Guralp Systems CMG-3TB - Manual (Page 45)

Operator's guide
As with AC installations, if the borehole is more than around 30 m
from the surface enclosure, you will need to provide a second earthing
point for the local ground plate.
DC power is most commonly available at self-contained installations
with power supplied from batteries, solar panels, or a wind generator.
In these cases, the power supply may already have protection from
transients installed, in which case you may not need such
comprehensive protection (although some form of protection is always
necessary.)
External lightning protection
The surface installation building, and if possible the borehole also,
should be protected by lightning conductors. These should lead to
ground well away from the borehole. As a rule of thumb, a lightning
mast provides a “zone of protection” within a 45 ° cone the height of
the mast.
If you are using two earthing points, for example in the DC installation
shown above, it may be convenient to connect the lightning conductor
to the supply-side earthing point. In any case, the lightning earth must
be well separated from the borehole (and its earth, if it needs one.)
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