Have taken the test box apart I use for testing caravan lights. Hoping to build one that’s more rugged.

Can you help me identify this component and what it is used for?

 

Its a PTC Polly switch, they come in diferent sizes for diferent current ratings. When overloaded they go opencircuit when heated up. When they cool down they conduct current again. Basicly a resettable solidstate circuit breaker.

 

It appears to be made by a Chinese company called Sea and Land Electronic Corp. I suspect it is rated for 400 mA, but I haven't found a datasheet with details.

Almost all such devices are copies of Raychem PolySwitch devices. Littelfuse acquired that division of Raychem some years ago.
https://www.littelfuse.com/products/resettable-ptcs/radial-leaded.aspx

You do need to be careful to select device with adequate votlage rating for the application. You can find lots of data and applications information at the Littelfuse site.