Testing for resistance to hygroscopic dust was removed for products operating at voltages proven to be immune to leakage current effects. Mixed Flowing Gas (MFG) Leveraging:
NEBS (Network Equipment-Building System) is the gold standard in North America, but its influence has spread globally. If your hardware is going to sit on a rack in a Telco office, it must pass GR-63-CORE. gr-63-core issue 5 pdf