History of cheap thermal fuses

Discussion in 'General Electronics Chat' started by Gdrumm, Apr 7, 2010.

  1. Gdrumm

    Thread Starter Distinguished Member

    Aug 29, 2008
    I've been repairing a lot of small appliances lately, and the most common cause of failure has to be thermal fuses.

    I have a old electric fan from the 1920's or 30's, and it has no thermal fuse. However, all modern day fans seem to have such a fuse, and it's usually rated pretty low, like 115 degrees, etc. Well, here in Texas, it sometimes gets to be 110 degrees in the shade, so naturally the fuses aren't going to hold up.

    I replace them with Radio Shack 224 degree fuses, and that seems to solve the problem with ease.

    Anyway, I just wanted to know if anyone knows about the laws concerning these things. I'm guessing they were added to prevent melting / burning plastic, which I believe melts at around 250 degrees or higher.

    Any insights would be appreciated.

  2. SgtWookie


    Jul 17, 2007
    You have likely essentially bypassed the safety feature that was provided by the original fuse.

    The fuse would likely have been at a place that was much cooler than the interior of the windings.

    What will now happen is that the windings will melt before the fuse does, protecting the fuse. :eek:

    You might have been OK by increasing the temp by a small amount, but you went quite a bit overboard. I hope that none of your fans catch on fire.
  3. Gdrumm

    Thread Starter Distinguished Member

    Aug 29, 2008
    These are usually mounted right against the windings themselves, and copper requires almost 2000 degrees to melt. Even plastics moldings have to reach 280 degrees to melt, so I'm wondering what else might have prompted the industries to adopt such strict limits for thermal fuses?

    I'll research some National Safety Board sites to see if I can find specific answers.

    I really believe I'm dealing with two issues, 1) overkill, and 2) cheap Chineese parts. I've also read that the wax pellets in these fuses can just become weak over time, and that causes them to trip.

    Thanks for you input.

  4. awright

    AAC Fanatic!

    Jul 5, 2006
    Gary, I agree fully with SgtWookie you are not using good judgement replacing the thermal fuse with such an overrated one. That is analogous to saying that substituting a quarter in your fuse box solved your problem with fuses burning out.

    You might be correct that your environment is a little hot for the thermal fuse rating selected by the designer but you are essentially completely eliminating the safety feature of the thermal fuse if you substitute a 225 degree unit.

    The thermal fuse is not selected on the basis of melting temperature of the parts of the appliance. It is selected based upon a reasonable margin above the normal operating temperature of the unit at the thermal fuse location with the appliance operating in the hottest anticipated environment. Any temperature significantly above normal is interpreted as a fault requiring shut-down of the appliance. Since you may be operating in an environment that is higher than the appliance was designed for it may be reasonable to use a slightly higher rated thermal fuse.

    Let's say the appliance was designed to operate in up to a 90 degree environment and you want to operate in a 110 degree environment. A reasonable ballpark estimate might be to say that, since you want to operate in a 20 degree hotter envoronment than the appliance was designed for, you would substitute a thermal fuse with a 20 degree higher rating than originally installed. (I have no idea if these assumed values are reasonable.) At least this approach has some logical basis and retains some protection from a serious fault.

    I have no idea what code or UL says about thermal fuses but, judging from their ubiquity, you can bet they are mandatory (and a good idea) in new appliances.

  5. Gdrumm

    Thread Starter Distinguished Member

    Aug 29, 2008
    I appreciate the feedback, and I will resolve the problems with these units, or disable them and throw them away.

    I have read a lot of discussions on this board about cheap Chinese products, so I will look into better quality, properly rated fuses as my source.

    Another cheap fix (with properly rated fuses installed) might be to drill some air vent holes in the plastic housing. It seems that would help keep the entire motor box cooler to start with, and lessen overheating in the first place.

    Fortunately, I haven't sold any of these units.

    I really appreciate you guys.

  6. KMoffett

    AAC Fanatic!

    Dec 19, 2007
    Another safety design violation! The plastic housings are there to prevent access to internal metal parts that could become electrical charged by a wiring insulation breakdown/fault. This is a "double insulation" design that allows the use of two-wire power cords.

    Ya can't win! ;)

  7. eblc1388

    AAC Fanatic!

    Nov 28, 2008
    Do thermal fuses rated in Fahrenheit? Or in Centigrade?

    Comparing 115°C to 110° F is pretty meaningless. :(
  8. Gdrumm

    Thread Starter Distinguished Member

    Aug 29, 2008
    Important update, and thanks for the last question.

    I went to Fry's electronics, and bought a couple of low temp rated fuses.
    They are mfg. by NTE Electronics. Each package came with an instruction sheet.
    It included a C to F temp equivalents table, and included installation instructions.

    It said "When replacing a thermal cut-off with a trip temperature less than 120 C, the replacement should have a trip temp within 4 degrees C of the original device. If the original device has a trip temp greater than 120 C, the replacement should be within 8 degrees C of the original device."

    Fortunately I saved several of the old bad thermal fuses I had removed, and in looking at them just now, I discovered that they were indeed of the 115 degree Celsius variety, therefore, they would withstand up to 239 degrees Fahrenheit.

    So, while I'm pretty sure that all of my installs meet that requirement, and I do feel much better, I still plan to go back and double check each one,. Since my supply source was Radio Shack, and since I was buying the lowest temp fuses they had in stock, I feel pretty confident that I will be in good shape across the board.

    This was a great learnng experience, and thanks for all your input.
    FYI, for reference, 100 degrees Celsius equals 212 degrees Fahrenheit per this table.....

  9. SgtWookie


    Jul 17, 2007
    I didn't even think to ask whether the specs were in C or F. :confused:

    Keep in mind that NTE does not manufacture any components themselves.

    They have other manufacturers make runs of components with NTE part numbers on them.

    NTE has a part number cross-reference database available online - but it only goes one way; TO an NTE part number. There is usually a substantial mark-up of the cost of the part.

    If you want an example, find a price on an LM3914N. Then look up the price on the NTE equivalent part.
  10. Mike33

    AAC Fanatic!

    Feb 4, 2005
    One thing that caught my attention a little, too, is that a thermal cutout placed in a certain location may register something like 200F, when the coils it is to protect may, internally, be at a MUCH higher (melting) temperature....glad that was resolved, though....115F seemed Ridiculously low to be tripping!!
    Metalmann likes this.
  11. BrianWren

    New Member

    Jun 25, 2014
    The protection is for the really thin 'paint' (lacquer, maybe?) that insulates the wire. Of course, using boiling eater as a benchmark, I am pretty sure that the insulation would not be harmed by 212º, and might be good to 300º.
  12. MrAl

    Distinguished Member

    Jun 17, 2014

    Yeah it is a good idea to use the same temperature replacement and also to mount it in the very same place and in the very same manner as it was in the original fan.

    The thermal fuse helps to prevent the wire insulation from burning up and shorting out, which causes more heating, which causes fire eventually. The windings can be very tightly wound so if the insulation fails, a short occurs in the windings and the operating current goes up meaning more heat.

    Overheating can also occur if the shaft freezes due to gunk buildup in the bearings which slows the fan speed and may prevent it from starting up again once it is shut off.
    So while it is apart you can clean and oil the bearings.
  13. alfacliff

    Well-Known Member

    Dec 13, 2013
    RichardO likes this.