Measuring Induced AC Voltage and 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:
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:
Airports utilize underground piping to transport large amounts of jet fuel, and this critical infrastructure requires cathodic protection to prevent corrosion. Since cathodic protection systems utilize insulated joints to sectionalize the piping, arcing at insulated joints presents a hazard to system operation and personnel, whether due to AC fault current, lightning, or static buildup.

A separate problem exists where the grounding conductor on motors or other electrical equipment on a cathodically protected pipeline shorts the CP system to the copper grounding system, resulting in inadequate levels of cathodic protection. While the grounding conductor is required by code, a means of addressing the CP current flow on this conductor is needed.

Solution:
Over-voltage protection of the insulated joints can be accomplished using Dairyland devices that are UL listed for hazardous locations. Most common has been the model OVP, which was developed in conjunction with the US Army Corp of Engineers to meet their need for a listed Class I, Division 2 product. The OVP, which is also certified for Division 1 hazardous locations, is specified by the Corp of Engineers for worldwide use for US military airport applications (see USACE Specifications). The newer model OVP2 now addresses Division 2 locations, while the OVP is used for Division 1 sites requiring explosion proof construction.

Where grounding conductors affect the CP system, the Dairyland model PCR has been most commonly used, although the SSD, with its compact design is also an option. The device is placed in series in the grounding conductor (and conduit if needed) to block the flow of DC current, while acting as a low impedance path to AC current. The devices are UL listed for meeting the electrical codes in the US and Canada for placement in a grounding conductor. See the schematic of a sample installation.

Implementation:

A. Insulated joint protection using an OVP
Select the OVP model with the FMFB mounting option, or the OVP2 with the MTF mounting option, if using the flange bolts for mounting the brackets. For other mounting methods, see the product literature.
Specify the hole size needed for the FMFB or MTF mounting kit. This hole diameter should allow for the flange bolt and insulating sleeve, plus clearance. Usually this value is 1/8" larger than the bolt diameter.

Applicable Products: OVP,OVP2

B. Decoupling electrical equipment from ground
The AC fault current in the circuit should be compared to the product ratings.
The device is connected in series in the safety grounding conductor, with the negative connection to the cathodically protected structure, and the positive connection to the panel ground.
Additional comments are provided in the application article regarding decoupling electrical equipment.

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.