Decoupling From Utility Grounding Systems

Decoupling From Utility Grounding Systems

Problem:

When a cathodically protected structure is tied into the site grounding grid, CP values may be unacceptably low, due to the bond between the site grounding system and the power company grounding system. The CP system attempts to protect the power company grounding system as well.

Solution:

The Dairyland model PCR can be installed by the power company at the transformer, to provide DC isolation and AC grounding between the two systems. The site CP system will not attempt to protect the power utility grounding system. This minimizes the CP current requirements and allows acceptable CP voltage for protection. See the typical schematic.

To implement a primary-to-secondary decoupling solution, the following issues need to be examined to determine product selection and ratings:

  1. Verify that decoupling the bond between the utility grounding system and user grounding system results in a change in the CP voltage and current. Accomplish this by temporarily disbonding the grounding connections between the two systems, using appropriate safety practices. At a small facility, this can be accomplished by turning off loads, then disconnecting the neutral and/or grounding conductor from the panel. The same isolation can be accomplished by the power company at the transformer. Keep in mind that other utility services, such as telephone, also connect between primary and secondary grounding systems and will act as a bypass unless addressed.
  2. Examine AC fault current exposure and select a rating in excess of the site conditions.
  3. Confirm acceptance by the power company of the proposed isolation. Note: for certain wiring arrangements this type of decoupling can occur within the user's electrical system. Contact DEI for more information.

See Application Note 3 for a full review of this application.