ISP FAQs

Frequently Asked Questions about Dairyland’s ISP

Dairyland’s ISP is the original solid-state AC conducting/DC blocking device designed for the corrosion prevention industry and features the most flexible and wide range of product ratings and applications available to the market. The ISP provides the widest range of features and ratings, including the highest levels of AC fault-handling capability – up to the highest typical transmission system faults making it the preferred product for power utility underground cable decoupling/bonding requirements.  The following are some of the most frequently asked questions about ratings, logic board functionality, and installation.

To learn about the ISP’s different operating modes and how to test them, please see this article.

What is the leakage current of an ISP?

A relatively small DC leakage current is present during DC blocking, AC continuous operation. Figure 5 below provides an estimate of expected DC leakage current, which is dependent upon the DC voltage available at the terminals. We advise users to expect around 25mA per Volt.

Figure 1: ISP DC Leakage Current

 

What is the importance of the logic circuit?

The most important component of the ISP is the logic-controlled circuit. This circuit determines exactly when the high current capacity thyristors should be turned “ON”. After a transient event, the circuit ensures the ISP returns to its normal operating mode, blocking DC while conducting AC. This prevents the thyristors from remaining in the ON state due to influence from a cathodic protection system or an external DC voltage source. The logic-controlled circuit, as with the entire ISP, is autonomous and does not require an auxiliary source of electrical power. It captures and stores the necessary power from any event that causes the voltage across, or the current through, the ISP terminals to reach the threshold level. Power is stored for a sufficient amount of time while the logic-controlled circuit deals with any foreseeable event.

The logic-controlled circuit includes a sub-circuit which provides a means for dissipating the stored inductive energy to prevent retriggering of the ISP. After this inductive energy is dissipated, the DC voltage will return to its normal steady-state value. The logic-controlled circuit then automatically goes dormant until the next triggering event occurs. The logic-controlled circuit differentiates between voltage rise caused by stored inductive energy, which should be safely dissipated, and external disturbances such as lightning surges or AC faults, where the ISP must trigger ON to protect itself and to limit voltage between its connected terminals.

Note on Revision 1 logic control circuit

The original logic-controlled circuit, designated as Revision 1, was introduced in the early 1990’s. This circuit revision does not protect an ISP from failure if the steady-state AC current is continually above rating. The Rev. 1 logic circuit does not include the following safety features:

• Prevention of capacitor overheating due to repeated retriggering if the ISP is operating above its steady-state ratings.
• Protection of the capacitor from excessive AC current that can occur below the absolute voltage threshold level of the ISP (applies when the 20V threshold level is selected).
• Provision for activating a flashing red warning indicator whenever the ISP is being repeatedly retriggered (due to application above steady-state rating).

ISPs with the original Rev. 1 logic control circuit were designed over 30 years ago, utilizing the most cutting-edge technology available at the time. Since that time, many technological advancements have been made. Dairyland now recommends that users of the older units consider upgrading to the newest Rev. 5 units.

Note on Revision 5 logic control circuit

Dairyland’s current ISP models utilize the latest Rev. 5 logic control circuit, which feature the following benefits:

• Inductive energy dissipation circuit that assures ISP remains OFF after fault event upgraded by 50% capacity for all models.
• Improved backup triggering of 26Vpeak if primary triggering fails.
• Prevention of capacitor overheating due to repeated retriggering if the ISP is operated above its steady-state ratings.
• Protection of the capacitor from excessive AC current that can occur below the absolute voltage threshold level of the ISP.
• 20V blocking voltage threshold standard.
• 90Arms steady-state AC current rating standard for models up to 68kA.
• 120Arms steady-state AC current rating for 118kA model.

 

What is the difference between 40 and 100 rating of logic control circuit?

These ratings indicate the maximum allowable DC current that can flow through the ISP during its conducting mode while still enabling the device to be automatically reset back to its normal mode after a fault condition. 40 is the standard rating, and it is capable of 40Adc. The 100 rating is optional, with capability of 100Adc.

Note that ISPs with Rev. 5 logic control circuit have 50% more capacity due to circuit improvements.

 

What is the impedance of an ISP?

For ISP units manufactured prior to 2020, the impedance value will vary depending on the steady-state current rating, as there were four options available. Impedance values are shown below:

Steady-State ACrms Rating	mW (60Hz)	mW (50Hz)
30A	265	318
60A	133	160
90A	90	110
120A	70	80

Table 1: Impedance values based on steady state current rating of an ISP

For ISP units manufactured in 2020 or later, only the 90A and 120A steady-state current ratings are available. The 90A and 120A impedance values shown above apply.

 

What is the difference in model numbers prior to 2020 and post-2020?

The ISP model number was streamlined in early 2020 for easier specification. Prior to 2020, there were numerous options that led to many variations of ISPs. The changes affected the steady-state current ratings, tamper-proof screws, and blocking voltage threshold.

The steady-state current ratings went from four options (30-120Arms) to only two options (90Arms or 120Arms), the blocking voltage threshold changed from two options (12.5V or 20V) to just the 20V option and finally, tamper-proof screws on the enclosure cover were made standard on all ISPs.

 

What is the purpose of the LED light?

The red LED indicator will flash approximately once per second if the DC voltage is above the voltage blocking level (12.5V or 20V), or if the steady state AC current is above the steady state rating of the ISP model (30A, 60A, 90A or 120A). Under normal operating conditions this LED should always be off.

If either of the above conditions occur, there will be an audible “clicking” sound as the thyristors are being switched.

 

Can ISPs be connected in parallel?

Single ISP models 35kA and 68kA should not be connected in parallel to double the AC fault current rating capability. The primary concern is that the logic control circuits are not synchronized to trigger at the same time during fault events. One of the units will trigger while the other one will not, thus the unit that triggers will see the full fault event and likely causing the ISP to fail.

For applications that require higher AC fault current capabilities, Dairyland recommends our highest rated ISP, the 118kA. This unit essentially has two single 68kA ISPs connected in parallel at the terminals and the logic controls circuits are synchronized together eliminating the issue mentioned above.

  

 Is an ISP listed for hazardous location use?

The ISP is a sealed, non-spark-producing device. This device meets the intent of ANSI/NFPA 70 (the U.S. National Electric Code) and CSA (Canadian Standards Association) requirements for Class I, Division 2 hazardous locations. The ISP has not been submitted for listing for hazardous locations because it is seldomly used in hazardous locations. If a listed device is required for hazardous locations, consider the PCR or PCRH, described in other literature sections.

Read more here about ISP’s different operating modes and how to test them.

If you have any questions regarding the information in this article, please feel free to contact us at techsupport@dairyland.com.