Thermal Fuse Covering

Thread Starter

draw2build

Joined Nov 25, 2022
19
I received 5 thermal fuses, 75C Normally Closed. They came with a clear soft plastic covering them.
What is this soft plastic covering used for?
I don't think it could be heat shrink, as the heat would pop the fuse.
Thanks in advance.
Screen Shot 2023-01-26 at 6.49.59 PM.jpg
 

MaxHeadRoom

Joined Jul 18, 2013
28,779
The type shown that I have used clamp the fuse to the bare metal being sensed?
Are you sure that is not just protective covering, unless intended for a specific purpose?
 

Tonyr1084

Joined Sep 24, 2015
7,964
A part number or data sheet will be most useful in determining a correct answer to your question. For now all I can do is guess that this may be a thermal cutoff switch that breaks contact at 75˚C and will self-reset when it cools to some lower temperature.

OK, someone just posted ahead of me. Let's see if we bumped heads.
 

Tonyr1084

Joined Sep 24, 2015
7,964
Thanks for the link. The plastic is superfluous. Not intended as part of the sensing of temperatures. The description calls it a thermal fuse AND a temperature switch. In the description section it describes the item as being good for 10,000 cycles. Fuses don't cycle, they blow. Once blown they're discarded and replaced when the cause of the failure has been fixed. What you have is a switch. And in the pictures there is only one photo that shows a plastic sleeve. I'm pretty sure it's only for protecting the product.
 

Tonyr1084

Joined Sep 24, 2015
7,964
Note: Edited post #5. Original statement said 1,000 cycles. Edited and corrected statement now says accurately 10,000 cycles.
 

Thread Starter

draw2build

Joined Nov 25, 2022
19
Thanks for the link. The plastic is superfluous. Not intended as part of the sensing of temperatures. The description calls it a thermal fuse AND a temperature switch. In the description section it describes the item as being good for 10,000 cycles. Fuses don't cycle, they blow. Once blown they're discarded and replaced when the cause of the failure has been fixed. What you have is a switch. And in the pictures there is only one photo that shows a plastic sleeve. I'm pretty sure it's only for protecting the product.
Awesome, thanks everyone for the great information. I wasn't aware it was a switch/fuse. So it looks like it should work. I chose this shape because it's flat, as I'm mounting it on a Peltier.
 

MisterBill2

Joined Jan 23, 2018
19,045
Probably that plastic covering is electrical insulation, because probably one or both sides of the connection may contact the metal housing, and thus create a shock hazard if the thermal fuse/switch touches an accessible part of whatever it is mounted in. Please consider that possibility before discarding the plastic cover..
 

MaxHeadRoom

Joined Jul 18, 2013
28,779
Probably that plastic covering is electrical insulation, because probably one or both sides of the connection may contact the metal housing, and thus create a shock hazard if the thermal fuse/switch touches an accessible part of whatever it is mounted in.
Every appliance I have seen these used in has had the fuse directly in contact with the metal body being monitored. :oops:
 

Tonyr1084

Joined Sep 24, 2015
7,964
Probably that plastic covering is electrical insulation, because probably one or both sides of the connection may contact the metal housing, and thus create a shock hazard if the thermal fuse/switch touches an accessible part of whatever it is mounted in. Please consider that possibility before discarding the plastic cover..
Take notice of note #3:
1675007260579.png
 

MisterBill2

Joined Jan 23, 2018
19,045
OK, so that plastic ccover is for shipping protection. I am not sure as to just what "the metal case is charged" is tellingme, but it seems to imply an electrical connection. So evidently the device needs a heat conducting electrically insulating mount.
 

Tonyr1084

Joined Sep 24, 2015
7,964
"the metal case is charged"
A guess at what this is saying is that one of the two leads is electrically connected to the casing of the switch. In the case of a Normally Closed switch then both leads would be conductive to the metal body of the switch. If it's intended to be in contact with something metal then that would seem unreasonable to have an energized source touching metal that may or may not be grounded. Perhaps - and this I don't know - if the switch is on the neutral side then there should be some way of determining which lead is always conducting to the metal body. One might assume that being in contact with a grounded frame could negate the safety feature. But again - in bold letters - I DON'T KNOW THIS FOR A FACT. It's just guesswork. I've seen them but I've never worked with them.

I know there is a thermal switch inside my treadmill motor(s). I don't think the metal housing of the motor is grounded. Though I haven't looked closely to determine if metal to metal contact is the actual way it's in there I would assume it would work better if it has a direct contact with the hot surface of the metal frame (permanent magnet motor). Depending on radiant heat or thermal conduction through the air, the switch will react differently. If by conduction through air then the switch will react much more slowly to an overheating condition.
 

MaxHeadRoom

Joined Jul 18, 2013
28,779
OK, so that plastic ccover is for shipping protection. I am not sure as to just what "the metal case is charged" is tellingme, but it seems to imply an electrical connection. So evidently the device needs a heat conducting electrically insulating mount.
Per spec: Can be fixed with 704 silicone RTV, which is High Temperature Resistant Silicone Rubber Heat Insulating Fixed Moisture Proof Sealant
 

Tonyr1084

Joined Sep 24, 2015
7,964
This is what I imagine this is what is meant by "the metal case is charged". One of the leads is electrically conductive to the metal case. Note the chassis ground symbol I've used. Keep in mind - I DON'T KNOW this to be the way it's constructed. This is how I perceive the term is referring to. Note that the two bars, one solid, one made from two dissimilar metals, are in contact at the far end. This is the normally closed position of this switch. The opposite would be true if the switch were a Normally Open (NO) type of switch.
1675053171674.png
 

