In a 24V DC power system, if neither terminal is connected to ground (i.e., the system is floating), the voltage between each terminal and ground is undefined and may fluctuate. However, once the power supply is connected to an instrument, the reference potential stabilizes, and this typically has no negative effect on instrument operation.
When one terminal of the power supply is grounded, the voltage with respect to ground becomes defined:
If the negative terminal is grounded, the positive terminal will be +24V with respect to ground. This is the most common configuration in industrial control systems.
If the positive terminal is grounded, the negative terminal will be -24V with respect to ground. This configuration is less common and typically used in specific applications.
Impact on the system:
Negative Grounding:
This is the standard method. It enhances safety, improves common-mode noise rejection, and aligns with the design of most instrumentation and control systems.Positive Grounding:
While technically possible, it may cause compatibility or safety issues with some devices. It is generally avoided unless specifically required by the application.