PCR • Polarization Cell Replacement

• Comparison of Dairyland PCR to liquid-filled polarization cells
• Comparison of Dairyland PCR to gapped arresters
• Comparison of Dairyland PCR to zinc grounding cells
• Comparison of Dairyland PCR to other solid-state devices

Comparison of Dairyland PCR to liquid-filled polarization cells

Polarization cell:
The polarization cell is an electrochemical switch comprised of pairs of stainless steel or nickel plates immersed in a solution of potassium hydroxide. It responds to DC current by polarizing the plates and reducing the flow of DC current. This occurs up to the threshold of about 1.2V, after which it effectively becomes a dead short. AC current can pass through the device. The electrolyte usually has an oil film on the top to prevent the evaporation. This electrolyte is a hazardous material, with issues of safety, disposal, and environmental concerns. Regular maintenance of the fluid level is needed, especially under conditions where high AC steady-state induced current is present, which can cause the fluid level to be reduced. Fluid level must be maintained to provide the nameplated AC fault current rating, otherwise the surface area for conduction is reduced. On some interval, the fluid is replaced with new electrolyte.

Dairyland PCR:
The PCR is a solid-state device which provides DC isolation and AC grounding, up to a threshold of about 3V. It has low AC impedance, allowing induced AC current to be shunted to ground, while providing a high impedance to DC current flow. The solid-state design results in very consistent device characteristics. The device was introduced in 1994 and is used on a wide-spread basis as a replacement for polarization cells. An extensive user base has applied the PCR to pipeline systems, tanks, power cable casings, and separation of grounding systems, for AC grounding and DC isolation of cathodically protected structures. Applications include AC mitigation, lightning over-voltage protection, AC fault conduction, stray current blocking, and grounding electrical equipment that are tied to CP systems. On this last application, the PCR is the only UL listed device for providing an effective grounding path for electrical grounding as required per NEC section 250. Any PCR user is reference for DEI. There have been no reported failures of a PCR for any reason.

Specific comparison of polarization cells to Dairyland PCR

DC leakage current
DC leakage current through the polarization cell is typically 1mA to about 40mA, while blocking 0.5V to 1.5V, respectively. This represents the amount of cathodic protection current that will be allowed to leave the pipeline through the device. Beyond about 1.2V, the polarization cell conducts heavily.

The PCR leakage current for the same voltage range as above is about 0.005mA to 0.5mA. It represents a reduction in leakage current to approximately 1/100th of that of a polarization cell, keeping the cathodic protection system current requirements minimized.

Lightning Surge Current Rating
PCR: 100,000A crest (8x20 microsecond waveform)
Polarization cell: no published value

AC Fault Current Ratings
PCR: Various, stated at 1, 3, 10, 30 cycles for each rating
Polarization cell: Various, 30 cycle rating

Blocking voltage:
PCR: 3V
Polarization cell: 1.2V typ.
The polarization cell is sometimes arranged with two or more in series to achieve a higher blocking level. Our solid state approach never requires this.

Failure mode:
PCR: Fail-safe; the device will always fail as a dead short
Polarization cell: fails open

Neither device is a one-shot device. The PCR can see an unlimited number of operations within its rating. Any failure is non-eventful, and the unit can be easily factory repaired and returned. There have been no failures in the history of the product. The "fail-safe" approach is a significant safety issue, protecting workers and equipment.

Hazardous electrolytes:
PCR: none
Polarization cell: Potassium hydroxide

Maintenance requirements:
PCR: none
Polarization cell: regular fluid checks. Occasional fluid replacement. Eventual disposal of electrolyte.

The cost of maintenance over the life of the polarization cell, coupled with the cost of safety matters, liability, environmental damage, and decommissioning, has driven a significant number of past users of polarization cells to replace them with PCRs. This is evident at most major power utilities, and a vast number of gas and liquid pipeline systems. Many major consulting engineers now specify our solid-state devices for DC isolation and AC grounding.

Third party listing:
PCR: UL and C-UL listed as an "effective grounding path," and can be installed in grounding conductors of electrical equipment. Also listed for hazardous locations: Class I, Division 2
Polarization cell: none
There are actually no other known devices that are third party listed in this area.

