Mains hum

I spent the day at a facility we built a year ago trying to find the source of mains hum on a lot of their technical distribution. I've blogged about such things a couple of times before - here and here and always hope that people will accept advice about best practise and how to avoid such things.
Anyhow - eventually I tracked down the source of their problem - I had installed small isolators on those monitors (plasma TVs actually) that were powered off the domestic mains but one wall-mounted display had managed to snag it's video co-ax on the mounting bracket and hence grounded the screen to domectic earth before the cable hit the isolator. Even though I could only measure about 0.15v between the two earths it was enough to put big hum-bars on pictures (and pretty much every feed in the building!). I fixed the cable but as a bit of a belt'n'braces move I put video hum-buckers right in every feed leaving the machine room to equipment powered by the "cooking mains". That way even if cable damage occours in the future the isolator will protect the integrity of the technical earth.
I was there for six hours and didn't get offered a cuppa once - and we were doing it as a friendly favour!