Determining AC Fault Current

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:
Small AC voltages on the power company's grounded primary neutral can appear on the customer's secondary neutral, as these are normally bonded together. This voltage may affect dairy cattle and other livestock.

Solution:
The DEI Variable Threshold Neutral Isolator (VTNI) blocks AC and DC current, when installed between the primary and secondary neutrals at the power company transformer by the utility. This results in the farm grounding system being isolated from the power company ground under normal conditions, but reconnected if the voltage rises on either system to the device threshold. The VTNI reverts back to the blocking mode when the event is over.

The VTNI is the preferred solution by utilities for primary-to-secondary isolation, based on its reliable solid-state design. It meets the U.S. requirements for an isolation product, as stated in National Electric Safety Code (NESC) 97D2.

Note: The DEI model VTNI is not authorized for use in solving nuisance shock to persons at swimming pools or similar human health situations where structure to ground voltage is present. Instead, proper bonding and grounding techniques need to be applied to solve such problems.

To implement primary-to-secondary isolation, the following issues need to be examined to verify product ratings:
1. Examine AC fault current exposure and compare to the VTNI ratings.
2. Install the VTNI between primary and secondary neutrals with no bypass around the device.
3. Test to assure that isolation has been achieved.

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 DEI 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.

Implementation:
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.

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 pipelines are in a common corridor with energized power lines, electric and magnetic fields can cause unwanted voltage to appear on the pipeline. This induced voltage requires low resistance grounding for mitigation, while cathodic protection demands complete isolation for the pipeline.

Solution:
DEI products provide continuous AC grounding for pipelines with induced voltage, while leaving the cathodic protection voltage unaffected. The device presents low impedance to alternating current and high impedance to direct current, and connects between the pipeline and a grounding system.

Mitigation designs for induced AC voltage are best done by specialists trained in the proper analysis techniques. Such analysis involves measurements and electrical modeling, using software developed for this task. While an overview of the issues involved is shown below, complete analysis may involve the use of such specialists.

Small projects can have reasonable estimates applied to determine product ratings and a basic system design. Examine the issues outlined below or call DEI for additional guidance.

To implement an induced AC voltage mitigation system, the following issues
need to be addressed.
1. A suitable low resistance grounding system is needed
2. The Measuring Induced AC Voltage and Current flowing to ground needs to be known
3. Determining AC Fault Current exposure exists and should be estimated
4. Is this site considered a Hazardous Locations Definition?
5. Mounting the DEI device for above-grade or underground connections

See Application Note 6 for a full review of this application, including mitigation considerations.

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.