally failing S/C and taking out the fuse and inrush limiter. I don't know if there was any lightning event at the time the unit actually failed, but while this is also a possibility, I doubt that it would have been the real cause of the failure.

If all your diodes test ok on the DVM I don't think anyone can know for certain what the failure mechanism was in this case. And since testing for diode leakage over the full working voltage range is not all that easy I can only suggest that you change the bridge rectifier (or all individual diodes in the bridge) just to be on the safe side.

: :Again, sorry if you thought I was being arsey with you, me old mate. I :really wasn't. :-) : :Arfa : : :

No problem Arfa...

We are pretty much on the same page with this I think d;~}

Arfa

That sounds like a 'Picofuse'.

--
You can\'t have a sense of humor, if you have no sense!