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Old 07-03-2007, 06:58 PM   #1
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Thermal fuses/cutoffs

How important is the current rating of a thermal fuse. Can I use a 2.5A/115C/250V cutoff in the place of a 2A/115-130C/250V fuse w/o worry?
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Old 07-03-2007, 07:15 PM   #2
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hmm, the 2.5A vs 2A gives me pause... the temperature doesn't concern me at all, though.
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Old 07-03-2007, 07:22 PM   #3
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I was thinking it'd result in the switch going off at a higher temperature, since the switch is rated for more current, and heat, than the fuse was. But if it's only a few degrees more I don't think it'd be a problem.
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Old 07-04-2007, 01:48 AM   #4
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How important is the current rating of a thermal fuse. Can I use a 2.5A/115C/250V cutoff in the place of a 2A/115-130C/250V fuse w/o worry?
omgwtfbyobbq,

Understand that the purpose of a fuse in general is to sacrifice itself to be burned up thus disrupting the electricity flow before the electrical device inside the house that caused the fuse to blow starts a fire inside the house. By replacing a 2A fuse with a 2.5A fuse, you're making the electrical branch that is connected by the fuse to allow more current to flow before disrupting the electricity flow. So that additional allowed current flow may start a fire inside the house as well as a tripped fuse which completely negates the purpose of having a fuse in the first place. Remember that a relatively "low wattage" heat source like a cigarette could start a fire. So from a "w/o worry" perspective, I'd be more worried.
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Old 07-04-2007, 12:48 PM   #5
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I'm looking at replacing a low current/high temp thermal fuse, with a higher current/lower temp thermal switch. Since temperature is likely a function of current, the thermal switch would probably go off at a higher temp all things being equal, but since it's rated for a lower temperature, in this case ~115C vs ~133C, I was wondering if that would make up for increase in rated current. Really, I'm just wondering what the relationship between temperature and current ratings are between thermal fuses/switches and Googling hasn't helped. Since I would imagine that a fuse/switch rated for a certain current/temperature would be equivalent to another fuse rated for less current/more temperature or vice versa. In terms of tripping fuses, it's for a fan, and I don't think going from 2A/250V/133C to 2.5A/250V/115C would trip any fuse in the house or start any fires since it's metal and plastic.
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Old 07-04-2007, 01:23 PM   #6
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Half an amp isn't going to matter. Now jumping from a 2 amp to a 20 amp, you'll have some problems. Fuses are rated below the capacity of the wiring they are protecting. The fuses are not there to protect your appliances, they are there only to protect the house wiring. The reason for the fuse is if the TV shorts out, it won't try to draw 500 amps through the wall socket, which a direct short will do. Overdrawing the in-house wiring will cause the wiring to get burning hot and cause the house to burn down. But just half an amp isn't going to do anything.

If anything, I'd question why such a small fuse was used in the first place, most housing fuses are 10 amps, but I'd also not replace the 2 amp fuse with a 10 amp fuse until I knew for certain that there wasn't a reason for the smaller fuse.
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Old 07-04-2007, 01:38 PM   #7
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Oh, it's not for the house, it's for a fan. Whoever makes 'em uses the thinest crap oil ever. So after a bit it'll bind and the thermal fuse will go once it gets to the right temperature. I'd like to replace the one time use fuse with a switch so if it does stop working I know to try relubing the bearings and don't need to crack open the motor again to replace anything.

Regarding the fuse rating, I just asked on another forum and the current rating is just the max current it can handle, while the temperature rating is where it will open, no relation between the two. It seems that even if I use a 10A fuse, as long as the temp rating is the same, it'll be just as safe. Even if it can flow 10A instead of 2A, it'll still open when the entire assembly gets to whatever temperature. It may get there faster if it shorts, somehow (can an AC motor short?), but the house fuse would protect everything in that case. I'm assuming the thermal is just so it doesn't get too hot and melt.
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Old 07-04-2007, 01:38 PM   #8
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Quote:
Originally Posted by omgwtfbyobbq View Post
I'm looking at replacing a low current/high temp thermal fuse, with a higher current/lower temp thermal switch. Since temperature is likely a function of current, the thermal switch would probably go off at a higher temp all things being equal, but since it's rated for a lower temperature, in this case ~115C vs ~133C, I was wondering if that would make up for increase in rated current. Really, I'm just wondering what the relationship between temperature and current ratings are between thermal fuses/switches and Googling hasn't helped. Since I would imagine that a fuse/switch rated for a certain current/temperature would be equivalent to another fuse rated for less current/more temperature or vice versa. In terms of tripping fuses, it's for a fan, and I don't think going from 2A/250V/133C to 2.5A/250V/115C would trip any fuse in the house or start any fires since it's metal and plastic.
omgwtfbyobbq,

Questions:
  1. Does the fan have an onboard fuse?
  2. What current rating does the fan have?

Another potential fire hazard may be the power cables in your walls. They may be rated at or around 2A that the current fuse is rated at. If you want to see the effect of current running through a metal wire, look at the filament of a clear incandescent light bulb. Now, imagine that happening to the power cables behind your walls.
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Old 07-04-2007, 01:41 PM   #9
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Check out my last post.
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Old 07-05-2007, 05:46 AM   #10
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I see. In that case, what is this fan used for, and is it necessary to keep it or could it be replaced with a better one? Sometimes it's cheaper to just replace an item rather than keep working on it. Don't forget to calculate time as money when deciding on the repair too. Time is a valuable commodity, and every second that passes means that much less of an irreplaceable, finite resource you have left. I normally use 60 bucks an hour and include parts chasing time when deciding on repair myself, hire someone or replace.
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