Torroid on power cord of electrical appliances

[boylesg]

There is often a single coil of the 3 core power cable through a toroid just inside tv sets etc.

What is the purpose of this?

 

[FvM]

It's a common mode filter to suppress high MHz interferences. It's often supplemented if the device failed EMC tests.

 

[smijesh]

Common mode choke with single turn. As a feed through filter. For high frequency(MHz) as single turn with both wire in same direction. But in low frequency noise filtering, the windings will be in opposite direction.