At what frequencies do clip on cable ferrites have an effect?

[Flyback]

At what frequencies do clip on cable ferrites have an effect? <30MHz?, >30MHz?, >100MHz? >200MHz, >1GHz?

I speak of such ferrites for clipping on to dc cables which are coming out of isolated offline SMPS containing enclosures.

 

[rjenkinsgb]

They typically have some effect from relatively low frequencies; it's an increase in impedance, so the attenuation also depends on other factors, as well as the core material.

eg. These all give maximum attenuation at very different frequencies:

 

[crutschow]

Generally the larger the ferrite, the lower the frequency for significant attenuation.

You might also consider common-mode chokes to suppress the SMPS noise.