Ground and screen connections
Ground connections
There are three different ground connectors in the SPN1 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 SPN1/w-05 is
connected to the SPN1 enclosure internally, creating a continuous screen
around the sensor electronics and output connectors. 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 SPN1 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 SPN1-RS232 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 SPN1-RS-10 is also screened. The
screen connects the M12 connector shells at both ends.
SPN1 User Manual v1.0
Introduction
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