Fuses, basic question.
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  1. #1
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    So tonight I turned the power On on the lathe and blew a fuse as soon as I went to engage the lever. Took me about 20 minutes to find the actual fuse as at first I thought it was the On or emergency switch acting up. Of course the lathe has 2 different fuse sizes and the only one I had in spare were not the type that blew. So tomorow I'll try to find some new ones.

    Question is: Do fuses sometimes just quit without having a surge? I've been running that lathe for about 2 years now. I did nothing out of the ordinary so I'm wondering. Why now? It's a 250V 20amp fuse. Lathe takes 2 of them.

    Thanks

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    I don't know if fuses actually lose there working rating after 2 years. I know a motor will typically have a rise in amps as soon as you turn it on, and then it will decrease a bit and smooth out. Maybe something has began bogging the motor to make it draw more amps.

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    I've had fuses get tired and blow for no apparent reason. I've pried open the can on some rather large fuses and was quite surprised at the flimsy little bit of metal that is taking all those amps.

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    Fuses, like light bulbs, will sometimes just blow for no apparent reason.

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    Maybe you had a surge, you can't see them. There are different kinds of fuses. Some blow quickly to protect electronics and some have a delay. An electric motor has an inrush current greater than running current to get started, generally considered as a rule of thumb to be six times nameplate current for a very short time. Dual element fuses which are time delay fuses are generally used ahead of an electric motor or else standard fuses have to be oversized. Add up the nameplate currents of all the motors on the lathe to see how much of the 20 amps is being used. A dual element fuse would be sized a little over the total current. If you have a wiring diagram see what size fuse was originally specified. Fuses are subject to "fuse creep", someone doesn't have the correct size and puts in the next bigger one they have. After a couple of those exercises you have no protection at all. 230 V dual element fuses are usually FRN -?? in American equipment. I don't think fuses wear out but sometimes they get loose in the clip which can get hot enough to melt them without an overcurrent.

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    I had a 10 amp fuse blow in my Hino box truck (protection for the dash/instruments lights circuit) Put another 10 amp in just like it, and it's been fine for 20 hours of driving at night so far. Truck has 360,000 miles, so I wonder if that fuse just got "tired" after all this time or what ? It did burn out upon turning the headlights/instrument lights back on after a rest stop...maybe there was an unusual surge just that one time somehow...dunno..

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    It's been my experience to blow fuses on equipment that cycles often, we have cooling towers that cycle 40 or 50 times a day in cool weather. They also pull locked rotor amp about 2 seconds per startup in the neigborhood of 150 amps, thats a lot for a 20 amp fuse, they go for about 9 months to a year. I suspect thermal stress from all the abuse. Our 600 volt ones on the large towers don't seem to suffer so much even though it's a big high mass fan, we run all our motors about 25-30% over nameplate FLA. I wouldn't suggest any more than that or you won't protect the motor.

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    Yes indeed, fuses do expire from old age.

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    Yes, fuses can get tired, but its equally possible you have an intermittent problem with the switch or perhaps the starting capacitor (assuming its not 3ph).

    First replace the fuse and see if you get lucky, then diagnose further if needed.

    Graham

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    Supposedly the metal used in the fuse bridge changes over time physically and chemically some amount dependent on how much it's heated, or loaded to near capacity.

    I remember reading that a long time ago, but I can't prove the physics of it.

    A crusty old electrician in a sawmill gave me his dissertation on electrical theory:
    "2 kinds of electricity boy, regular and extra crispy" and "electrons go through fuses no problem, fagatrons are scarce but they happen on occasion; a fagatron will blow it every time, yep"

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    Just going over the number of parts I made in the last 2 years. I figure I probably turned that machine on around 25k to 30K times. So I guess that high amp draw over so many times would do it.

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    One other thing that I didn't see mentioned--either "plug" (screw in) or cartridge fuses--if the fuse holder clips are loose or the fuse gets loose in the socket, it can build heat which of course will cause a premature failure.

    This also applies, of course to the wire connections to the fuseholder.

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    Although fuses do seem to get "tired",the one thing that I have noticed is how the voltage fluctuates over the course of the day due to power demands on the electrical grid.As the voltage drops the amperage must go up to keep the motor runing with the same load.
    It's interesting to hook up a voltage meter with a min&max setting and see how much the voltage varies during the day,especialy when your own equipment isn't running,to eliminate the effect of heavy local power draws on your own system.

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    The effect I have seen is from cumulative surges.

    The fuse material works by heating. Current heats it up.

    When it heats it expands. This causes it to flex a small amount with each surge.

