Sharp 1840TC-FC Instruction Manual And Parts List page 12

(3) A separate grounding wire should be used, one whose length is as
short as possible.
(4) Check the resistance to ground by actual measurement. This should
measure less than 100 ohms if the single device is connect to its own
grounding rod.
DESIRABLE INDEPENDENT GROUNDING:
COMMON GROUNDS:
NEVER GROUND EQUIPMENT AS SHOWN IN THE FOLLOWING FIGURE:
Earth resistance:
Less than 100 ohms
Resistance to ground=100/
the number of devices
connected to the ground (Ω)
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