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Comparison of Dairyland PCR to gapped arresters

Arrester:
Typical terms: gas tube arrester, spark-gap arrester, surge diverter

Reaction:
If the voltage across the gap gets high enough, an arc bridges the two electrodes, establishing a current path. From typical data sheets, it takes an AC or DC voltage of many hundreds to about one thousand volts to flash over, and during lightning conditions, a voltage of about 800V to several thousand volts. The arrester can usually carry 30,000 to 100,000A of lightning surge current (this is the peak value of an 8 x 20 microsecond waveform, which is the industry standard waveform for testing for lightning conditions). Manufacturers do not usually publish AC fault current data, as a gapped arrester cannot carry AC fault current for any significant length of time before failure would occur. The device cannot be used to provide induced AC voltage mitigation, as the device is a permanent open circuit. Failure mode is that the device remains a permanent open, with the electrodes burned back. The device then provides limited or no further over-voltage protection for the joint or structure. The voltage allowed on the structure can then rise to high levels.

Dairyland PCR:
The PCR will handle 100,000A of lightning surge current (8 x 20 microsecond waveform.) The device goes into conduction at a much lower voltage than a gapped arrester, keeping the voltage across the insulated flange to a low value. For steady-state conditions, the PCR goes into conduction at about 3V. For lightning surge conditions, the voltage across the PCR would be about 100V. The voltage allowed across a gapped arrester, in comparison, will reach unsafe levels.

No arcing occurs within the PCR - it uses large solid-state components for conduction. Notably, the PCR has the ability and ratings to handle AC fault current, with published rating data. No other product has any significant AC fault rating. During any AC fault condition, the PCR would have less than 10V across it. Devices such as gapped arresters do not have published AC fault ratings as they are not intended for such service, yet AC conditions on pipelines are common.

AC mitigation is performed by the PCR, since it is a low impedance to AC current, but a high impedance to DC current from the CP system. It collapses induced AC voltage down to a low level. This function is not available on a gapped arrester.

When protecting insulated joints, many manufacturers of insulated joint kits do not publish satisfactory voltage withstand data such that customers can consider if their protective device can adequately protect the joint. With our known, low threshold voltage, the PCR will provide the best protection for insulated joints or any other connection points where it is applied.

Another very important issue is regarding the design of the PCR, which is considered "fail-safe." The PCR, if exposed to values beyond our already high ratings, will always fail in the shorted mode (fail as a dead-short). This assures that over-voltages will be clamped to the lowest levels, and if current beyond our ratings is present, the device will safely and non-eventfully fail, bonding the two points together for safety. A gapped arrester has an open gap, which will always remain an open gap. If the arrester were to fail, it would be as an open circuit.

Regarding independent product approvals, DEI has the most extensive approvals of any company. The PCR is UL listed as an “effective grounding path,” so that it can be used for motor operated valve isolation in grounding leads, or other grounding conductor applications. It also is UL listed for Class I, Division 2 hazardous locations.

In summary, our benefits over arresters are:
1) The PCR has a much lower threshold voltage.
2) The PCR can handle AC fault current of significant values.
3) The PCR clamps over-voltages to much lower levels than any MOV or gapped arrester.
4) The PCR is a fail-safe device.
5) The PCR has known, fixed parameters.
6) The PCR is third-party listed to applicable US standards.

Please contact us if we can answer any questions about these issues.

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Comparison of Dairyland PCR to zinc grounding cells

Zinc grounding cells
These cells consist of two zinc rods, with insulation material to separate them, in a conductive backfill material, packaged in a bag. The insulation or separator leaves the zinc rods reasonably close together without touching. The backfill material, such as bentonite, promotes conduction and contact with the surrounding earth, as the package is buried near the points it is connected to. A wire lead attaches each rod to a point of interest, such as across an insulated flange, or between a pipeline and ground. The concept for protection is that the zinc rods will allow separation of the two structures, and minimize the DC cathodic protection current flow under normal conditions. Under over-voltage conditions, the small separation between the rods, and the conductive backfill will allow current to flow between them, draining the unwanted current.