MrAl

Joined Jun 17, 2014
11,583
I received 5 thermal fuses, 75C Normally Closed. They came with a clear soft plastic covering them.
What is this soft plastic covering used for?
I don't think it could be heat shrink, as the heat would pop the fuse.
Thanks in advance.
View attachment 286209



Hi,

Looks like a protective sleeve, although i dont know why it would need that.
It could also be to aid in mounting to a particular application. If you take it as is and slip it around something else that is shaped like a thin shaft it may be the way you mount it.
The ones i have gotten in the past were either plastic or some type of metal sleeve. You mount them in direct contact with the part to be sensed. One application for example is in a power strip surge protector which uses spike absorbers to absorb the extra energy from a line surge. The absorbers can get hot and cause a problem so they mount a thermal fuse in direct contact with the absorber so if it gets too hot it opens up the entire circuit completely.
Another app is in a line powered fan. If the fan metal gets too hot it opens up the circuit and the fan shuts off.
 

Alec_t

Joined Sep 17, 2013
14,382
Unlikely, but there might be some unusual application in which the sleeve provides a thermal delay, e.g. to prevent nuisance tripping from brief temperature excesses.
 

MisterBill2

Joined Jan 23, 2018
19,045
Hi,

Looks like a protective sleeve, although i dont know why it would need that.
It could also be to aid in mounting to a particular application. If you take it as is and slip it around something else that is shaped like a thin shaft it may be the way you mount it.
The ones i have gotten in the past were either plastic or some type of metal sleeve. You mount them in direct contact with the part to be sensed. One application for example is in a power strip surge protector which uses spike absorbers to absorb the extra energy from a line surge. The absorbers can get hot and cause a problem so they mount a thermal fuse in direct contact with the absorber so if it gets too hot it opens up the entire circuit completely.
Another app is in a line powered fan. If the fan metal gets too hot it opens up the circuit and the fan shuts off.
Interesting and useful about the fans. I have been asked to fix a few of them that "just stopped" and while on some it was the cord breaking inside where it flexes as the fan swings back and forth. So a thermal fuse buried inside the motor may be the culprit. Now I know another cause to search for when a motor is just open circuited..
Thanks for the clue!!
 

Tonyr1084

Joined Sep 24, 2015
7,964
I received 5 thermal fuses, 75C Normally Closed. They came with a clear soft plastic covering them.
What is this soft plastic covering used for?
I don't think it could be heat shrink, as the heat would pop the fuse.
Thanks in advance.
View attachment 286209
Bottom line - unless there is good reason to use them, the intended purpose is for the metal body to be mounted against the source of heat you want to protect from excessive heat. In the case of a fan or any other motor, such a switching device is a good idea. If you've ever opened up a refrigerator, you've seen a thermal switch on the side of the compressor. If there's a brief interruption in power while the motor is running, the interruption will cause back pressure to stall the motor. The motor will quickly heat but the thermal switch will protect it from excessive heat. Power to the compressor will be interrupted. Only when the motor has cooled enough will the switch close again and allow the compressor to try to start again. If it is too soon and back pressure has not sufficiently been relieved the motor will remain stalled until pressure drops enough. The thermal switch may cycle several times before the motor can successfully start.

In the case of personal fans such as desk fans or box fans, they have a fusible link in direct contact with the motor. If for some reason the motor heats excessively, either due to a blocked fan blade where the motor can not spin or if the motor bearings become dry and frozen the motor will overheat. The fusible link will blow. It is a one time device. It's purpose is to prevent home fires. I've seen many fans with blown fusible links. Same is true of other devices as well. Mother-in-law had a blown fusible link in her washing machine. The machine could not switch from agitate to spin because of the blown fuse. It had to be replaced. It doesn't self-reset. It's not a switch.

What you have is a switch. In my drawing in post # 15 you see the bent bar drawn with three lateral lines. That's because a bi-metallic strip consists of two different types of metals with different heat expansion characteristics. One bar expands more than the other at higher temperatures. Depending on the exact makeup and sizing of the two metallic strips you can get them to react at a specific temperature. In the case of the drawing there are two contacts that separate when sufficient heat has been applied. The intention is to protect machinery from overheating and possibly causing a fire.

Dryers are another home item that have thermal switches. One is designed to prevent overheating while another is designed to interrupt the heat cycle at a specific drying temperature. The first will shut the whole machine down. The second will continue to run the machine and control the drying temperature. There are lots of uses for thermal switches. For many years bi-metallic springs have been used to regulate the temperature in the home. Of the various types, they all employ the same principal of thermal expansion at given temperatures. In the thermostat there's a coil with either a set of contacts or perhaps a glass bulb filled with mercury and two electrodes. As the room cools the coil retracts and tips the mercury into contact with contacts, turning the heater on. When the room warms sufficiently the coil expands and tips the mercury away from the contacts.

That's my brief summary on bi-metallic strips. Whether we're talking about using them in a fan or in some other application which I don't recall the TS stating a particular project, all we can say for sure is that you have a thermal switch. And in all likelihood the rubber coating may be nothing more than a protection so the casing does not get damaged in shipping. However, none of us can say for sure that the sleeve has or doesn't have a purpose beyond protecting the product (PTP).
 

MaxHeadRoom

Joined Jul 18, 2013
28,779
There is also one in coffee pots, if the boil element stays on, instead of switching down to the base warmer, it can cause a fire, hence the thermal cut out.
BTW, one of the largest cause of residential fires in N.A. is unattended kitchen appliances!.
 
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