This approach could be adjusted to various control systems, but the principle remains the same. These are the important features of this design:
- Single-point grounding architecture easy to maintain over a local area or department.
- All communicating control systems are located on the same reference potential. No induced ground current in any communication link.
- One ground reference per area, with ground isolation between each area, ensured by fibre optic communications media to connect systems belonging to different ground references.
- CSG safely connected at a single-point to the main safety ground, thus minimizing the ground differential voltage between them.
- Ground cabling well segregated for each system and floor, minimizing troubleshooting time.
- No current flows through the equipment backplanes of any equipment installed on single-point ground reference control panels, thus preventing damage to sensitive components.
This grounding architecture has been implemented and tested for many years with no reported grounding-related communications problems once installed.
It was also implemented to correct problematic sites and proved to be very robust and easy to maintain.
Checking the condition of your own CSG
First, analyze your single-point ground installation to detect any sign of corruption. If you are shut down and want to test a CSG (single-point), the best method is to disconnect the CSG link to the main safety ground and to use a Megger (50 V max) to test the impedance of the CSG, relative to the safety ground. This will allow you to see whether you have a high impedance value. A value of a few mega-ohms is enough to determine whether you have an isolated CSG (a capacitive effect arises from the various isolated ground cables running along the building structure and cable trays, which lowers the impedance). If you do not detect a high impedance, then you know that your CSG is somehow corrupted, and that you have to take action to resolve the situation.
Once the corruption is eliminated, you can safely reconnect the link to the safety ground.
This procedure can be performed every time your plant is shut down in order to maintain the good working condition of your CSG.
Using the ground current level method
If your facility never shuts down, you can use the following ground current level method to determine whether or not your CSG is corrupted.
Start by measuring current in the main connector of the safety ground circuit. If the current flow exceeds the acceptable value in terms of the capacity of the grounding installation, analyze the other branches until the corruption points are found. The magnitude of the acceptable current (mA) depends on the capacitance of the isolated ground system. In this case, experience helps determine the current portions that relate to circuit capacitance versus corruption.
Either the CSG is well engineered, installed and maintained, or there is a multitude of corruption points. There were a multitude of corruption points in 100% of the problematic sites surveyed.
The effect of this corruption greatly depends on the location of the corruption points and the control systems capability to cope with the induced disturbance.
The ideal method consists of measuring the impedance of the CSG pertaining to the safety ground during a shutdown. The later method describing the current magnitude analysis helps identify obvious corruptions, while at times, proving to not be good enough to confirm the absence of corruption. In that case, impedance measurement is the best alternative.
As you can see, evaluating the condition of a CSG looks simple, but is in fact a complex task when performed without the proper knowledge and instruments.
BBA would be pleased to assist you in performing an effective CSG health check!