Determining Maximum DC Voltage

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.

Problem:
Electrical equipment, such as motor operated valves, on a cathodically protected structure requires safety grounding according to the electrical codes, however a direct bond will cause a short-circuit on the CP system. Likewise, tanks with electrical equipment can be affected by the bond to ground.

Solution:
Using a DEI product certified for use in a safety grounding conductor is an authorized method of providing DC isolation and simultaneous AC grounding for motors and other equipment. In the case of a valve motor, this eliminates the need for insulated joints on either side. Instead, the motor is grounded via the DEI isolation device. Certified (listed) products are needed for this application, to comply with electrical codes. See the schematic of a sample installation.

To implement an equipment/tank isolation system, the following issues need to be examined to determine product selection and ratings:
1. The maximum DC Voltage present between the structure and ground
2. Whether induced AC voltage is present at the site
3. Examine AC fault current exposure
4. Is this site classified as a hazardous Location?
5. Install the device in series in the grounding conductor, and determine that there is not an electrical bypass around the device.

See Application Note #2 for a full review of this application.

Problem:
When an insulated joint is used to electrically isolate sections of pipeline, over-voltage protection of the joint insulation may be necessary. The insulation can fail, due to lightning or AC fault current, with potentially disastrous results. Arcing across the joint will cause insulation failure and possible ignition of flammable gases.

Solution:
A protection device connected across the insulated joint will limit the voltage to safe levels, and provide a conduction path around the joint, while maintaining cathodic protection. Products listed for use in hazardous locations will address the over-voltage problem while assuring safe operation.

To implement an insulated joint protection system, the following issues need to be examined to determine product selection and ratings:
1. The maximum DC voltage present across the insulated joint
2. Whether induced AC voltage is present at the site
3. Examine AC fault current exposure
4. Location of the device, and the resulting conductor length
5. Is this site classified as a hazardous location
6. For mounting options, see data on each product page

See Application Note 1 for a full review of this application.