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Evonet

Cambridge
CB24 8TX, UK.

The Ethics of Thermal Fuses

26/08/2014

The thermal fuse is an inexpensive electronic component, that has become commonplace in many household appliances. It is frequently added to prevent a faulty or misused appliance from becoming hazardous or causing a fire. It seems hard to imagine there might be ethical issues surrounding such a humble innocent device – but there are.

A significant percentage of the appliances brought to Repair Cafes for repair have tripped thermal fuses. We should be questioning why this must be.

What is a thermal fuse?

These (roughly life-size) are what thermal fuses usually look like:

They cost about 15p each in quantity. Inside each thermal fuse, there is a strand of wire made of an alloy with a low melting point – typically somewhere in the range 90-140°C. If the temperature of the device gets above the melting point of the alloy, the wire coalesces into a blob at one end, and the circuit is broken. The manufacturers of domestic appliances are rightly concerned that under fault conditions, the temperature inside the appliance could rise without limit, and cause a fire. A thermal fuse ensures that this doesn’t happen.

Common appliances containing a thermal fuse include hair dryers, sandwich-toasters, blower heaters, paper shredders, vacuum cleaners. Sometimes devices like hi-fis and televisions containing a mains transformer have a thermal fuse included in the primary winding of the transformer.

So what is the ethical issue?

The ethical issue is that the user is not provided with any information about the presence of a thermal fuse, and they do not have the means to replace it if it blows. Because they don’t know of its existence, they don’t know that the appliance can be repaired very simply by replacing the thermal fuse. The appliance almost invariably gets thrown away, for the want of 15p and a little time.

Consider for example a hair dryer. It is so easy to obstruct the flow of air, by hair over the inlet, or a hand or clothing over the outlet. In a few seconds, the thermal fuse will trip, and the hair dryer rendered useless – and probably consigned to the dustbin.

Thermostatic Switches

The produce manufacturers have a simple alternative. A thermostatic switch breaks when the temperature gets too hot, and remakes when the temperature cools down. Here (also roughly life-size) are a couple of common formats in which thermostatic switches are manufactured:

They are slightly larger than thermal fuses, but with a little design work, this could be overcome. The real killer is the price – they cost around £2.30 each in quantity.

There are implications inherent in a self-resetting device. The user manual must make it clear that an appliance that turns itself off must be left for a few minutes to cool down. They must understand that this is indicative that the appliance needs attention – for example to clean filters, remove a blockage etc.

There is a much reduced but still real potential for a self-resetting appliance to cause a fire. For example, if one were to wrap a hair dryer containing a thermostatic switch in clingfilm (an extreme form of abuse!) and leave it for a long period of time, it would continuously cycle on and off, and eventually something else would fail, possibly causing a fire. This could be countered by putting a thermal fuse with a higher fail temperature in series with the thermostatic switch. (Safety testing to confirm the efficacy of this would be straightforward to do.)

Conclusion

It is very hard to escape the conclusion that there is no incentive for the manufacturer to incur the additional manufacturing cost of using a thermostatic switch, especially when the cheaper thermal fuse will result in more sales of his product. This is a clear example of built-in obsolescence.

At Repair Cafes, we can improve the situation. We can:

  • Inform users about the presence of thermal fuses, explain to them what has happened, and make a recommendation to reduce the likelihood of it happening again.
  • Provide a service to replace thermal fuses where necessary
  • Name and shame product manufacturers who use these devices without telling their users, and thereby encourage them to consider whether the additional cost of a thermostatic switch might be justified to maintain the reputation of their brand

In this way, we may reduce the number of otherwise perfectly good appliances that may otherwise end up in landfill, for the want of a 15p part.

Technical Note

When replacing a tripped thermal fuse, some precautions are required:


  • Make sure the key parameters of the replacement thermal fuse are as near as possible to the original (though it’s impossible to keep a supply of every type):

    • Trip temperature

    • Current-carrying capacity

    • Maximum voltage when broken

  • Same package style – and if original thermal fuse was bonded to something that might overheat, bond the replacement as well.

  • Don’t use solder on the leads, unless you are confident that there is no possibility of the local temperature being hot enough to melt solder, before the thermal fuse breaks.  A better strategy is to make some crimps from copper tube 2mm OD, 1mm ID (available from good model shops[1]) – cut into short lengths by rolling with a Stanley knife.  Push the two wires into the tube, one from each end, and squeeze with a pair of wire cutters:



  • Whatever you do, do not simply short out the broken thermal fuse!   The appliance will appear to work fine – but if there’s a fault, you could cause a fire.




Feedback

Please give some feedback on this blog:

This is great, you should find a publisher!

I appreciate this, and it has made me think anew about something.

It's interesting and provocative, and I mostly agree with the opinion expressed.

It's interesting and provocative, but I mostly disagree with the opinion expressed.

I completely disagree with what you're saying.

It was a waste of my time to read this rubbish.

This is dangerous talk, and the world would be a better place if it had never been said.

(Please note that you can only vote once for each article from a given IP address.)


 

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