did this fuse blow funny?

[[email protected]]

i have a 1 amp , glass, fast blow, 1 1/4" fuse on the hot wire going
into a 65W SMPS. It blew on several units being tested, but its not
clear to me why. And the way the fuse "blew" was strange. Its seems
more like it "broke" off at the "weld" inside one of the metal cups.
You cant see where the fuse blew, but if you shake the fuse the
element bounces oscillates inside like a tuning fork. Its still
connected to one of the metal cups inside. Anyone ever see something
like this? There is no blackening of the glass or any other indication
the fuse blew.
And its not clear why a 1A fuse would blow on a power supply whos max
input current is supposed to be 72W. (60A inrush)

Its the LPS53-M by Astec

 

[John Popelish]

i have a 1 amp , glass, fast blow, 1 1/4" fuse on the hot wire going
into a 65W SMPS. It blew on several units being tested, but its not
clear to me why. And the way the fuse "blew" was strange. Its seems
more like it "broke" off at the "weld" inside one of the metal cups.
You cant see where the fuse blew, but if you shake the fuse the
element bounces oscillates inside like a tuning fork. Its still
connected to one of the metal cups inside. Anyone ever see something
like this? There is no blackening of the glass or any other indication
the fuse blew.
And its not clear why a 1A fuse would blow on a power supply whos max
input current is supposed to be 72W. (60A inrush)

Its the LPS53-M by Astec

My guess is that the input side of the supply is a rectifier
and filter capacitor, with little to limit the inrush
current that charges the cap very fast if the power is
applied at the peak of the line cycle. The pulse does not
last long enough to melt the fuse element, but gives it a
large mechanical sag-and-retighten because of the rapid
temperature change. This repetitive bending and tensioning
at the wire ends, eventually fatigues the metal and it lets go.

Most such supplies require a slow blow fuse, possibly one
with a spring mechanism inside, that absorbs the expansion
and contraction of the element with a material with a high
fatigue resistance.

 

[[email protected]]

Most such supplies require a slow blow fuse, possibly one

with a spring mechanism inside, that absorbs the expansion
and contraction of the element with a material with a high
fatigue resistance.

I keep reading and getting told by Astec that I should use a fast blow
fuse. Whats the deal here? "slow blow does not adequately protect a
SMPS". Is this true? I would think that with the inrush current specd
at 60A, that a fast blow would be too sensitive, and a slow blow more
appropriate.

 

[John Popelish]

I keep reading and getting told by Astec that I should use a fast blow
fuse. Whats the deal here? "slow blow does not adequately protect a
SMPS". Is this true? I would think that with the inrush current specd
at 60A, that a fast blow would be too sensitive, and a slow blow more
appropriate.

If Astec is telling you that the fuse is there to protect
the supply, they are admitting that the supply is poorly
designed. Fuses are normally used to protect the wiring
from overload after the supply has failed, shorted, and to
protect the surroundings from fire caused by supply failure.
Evidently, Astec is having lots of trouble with this model
failing and they are trying to improve its chances of
survival by using a twitchy fuse, instead of adding the
proper inrush limiting system.

Sounds pretty futile, to me.

 

[Eeyore]

i have a 1 amp , glass, fast blow, 1 1/4" fuse on the hot wire going
into a 65W SMPS. It blew on several units being tested, but its not
clear to me why. And the way the fuse "blew" was strange. Its seems
more like it "broke" off at the "weld" inside one of the metal cups.
You cant see where the fuse blew, but if you shake the fuse the
element bounces oscillates inside like a tuning fork. Its still
connected to one of the metal cups inside. Anyone ever see something
like this? There is no blackening of the glass or any other indication
the fuse blew.
And its not clear why a 1A fuse would blow on a power supply whos max
input current is supposed to be 72W. (60A inrush)

60A inrush and you're surprised it blew ?

I'm sure it should be a slow blow. Who specced the fuse ? Is it mounted in / on
the PSU itself ?
http://www.astecpower.com/products/datasheets/lp/lps50-M.pdf

It's not is it ? Was it your idea to use a fast blow fuse ?

Graham

 

[Eeyore]

I keep reading and getting told by Astec that I should use a fast blow
fuse. Whats the deal here? "slow blow does not adequately protect a
SMPS". Is this true?

Bollocks.

The job of a fuse is to prevent fire, not protect electronics.

Graham

 

[Eeyore]

If Astec is telling you that the fuse is there to protect
the supply, they are admitting that the supply is poorly
designed. Fuses are normally used to protect the wiring
from overload after the supply has failed, shorted, and to
protect the surroundings from fire caused by supply failure.
Evidently, Astec is having lots of trouble with this model
failing and they are trying to improve its chances of
survival by using a twitchy fuse, instead of adding the
proper inrush limiting system.

Sounds pretty futile, to me.

Idiotic in fact.

Graham

 

[John Popelish]

Idiotic in fact.

Or more generously, desperate, and grasping at straws.

 

[Rich Grise]

Idiotic in fact.

Idiotic all the way to the bank, as long as they can get away with it. ;-)

Cheers!
Rich

 

[mpm]

i have a 1 amp , glass, fast blow, 1 1/4" fuse on the hot wire going

In case anybody "cares", you do know you can unsolder the outside end
caps on those glass cartridge type fuses and remove the remains of the
fuse element wire....

