Grounding (FAC2100)

Owned by Gary Cane
Last updated: Jun 30, 2014 by Mark Krueger (Unlicensed)

Grounding


 As with all electrical devices, it is important to keep in mind the layout of the grounds when wiring a control circuit. There are three types of grounds to keep track of (see figure below):

  1. Earth / Case Ground,
  2. Isolated ground for the controller, and
  3. External power supply ground or a non-isolated ground for the controller.

Ground Symbols and Associated Labels


The ground symbols shown above are used throughout the installation figures in this section. Like symbols in each of the figures represent the same circuit point. Make sure that joining these common grounds to different voltage potential levels does not create unwanted shorts.


The circuitry ground and the auxiliary output ground are both isolated from the input power source, and as a result, are isolated from the external world. Thus, the designer only needs to be concerned with not mixing grounds within the same controller.


Connections to the analog inputs and analog outputs are all referenced to the same isolated internal circuitry ground. The communications, relay, digital inputs, and digital outputs are optically or mechanically isolated and therefore pose no ground-mixing risk.


Consider the example connection diagram shown below, where both analog inputs are connected to the same signal. In the figure, a loop-powered transmitter is connected in series to both analog input 1 and analog input 2 of the same controller. Inside the FAC-2100 silhouette, the equivalent circuit of each analog input is shown. Notice that the negative analog input terminal is connected to the internal circuitry ground symbol. This is true for all analog inputs.

 

Incorrect Grounding Scheme Example


Since the ground (Gnd) terminal of each analog input is at the same ground potential, the wiring below is equivalent to the above example. Notice that in the equivalent circuit, the analog input 1 leads are shorted together, thereby totally bypassing the input. This problem can be resolved by wiring analog input 2 as a voltage input and wiring it in parallel to analog input 1 (reference Voltage Input Mode).

Incorrect Grounding Scheme Equivalent Wiring Diagram

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