Jul 05, 2017

Is your control system ground in good working condition?

  • Article
  • control system
  • advanced control system
  • ground loop currents

It’s difficult to answer this question without proper measurements.

However, we know that industries are losing millions of dollars yearly due to unplanned shutdowns related to communication disruptions created by ground loop currents.

  1. The following situations are all symptomatic of a faulty, poor or corrupted control system ground (CSG):

    • Communication error rates are inexplicably high
    • Disruptive events are somehow more frequent at a certain time of the year
    • Communication cards or power supply modules were damaged by a lightning strike
    • Cable shields are grounded at both ends

    These and other disruptions can point to a dysfunctional or corrupted CSG.

    In fact, in the past 30 years, we were involved in many CSG site survey and have never found one in good working condition.

  2. Not so simple…

    The apparent simplicity of a single-point CSG network design can be misleading. The engineering required for a CSG to function properly is crucial and complex. Many things have to be properly planned and performed, including design, installation and maintenance. One bad connection can put the entire installation at risk.

    Also, control system manufacturers have various grounding requirements, which complicate implementation. Respecting all requirements of all individual manufacturers and integrating those into a functional CSG can turn out to be quite a challenge.

    A bit of history

    Within the process industry, single-point ground networks for control systems came with much fewer requirements in the early 80s. DCS (distributed control system) manufacturers had the most stringent CGS requirements: their system had to be installed with a dedicated ground system, which had to be kept isolated from any other electrical ground system in the plant. They wanted to keep their system free from any outside (ground) disturbances or corruptions.

    This was challenging, mainly for safety reasons, because high voltages could develop between the respective groundings of the DCS and the surrounding building (main safety ground) within large installations. Even in a control or I/O cabinet, there could be voltage between the DCS ground and the cabinet enclosure (if metallic), which posed a threat to employees, primarily during an equipment failure or a lightning strike. Surge protection devices (large capacity Zener diodes for example) were installed to connect the DCS ground and the cabinet safety ground locally in order to clip the potential to a value deemed safe at the time (typically less than 50 V).

    In order to protect plant personnel from electric shocks, some consultants came up with a different approach: connecting the DCS and the main safety grounds with an easily accessible copper strap. The goal was to minimize the risks of high voltage between the DCS ground and the main safety ground.

    When a manufacturer blamed a DCS malfunction on the absence of ground isolation, it was then easy to simply cut the copper strap to isolate the two ground systems (DCS and main safety) (See fig. 1 below).

  3. Fibre optics brought major changes

    The introduction of fibre optics changed grounding requirements drastically. It became possible to have a local grounding approach, and manufacturers adjusted their requirements to the new technology. For example, fibre optics helped reduce grounding-induced current problems when used to connect two systems, as this would prevent any grounding-related current loops on that ground link.

    The use of fibre optics led to a significant reduction of grounding-related problems. CSG-related issues were, from then on, given less attention. So much so now that the technical expertise required to troubleshoot CSG problems that are still occurring is less and less available.

    Currently, the use of fibre optics within an inefficient CSG network has the following downside: the ground current becomes concentrated into the remaining copper ground links. It can facilitate the problem troubleshooting with high ground current. However, it increases the risks of damaging sensitive electronic components.

    One solution is to ensure the proper engineering, installation and maintenance of the CSG (single point ground) to eliminate induced ground currents in copper ground links by using fibre optics, when necessary. This is easier to implement in a greenfield project, but can also be retrofitted into an existing installation.

    The following CSG approach has been applied to many sites and has generated excellent results (See fig. 3 below).

  4. 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!

This content is for general information purposes only. All rights reserved ©BBA

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