Installing the 3TB in a borehole
The best local earth point in many installations is the borehole itself. For this to
work, the borehole must have a conductive casing and be situated close (<30 m) to
the surface installation. In such an installation you need only connect a cable (green
wire in the photograph below) from the local ground plate to the borehole casing.
An earth strap can be used to ensure a good connection.
If the lower borehole is filled with salt water, the instrument will be adequately
grounded without any further action. Fresh water is an inferior conductor.
In a dry or sand-filled borehole, or one with a non-conducting casing, you will need to
ensure the sonde is grounded by some other means. The best option is often to
attach the sensor housing to an earth line brought out to the surface and attached to
a metal stake driven into the ground nearby.
The sensor's load bearing cable is suitable for this purpose, provided it is secured to
the sensor's lifting loop with a metallic clamp as shown below. This provides an
additional firm electrical contact between the sonde and the load-bearing cable.
If your system has a separate surge-suppressor sonde, this technique should be used
to ground the load-bearing cable to the surge-suppressor. A separate grounding
cable must then be provided between the surge-suppressor and the sensor. The
short, intermediate load-bearing cable between the two units can be used for this
purpose.
Installations with down-hole digitizers will need similar arrangements at the top
and bottom of the digitizer module, or a separate cable for this purpose.
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Issue G - November 2019
