Thermal shutoff in heater, keeps blowing

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Hi, guys. I have a thermal shutoff in an electric storage heater which failed suddenly with burn marks on one side and onto the connecting wire (pic). I ordered the replacement component and connected it in, all the wire cut back and cleaned, etc.

As soon as I turned the heater back on, it popped again, this time scorched on both sides.

This is the first component in the whole circuit - the live wire comes straight from the wall and into this. There's nothing before it that could be failing/missing.

In the picture, below/off screen is the heating elements. The face of the switch is exposed to them in order to sense the temperature. It's all behind a heat shield - the burning is definitely from it "blowing" and not from the heating element.

Any ideas?

Thanks!
e2984813-7248-4765-8353-09009e9be51e.jpg
 
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If it failed instantly then most likely there's some sort of overcurrent fault going on (element shorting to ground or to itself would do it or maybe an issue with the fan).
Have a look at the rest of the heater, if you have a multimeter do some comparisons with a working heater
 
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Did you replace the slide on connectors with new ones or did you re-use the old ones ?

The original burn ( as in the photo ) is almost certainly due to a defective crimp between the slide on connector and the brown wire.

The heat from that burn will have softened the metal of the connector and it will no longer be a tight contact with the blade of the sensor. That poor electrical contact will generate heat
 
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Is the crimp on the white wire on the thermal cutout shorted to the chassis to its rught?
Any other similar possibilities for shorts?
 
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The crimp on the brown wire is completely failed and will never make a useable connection.

This was caused by a bad joint to the old cutout and caused the cutout to operate. I wasn’t caused by the operation of the cutout.

You need to remove the crimp and as much of the brown wire as has been damaged by the bad connection and subsequent over heating and replace it. If you have used this connector to supply your new cutout then that will now need replacement also.
 
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Measure the resistance of the heating elements.

OK - what am I looking for there? Although, the heater itself was off (on its control panel) when I turned it on at the wall, so the elements weren't in the circuit anyway at the time.

Is the crimp on the white wire on the thermal cutout shorted to the chassis to its rught?
Any other similar possibilities for shorts?

No, that off-white part of the chassis is actually plastic, there's no metal contact there.

The crimp on the brown wire is completely failed and will never make a useable connection.
Did you replace the spade terminal?
Did you replace the slide on connectors with new ones or did you re-use the old ones ?

Yes - I cut back the brown wire completely. I did actually solder the wire onto the new component, could that possibly be an issue? I couldn't find my crimps set and just went for it. It was a good solder connection, and how it blew the second time was sudden, immediate, and not just on the live side.


If it failed instantly then most likely there's some sort of overcurrent fault going on (element shorting to ground or to itself would do it or maybe an issue with the fan).
Have a look at the rest of the heater, if you have a multimeter do some comparisons with a working heater

Yea the way it popped was reminiscent of when we used to blow up LEDs by connecting them a power supply at school, was definitely "too much current" from a layman's understanding of it.

Thanks for the replies, all - I'll get back to you.
 
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I did actually solder the wire onto the new component, could that possibly be an issue?

Yes, that would indeed be an “issue”.

It’s a thermal cutout. It’s not going to enjoy being heated to 200 degrees by a soldering iron!

Get another cutout, and connect it with a new crimped spade terminal.
 
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Yes, that would indeed be an “issue”.

It’s a thermal cutout. It’s not going to enjoy being heated to 200 degrees by a soldering iron!

Get another cutout, and connect it with a new crimped spade terminal.


Hmm, really? Its switch temperature is 175 degrees already, so I'd have thought it would cope with that - and also the body is plastic (spade terminals pass into the plastic body) and none of that was damaged by the soldering. I'd be very surprised if the soldering heat damaged the component (didn't even melt the wire sheath near by). I'm pretty neat at soldering, never seemed to damage even very delicate components.
 
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Is the other side of the cutout in contact with a metal part of the heater?
Check continuity between the terminals and earth.
Please post more photos. Maybe there is a fault in the control panel.
 
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Is the other side of the cutout in contact with a metal part of the heater?
Check continuity between the terminals and earth.
Please post more photos. Maybe there is a fault in the control panel.

There's definitely no shorting or contact with metal parts of the heater, no. The white stuff around it is plastic.

Did any fuses or MCBs etc. trip when it failed?

Nope.
 

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