Guralp Systems CMG-3TB - Manual (Page 44)

CMG-3TB
For boreholes with a metallic casing at the bottom and plastic above,
we recommend connecting a cable between the sensor housing and the
ground plate so that the lower borehole casing acts as the earthing
point. Again, the
If there is a significant distance (>30 m) between the borehole and the
surface installation, the resistance of the earth cable may make it
impractical to use the borehole as an earthing point. In these cases,
you will have to connect the local ground plate to an earth stake near
to the enclosure; any coupling between this sensor-local earth line and
ground lines for other parts of the system must be minimized.
Installations with DC power supplies
Güralp sensors require a 24 V DC power supply. In most cases, this is
provided by an isolating DC/DC converter installed at the surface. This
converter can be earthed to the local ground plate as above.
However, DC/DC converters contain sensitive electronics, which must
be protected thoroughly. We recommend installing a full surge
protection unit in addition to the spark gap protector. This protection
is installed on the supply side of the isolator, so it must be earthed
separately from the borehole installation. Otherwise, transients in the
power supply will couple to the sensor.
44 Issue C