Decoupling Power Cable Sheaths/Casings

Decoupling Power Cable Sheaths/Casings

Problem:

A cathodically protected pipe-type cable casing or lead sheath must have solid grounding to assure safe conditions for personnel and equipment, however the large copper substation grounding system would adversely affect the cathodic protection values if directly connected.

Solution:

The DEI model ISP (and in some cases the PCR) can be installed in series in the safety bond between the pipe casing/sheath and the grounding grid. The fail-safe nature of the DEI design assures safety under all conditions. The device has a low AC impedance and can pass induced or imbalanced currents, while blocking DC current, and is chosen to have an AC fault current rating in excess of the available system fault current.

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

  1. Verify the voltage threshold needed, considering any stray DC sources.
  2. Examine AC fault current exposure.
  3. Determine the maximum steady-state AC current that can flow through this grounding connection.
  4. If the site is below grade or in a vault, select a device with a submersible rating.

See Application Note 4 for a full review of this application.