Model ISP Isolator/Surge Protector by Dairyland Electrical

Solid-state AC Conducting, DC Blocking Device for Cathodic Protection Applications

ISP Overview

The Dairyland ISP is the original solid-state AC conducting / DC blocking device designed for the corrosion prevention industry. The ISP product line features the most flexible and wide range of product ratings and applications available to the market. Additionally, the ISP provides the highest levels of AC fault-handling capability – up to the highest typical transmission system faults. With its design features and ratings, the ISP is the preferred device for power utility underground cable decoupling/bonding requirements.

Features & Characteristics

  • Solid-state design replaces polarization cell technology.
  • No maintenance needed.
  • Fail-safe design.
  • Self-protection features built-in.
  • Simple field testing procedures with standard equipment.
  • Comprehensive testing available with In-Situ Tester (IST)
  • Flashing LED indicates device operation.
  • No energy/power requirements.
  • Unlimited number of operations.
  • Submersible enclosure.




Threshold Voltage
10 or 17V
Other voltages optional

AC Steady-State Current (amperes – rms) 60 Hz
30, 60, or 90A

AC Steady-State Current (amperes – rms) 50 Hz
25, 50, or 75A

Lightning Surge Current
50 or 75kA crest (8 x 20 µs waveform)
100kA (optional)

AC Fault Current (Amperes - rms) 60Hz

 Catalog Code

1 cycle 3 cycles 10 cycles 30 cycles


AC Fault Current (Amperes - rms) 50Hz

Catalog Code

1 cycle 3 cycles 10 cycles 30 cycles



Environmental rating:

NEMA 4X or 6P

Hazardous (classified) areas:
Not listed, but uses same general construction as our listed Division 2 (or Zone 2) products

Dimensions & installation
See ISP drawings in pdf format

Technical Comparisons

Comparison of Dairyland ISP to Polarization Cells

  • The ISP is a solid-state device, flexible in design and ratings, which requires no maintenance over the life of the product.
  • With an ability to block voltage up to 12.5V (optionally 20V), a single ISP can always handle the voltage blocking level needed, as compared to a polarization cell, which blocks up to about 1.2V. This can result in the need to place several polarization cells in series to achieve the required blocking level.
  • Leakage current through an ISP is very low - it presents a high impedance to the flow of DC current, limiting this current to values much lower than polarization cells.
  • While blocking DC current, the ISP appears as a low impedance AC path. Any induced AC current can be safely shunted to ground, resulting in a low induced voltage on the structure.
  • The ISP has various AC fault current ratings, appropriate for the available AC fault current from the electrical system. Device switching is instantaneous, presenting a virtual dead-short between the connected points.
  • The ISP is generously rated for lightning surge current conditions. Polarization cells do not have a published rating for lightning.
  • The ISP is fail-safe, in that if currents beyond its rating cause the device to fail, it will always fail as a short circuit, safely grounding the structure. Failure is non-eventful, and the device can be easily restored to service after factory rework. The ISP is even self-protected for steady-state AC current beyond the rating. Any failure of the polarization cell would result in an open-circuit, with no further safety grounding available for the structure or personnel.
  • Worker safety, environmental concerns, and maintenance issues are removed via the solid-state design of the ISP. Polarization cells contain a hazardous electrolyte.
  • Comprehensive field testing is available when an ISP has the optional Test Point, allowing the In-Situ Tester to perform a number of status checks, measurements, and force the ISP into various operating modes. Basic tests to confirm the status of the device can always be done with simple field equipment, such as a multimeter.
  • All anticipated system conditions have been factored into the logic design of the ISP, allowing it to be self-protected for certain conditions, and correctly operate and reset for all operations. The fail-safe nature is key for safety grounding of structures.


Comparison of Dairyland ISP to other Solid State Devices

The Dairyland model ISP was the first solid-state device offered to the corrosion prevention industry in the late 1980s. It offered a solid-state, maintenance-free design, with ratings suitable for any application. The ISP design led to many other innovative Dairyland products for the energy and transportation industries. It still provides the most flexible package for customization, including such features as:

  • High AC fault ratings.
  • High AC steady-state current ratings.
  • Voltage thresholds of 10V and 17V standard, customizable to higher values.
  • Patented logic control for self protection and turn-off.
  • Triggering based on both current and voltage thresholds.
  • In-situ testing of the ISP, through the Test Point option.

The ISP is the standard product used by power utilities on pipe-type underground transmission cable, and many other pipeline, power, and transportation applications. Other designs have surfaced which mimic the ISP in general layout, but none contain the high quality manufacturing techniques developed by Dairyland, nor the patented logic control functions which are vital to correct operation of the device under all system conditions.

Unique to the ISP, the device is self protected against steady-state AC current in excess of the device rating. Should excessive current flow, the device enters a safe switching mode that will protect the unit. Other devices do not offer this type of protection. Further, the ISP logic features an energy dissipation system that assures correct turn-off of the device under all system conditions, including those that would attempt to force the unit to become stuck in the conducting mode. Only Dairyland offers such protection.

Another sophisticated function available only from Dairyland involves testing the ISP while in service. With the Test Point option, the ISP will accept the Dairyland In-Situ Tester (IST) to accomplish tests including:

  • Verification that the ISP is not in the shorted mode.
  • Measurement of the steady-state values of voltage and current.
  • Applying various waveforms of both polarities to force the unit into and out of conduction, assuring correct operation.
  • Verifying the limits and tolerance of the main components.