BF5 User Manual v 2.3
Use with a data logger
27
Ground and screen connections
Ground connections
There are three different ground connectors in the BF5 analogue output
cable (and another one in the serial cable), so it is possible to create
offsets in the readings if these are used without care, especially with long
cables. You also need to think about how the data logger ground is
connected internally.
Some key principles to remember are:
Ensure that no return currents flow in the SigGND wire. This should
only go to the –ve terminal of a differential voltage input channel.
Ensure that the current returns for the DL-Pwr wire and the Sun wire
are through the DL-Gnd wire.
Ensure that the current return for the Heater current (Htr+) is through
the Htr- wire.
If you use a logger and a separate power supply, then one of them
must be able to float relative to the other, or they must share a
common ground connection.
Earthing and screen connections
The braided screen of the 8-core analogue cable type
SP-BF/w-05
is
connected to the BF5 electronics internally. The screen is connected to
the sensor ground by an 18V transient suppressor - to minimise the
possibility of high voltages relative to the internal circuitry if there are
nearby lightning strikes.
In general the screen (black wire) should be connected to local earth near
the logger. The BF5 may also be connected to earth at its mounting point.
If there is a lightning strike nearby, there may be large transient voltages
induced between earth points, and in the sensor cabling. Some of this
may also appear on the signal wires. Because of this, the logger should
either be free to float relative to local ground, or should have some form of
transient protection on its inputs to avoid damage.
The 5-core serial cable type
SP-BF-RS01
is screened. The screen is
connected to the 5-pole M12 connector shell but not to the DB9 shell.
The 5-core serial extension cable type
SP-BF-RS10
is also screened. The
screen connects the M12 connector shells at both ends.