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Originally Posted by Monkfish
You're doing wonders for my confidence in NTL. 
I now wonder what they've done in our street cabinet?
Anyway, the screen is obviously not isolated in the cabinet, otherwise I'd only get aerial pickup and not this 50Hz earth loop problem.
Listening to your horror stories my 4V ac interference seems tame.
I wonder whether wrapping ptfe plumbing tape around the coax thread on the back of my modem would act as a highpass filter; i.e. pass through the cable signal but open circuit the 50Hz? A coax is a waveguide after all.
Maybe I'll just get an engineer out.
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Yes, you are correct the screen is not isolated in the cabinet.
The screen would be connected directly to one of the F-sockets on the customer tap bank, this would be grounded by its connection to the amplifier and the amplifier and tap banl should have their own earth wire wired to the earth spike.
Wrapping ptfe tape around the thread would mean loss of signal, the higher frequencies would jump across but the lower ones would be severely attenuated, the return path Upstream would be non-existent.
I dont think isolating would cure the volts you are seeing on the screen, I think most of it is probably due to different earth potentials and pickup. If grounding the screen causes it to dissapear, I would tend to think its too high in impedence to cause any problem. Also remember that whilst it is easy to earth a 50Hz signal (mains power) Its very difficult with RF. The RF earths need to be very short to avoid pickup on the screen, if the screen is multiples of a wavelength, (bound to be with such a wide freq range) it will still act as a good antenna. A coax drop cable say 20 meters long earthed at the cabinet end will still be a very effective HF antenna, it may look like a short to earth less the cable resistance at 50HZ but to RF it's another matter.