I "invented" this fuse repair job on a site so remote, that if it
wasn't already on the truck, you really didn't NEED it....

-mpm

 

[[email protected]]

i have a 1 amp , glass, fast blow, 1 1/4" fuse on the hot wire going
into a 65W SMPS. It blew on several units being tested, but its not
clear to me why. And the way the fuse "blew" was strange. Its seems
more like it "broke" off at the "weld" inside one of the metal cups.
You cant see where the fuse blew, but if you shake the fuse the
element bounces oscillates inside like a tuning fork. Its still
connected to one of the metal cups inside. Anyone ever see something
like this? There is no blackening of the glass or any other indication
the fuse blew.
And its not clear why a 1A fuse would blow on a power supply whos max
input current is supposed to be 72W. (60A inrush)

Its the LPS53-M by Astec

Just a thought. If there is a poor connection on one end
of the fuse, it could overheat on that end and blow it.
I have an inexpensive IR thermometer that I use all the
time to check equipment with. I can find an overheating
fuse, breaker or connection quickly with this little jewel.
I paid around $20.00 for it.

http://tinyurl.com/ytzov3

[8~{} Uncle Monster

 

[joseph2k]

Idiotic all the way to the bank, as long as they can get away with it. ;-)

Cheers!
Rich

Sounds like a good way to Astec to lose UL, CSA, TUV, etc., safety labels.
That's why accountants aren't allowed to do engineering.

 

[legg]

60A inrush and you're surprised it blew ?

I'm sure it should be a slow blow. Who specced the fuse ? Is it mounted in / on
the PSU itself ?
http://www.astecpower.com/products/datasheets/lp/lps50-M.pdf

It's not is it ? Was it your idea to use a fast blow fuse ?

Graham

I see an F1 located on the artwork...........

I'd be more interested to note markings on the fuse. (mfr - series -
rating) . As you've stated, the fuse is for fire protection/open trace
protection/single-fault abnormal test compliance. If it's certified
with one type of fuse that doesn't allow reliable operation at high
line, then the whole product series is a dog and should be avoided

Adding external inrush limiting might cut down on returns of current
inventory. It looks loke hey are counting on the series resistance of
the line filter components alone to do this. Normally a commercial
design would try to satisfy the 35Amp peak ratings for commodity line
switch-gear. This product's spec sheet shows that this has not been
done and it's a case of buyer beware in that case. Safety approved
NTCs in the 10 ohm range are available for PCB mounting - but would be
a headache to incorporate as a retrofit nto external field wiring.

Depending on how the fuse is mounted, Astec could be damaging it in
the lead-forming or the wave-soldering process. Does it break always
at the same end?

Perhaps a more reliable mfr of fuse, with similar safety
certifications is called for if you are stuck with the model. You'd
need accurate info from the listed safety critical components in the
mfr's actual certification folder to determine that the part you use
complies. Rough to have to rework this prior to application.

I've never stuck fast-blow fuses smaller than 2A into low power
off-line switchers, and then it may be questionable whether a
glass-bodied low-interrupt rating part is safe from explosive
behaviour, when carefully tested to cover single-fault abnormals.

RL

 

[Eeyore]

I see an F1 located on the artwork...........

So there is.

T3.15A.

That makes more sense.

Graham

 

[Eeyore]

Adding external inrush limiting might cut down on returns of current
inventory. It looks loke hey are counting on the series resistance of
the line filter components alone to do this. Normally a commercial
design would try to satisfy the 35Amp peak ratings for commodity line
switch-gear. This product's spec sheet shows that this has not been
done and it's a case of buyer beware in that case. Safety approved
NTCs in the 10 ohm range are available for PCB mounting - but would be
a headache to incorporate as a retrofit nto external field wiring.

There is an NTC.

It's marked TH1. As in thermistor.

Graham

 

[legg]

60A inrush and you're surprised it blew ?

I'm sure it should be a slow blow. Who specced the fuse ? Is it mounted in / on
the PSU itself ?
http://www.astecpower.com/products/datasheets/lp/lps50-M.pdf

It's not is it ? Was it your idea to use a fast blow fuse ?

I note also, on the board layout, that the printed fuse rating is
"T3.15A". This is a slow-blow 3.15A fuse. I expect that's a mistake of
some kind or another. It certainly shouldn't produce nuisance
tripping, if used.

2.5A also shows up in the neighbourhood. This kind of ambiguity in a
fuse label would make certification a pain.

I think you really need to examine the actual product in hand and get
a copy of the test files for the model in question.

RL

 

[Eeyore]

I note also, on the board layout, that the printed fuse rating is
"T3.15A". This is a slow-blow 3.15A fuse. I expect that's a mistake of
some kind or another. It certainly shouldn't produce nuisance
tripping, if used.

2.5A also shows up in the neighbourhood. This kind of ambiguity in a
fuse label would make certification a pain.

It wouldn't get past proper certification with an ambiguity like that.

Graham

 

[neon]

a fuse of 1A never blows exactly at 1A it varies with imlementation heat sink and others. a power supply of 72w will require a slow blow fuse because of inrush current durng start up. it may survive once or twice depends on when you turn on the power ac wise.

 

[joseph2k]

It wouldn't get past proper certification with an ambiguity like that.

Graham

Maybe the production units are not quite the same as the the qualification
units. If so UL takes a very dim view of such. It wouldn't be the first
time though, especially with accountants twisting managements arm.