    Over time, the flexing can cause cracking, since the fuse material isn't a structural alloy. With "wire" element fuses this is less of a problem, and with the "stamped" flat sheet elements it is more of a problem.

    When a crack gets to a certain point, the local resistance goes up, and the area heats more, tending to blow. or, it cracks through, and the resulting arc melts the element.

    The key rating is called the "I^2-T" rating. It is the product of the effective current in the surge, squared, times the duration, and is equivalent to energy. It has NO direct relation to fuse rating , and may be LOWER in a fuse with a higher current rating.

    If the manufacturer keeps the surge energy down to about 1/4 of the I^2-T rating, the fuse will last 100,000 surges. If it is close to the same as the rating, the fuse will blow in less than 10 surges, so it is non-linear.

    I don't know if the I^2-T rating takes the mechanical effects into consideration, but if you proportion correctly, you will have no problems in normal use. The 100,000 surges is a lot of switch flipping.... a bit more than twice an hour 24/7 for 5 years.

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    Fuses do seem to weaken with age.

    Ever had a light globe blow as you switch it on?

    That's because you have had the good/bad fortune to have switched it at or near the voltage peak; 340 volts for us Aussies, 1.41 X 110 volts for most of you.

    Cold (low-resistance) filament + high voltage/current = "Plink".

    With the number of startup (high current) cycles reported by SND, it's likely to be a combination of ageing & surge.

    Try to always switch-on when the AC cycle is at zero. [img]smile.gif[/img] [img]smile.gif[/img]

    There are two fundamental fuse types; slow-blow and fast/quick-blow.

    Slow-blow would be most suitable for machines such as lathes. Quick-blow (often in picoseconds) are used to protect really sensitive items.

    Also, there is, in many cases, no guarantee that the correct fuse is in place; it may have been replaced at some stage and, to many people, esp. those in a hurry, a fuse is a fuse.

    It pays to check the machine maker's specs if possible.

    Fuses should be de-rated for high temperatures;

    http://www.circuitprotection.ca/fuseology.html

    And manufacturers of machinery etc can themselves be guilty of not fully understanding fuse behaviour.

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    If it never happened before then what I'm gonna say isnt the cause. But one place I worked we had a lathe, cannot remember brand, anyway the spindle lever if you were not firm with it would dance between reverse and forward a bit right at "OFF"...this would blow 2 of the 3 fuses....plugging the motor didnt...but jacking it back and forth between forward and reverse like that WOULD. I have never had a fuse get "old".....maybe not been around enough equip long enough to see it.

    Working on a project to convert a BP mill to CNC I have been looking at fuses, the 1/4" x 1-1/4" are sure a great value at $.25 each,

    the buss "little fuse"(the little 1/2" dia puppies) I saw a PAIR of 20 amp ones at Menards store for $13..thats right $6.50 EACH....at Lowes they were a couple bucks each.


    Bill

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    Fuses are really designed to blow before the wiring gets too hot. They are not designed to protect motors because of the starting current surge, compared with the typical overload current of an overloaded motor.
    The standard English 13A cartridge fuse will blow at 65 Amps in 5 seconds. What sort of motor protection is this?
    I agree with J Tiers about fuses ageing, especially wired in fuse links they are cheap and cr***y but they will protect the wiring.
    Wiilbird I suggest that you are getting what you are paying for. Most 1/4" x 1 1//4" fuses are not vacuum sealed and their holders in general are simply not up to it, also in a 20 A current rating, I am not sure that they are rated at mains voltage - should be in a ceramic tube not glass.
    At college we were shown the "works" of a 600A, 600V HRC (High rupture curent) fuse. Its a work of art, there were 20 odd silver plated wires in parallel, wired to individual siver plated fingers, each with a tiny little blob of metal on it. The whole works were then filled with some high grade sand. Thats why decent cartridge fuses cost money but they do conform to a spec., so they will protect a piece of kit and last a long time.
    Frank

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    "as soon as I went to engage the lever"

    Was it the switch lever or a clutch lever? A spindle locked or other excessive load would pop a fuse if it was the clutch...Joe

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    I always thought it would be good to be in the fuse business. No competition from used or rebuilt equipment. Don't have to fool with tradi-ins. No heavy lifting. Very few returns saying it didn't work right. No complaints about dimensions, color, noise or appearance. A lot of people working for you blowing the wrong kind of fuse until they figure it out. A lot of product laying around in drawers never being used. What a great business. I think we have two strictly fuse stores in Houston.

  20. #20
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    Well I bought a box of 10 fuses this morning. 20amps, same as specified. That should last me a while. I put them in tonight and it started right up and I ran it for 4 hours with no problems.

    Thanks everyone!


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