Mu metal shielding. Your problem is probably magnetic rather than electrostatic. Mu metal should handle the field at 7 meters. Perhaps
even a couple layers of mild steel properly fabricated, (complete magnetic circuit) would do. Shield then degauss.
Back when I worked at a primary aluminum plant (with 90kA busbars), we used heavy steel boxes around the monitors, which made an unuseable
monitor just tolerable.
The real answer is TFT monitors. I wouldn't even mess with a CRT.
Eric M. Klintworth, PE
Columbus, Ohio USA
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You can use enclosure made of mu metal , this will help , Also enclosure from Rittal can be used , they have a range of EMC enclosures.
Personaaly I feel LCD (TFT) monitors would be the best choice.
Use an LCD monitor such as Philips 105B, they are coming down very nicely in price, a supplier recently had them on offer for R3999 in South Africa, about 500USD