The product must only be connected to a power source that has a safety electrical earth
The earth connection to the rack must be checked before switching on by an electrical engineer
who is qualified to the appropriate local and national standards to perform the check.
IBM System Storage DCS9550 1S1 Storage Expansion Unit Installation, Service, and User Guide
If more than one product is fitted in a rack, the earth connection to the rack is
even more important, because the rack will then have a high "EARTH
LEAKAGE CURRENT" ("TOUCH CURRENT").