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This is a recessed light I pulled out of my ceiling. There is a black cylindrical thing attached to the junction box and wired in series with the live wire to the bulb base. What is it?

The junction box and the black thing are both an integral part of the light fixture, ie they came with it.

The cylinder has "A10A9 Mexico" marked on it. This is in NJ, USA and the light fixture is probably around 1970s or 80s era.

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Harper - Reinstate Monica
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jay613
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    Do you know the type of light bulb it uses? Might narrow down the choices. Maybe a temperature limit switch if haligon – crip659 Apr 17 '22 at 22:27
  • It is designed for an incandescent reflector flood, like an R30. Replacement CFLs and LEDs can be used too. It would be a strange place for a temperature switch. The junction box is connected to the actual high hat by a 6-inch *horizontal* metal arm. You can see me holding that in my hand. The temperature would have to be carried to it via the wires. It might be a fuse but I've never heard of a fused fixture for incandescent lights. – jay613 Apr 17 '22 at 22:49

1 Answers1

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It is a thermal protector for a recessed ceiling light. It will cut out the light if the temperature of the outside of the fixture exceeds the rating of the protector. All UL approved fixtures will have them. If the protector fails, in many cases,the bulb will turn on and off signaling a bad protector.

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Picture from Lighting Supply.

JACK
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  • This is clearly correct. It doesn't look to me as though heat would be conducted properly from the high hat to the protector device, but it must be that heat is conducted through the wires, which travel through a conduit in the horizontal metal arm, and then through a wire nut and then to the protector. I don't understand the design. – jay613 Apr 17 '22 at 23:18
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    @jay613 The heat could also be conducted through the metal tabs that secure the protector to the housing. – JACK Apr 17 '22 at 23:32
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    @jay613 Critical to understand is that recessed lights come in two major flavours - IC (insulation contact) rated, and non-IC fixtures. What the question shows is a *non-IC* fixture - it's meant to be installed in an empty ceiling cavity. If insulation is packed around it then it will become too hot for the wires. That protector is there to cut power to the light if that happens. It's not there to measure the bulb or the cone, but the ambient temperature around the fixture, which must remain low and in free air so that the rest of the fixture can shed heat. – J... Apr 18 '22 at 13:38
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    @jay613 The sensor has a heater built-in and is designed to sense the ability of that heated element to shed heat. See [here](https://rexel-cdn.com/Products/LithoniaLighting/RK1CE00210THERMALPROTECTOR.pdf?i=335452EB-E9CB-43E3-90B7-5B03DF995E30) for more detail. I quote : *"The heater is so effective that when the RP is inserted in insulation without the lamp installed, the bimetallic protector will operate. In this manner, the RP functions as an insulation detector."* – J... Apr 18 '22 at 13:41
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    @J... brilliant ... thanks for the education! – jay613 Apr 18 '22 at 14:41
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    Ah. I'm used to the IC type, where it's a simple in-line switch, typically at the top of the can, that turns off when it overheats. – Ecnerwal Apr 18 '22 at 15:42
  • @Ecnerwal Same here... we have insulation everywhere. – JACK Apr 18 '22 at 23:51
  • Here's another [nice article](https://iaeimagazine.org/electrical-inspections/thermal-protection-in-recessed-luminaires/) from IAEI on these thermal protectors. – J... Apr 19 '22 at 15:40
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    @J... Great article. Thanks for sharing. – JACK Apr 19 '22 at 15:52