There are several ways in which the zinc grounding cells will not perform very well for this application. First, the package must be buried. If the application is protection of insulated joints from over-voltage (lightning, for example), and the joint is above ground, the leads that connect to the joint will be at least several feet each. Under lightning surge conditions, this affords little or no over-voltage protection to the insulated joint, as leads have inductance, which lightning sees as a high impedance, and therefore creates a large voltage drop in the leads. Lightning protection for insulated joints is best done by locating a device directly across the joint, which cannot be done with zinc grounding cells, as compared to the Dairyland PCR.

Another electrical problem comes from AC power system fault current. Zinc grounding cells are generally not sized for handling any significant fault current.

Power laboratory testing to assure the performance of this product would be challenging, as this product is placed in service by burying in the earth, which would be difficult or impossible in a lab setting that could provide the needed values of lightning surge current or AC fault current. We have seen no test data for these products.

By separating the zinc rods in the package to achieve isolation, the question arises as to what voltage is allowed by the cell. Separation of two metallic surfaces by a medium (air, gases, solid insulation, semi-conductive backfill) has been common for over-voltage protection, but consistency of performance is an issue. Having the separation be consistent, producing a known, fixed clamping voltage is difficult if the product can vary in dimensions or construction. Consistency in over-voltage protection using a zinc grounding cell is in question, just as a spark gap arrester in air can vary during atmospheric changes due to moisture.

In addition, where the desire is to provide the lowest possible over-voltage on the structure during an event, a device should have a low impedance for conduction of current. This is not the case with a zinc grounding cell.

Dairyland PCR
As a comparison to the above description, consider the characteristics of the PCR. It is an AC conducting, DC blocking device in the normal mode. For DC (cathodic protection) current, it presents a very high impedance, while appearing as a 10 milliohms path to AC 60Hz current. The PCR blocks DC voltage up to 3V, then appears as a short circuit for disturbances. When the event is over, the device automatically reverts to blocking DC current. There is no internal gap in the device, so over-voltage clamping occurs at a very low voltage - much lower than any other technology. Consistency in performance is assured through the use of tested power semiconductor components. The device has been designed so that it will even fail as a short circuit if exposed to currents beyond the rating, providing a fail-safe nature. (There have been no failures of a PCR to date, for any reason.) The PCR has been tested in a power laboratory and is rated for lightning surge current and AC fault current.

Problems such as induced AC voltage on pipelines can be handled by the PCR as well, since it appears as a low impedance AC path while blocking DC current. This collapses the induced voltage down to insignificant values. Other major applications of the PCR include protection of insulated joints, electrical isolation of equipment on pipelines from station ground, isolation of station ground from the power utility ground, on structures such as power cable casings, pipelines, tanks, grounding systems, and other cathodically protected systems.

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Comparison of Dairyland PCR to other solid-state devices

Since its introduction in 1994, the DEI model PCR has been the standard for over-voltage protection products in the corrosion prevention industry. The PCR compares favorably to other solid-state products that have appeared in recent years. As these other products attempt to mimic the PCR design, they may appear similar, but significant differences exist. These include:
• Unique, patented construction that assures maintenance-free operation
• Assured fail-safe design, independently tested
• UL, C-UL, and CE marked and tested
• Proprietary manufacturing methods that assure corrosion resistance, high AC fault capability, and low leakage current

The PCR has extensive certification. In fact, no other products of this type on the market have any known certification to any standards whatsoever. Consider the following PCR certifications:
• UL listed as an "effective grounding path" per the US National Electrical Code (NEC)
• UL listed for blocking cathodic protection current per the US NEC
• UL listed as meeting the lightning surge current standards
• UL listed for Class I, Division 2 hazardous locations
• UL listed NEMA 4X and 6P environmental ratings
• C-UL listed as an "effective grounding path" per the Canadian Electrical Code
• C-UL listed for Class I, Division 2 hazardous locations
• CE marked per a Type Examination by DEMKO as meeting the ATEX directive for Zone 2 hazardous location use in
Europe

For more information on PCR certification, see the full data sheet.

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