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I'm tracing a circuit. What is this part?

one side of component

This is back of it.

other side of component

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It's a 1A 125°C (nominal) thermal fuse, rated to fuse at 121°C +/-2°C

Operating temperature is up to 100°C.

Made by Xiamen SET Electronics Co., Ltd in Xiamen China.

enter image description here

It is not resettable. Needs to be replaced.

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  • \$\begingroup\$ Why can't I measure conduction in the two legs? \$\endgroup\$ – Jtl Jan 15 '19 at 4:58
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    \$\begingroup\$ Maybe it's already burned out. Can't really tell from that photo- it should measure very low resistance. Or maybe you're not making good contact because the board is coated or whatever. \$\endgroup\$ – Spehro Pefhany Jan 15 '19 at 5:00
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    \$\begingroup\$ That would explain what looks like a bubble in there. \$\endgroup\$ – Mazura Jan 15 '19 at 14:14
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    \$\begingroup\$ Before replacing it, consider looking for the reason why a device that is supposed never to go over 120°C, but needs costly measures to be really, really kept from going over 120°C, went over 120°C \$\endgroup\$ – rackandboneman Jan 15 '19 at 15:04
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    \$\begingroup\$ Follow rackandboneman's advice "Before replacing it, consider looking for the reason why a device that is supposed never to go over 120°C, but needs costly measures to be really, really kept from going over 120°C, went over 120°C" Judging by that solder job it appears to have been replaced before. \$\endgroup\$ – cardstroker Jan 15 '19 at 16:37
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It is called Thermal cut-off (TCO) / Thermal fuse.

in the "SET F3" 3 indicates that it is rated to cut off at 121+/-2 ℃, 125 ℃(nominal temperature) rated for 100 ℃. where as F indicates rated for 1A load current.

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  • \$\begingroup\$ If more than 1A passes through it. It can open too or only from temperature? \$\endgroup\$ – Jtl Jan 15 '19 at 6:21
  • \$\begingroup\$ In the original post and picture, the legs are no longer conducting and I checked inside. It has opened already. So this can occur without any obvious ambient rise in temperature. Isn't it. \$\endgroup\$ – Jtl Jan 15 '19 at 6:31
  • \$\begingroup\$ Its already burned out. these are single time operation devices. yeh may be. \$\endgroup\$ – Satish Singupuram Jan 15 '19 at 6:36
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F1 indicates a fuse on the silk screen. eg https://www.digikey.com/product-detail/en/littelfuse-inc/80812000440/WK6289-ND/2206338

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    \$\begingroup\$ Seems to be a thermofuse to me. No current noted, but 125°C. \$\endgroup\$ – Janka Jan 15 '19 at 4:28

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