Hi Fman,
Iknew i should have removed that shrinkwrap & took a better look
The following are just my observations & i stand corrected if i'm wrong (which is a lot

)
I thought ferrite rings were installed on devices to arrest any RFI from & to the equipment in question.
I also thought that you can achieve the same RFI arresting qualities by using a well designed cable with a plaited configuration.
Sky digi boxes are built to a price (cheap as chips) & if a manufacturer has the option of using a £1 ferrite ring or a £25 mains cable.......well i know which he will choose.
Digiboxes (in my experience) are one of the main sources of RFI, so my guess would be that the rings are there to stop the digibox polluting the rest of your kit. I also read that the EEPROMS inside a digibox are susceptable to RFI, so are the rings there to protect the delicate electronics from it's own (unshielded) power supply?
With digiboxes being widely avalable in Europe they have to contend with varying & variable voltages so they have to made tough, so IMHO the rings are not there to stop voltage spikes getting to the digibox.
But then again i could be talking cobblers
Any electricians care to enter the fray?
Cheers.
Tim.