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Farwest places a high value on getting our customers the information they need. If you have questions of a technical nature regarding our products or applications and cannot find the information on the website, please phone us or email us at sales@farwestcorrosion.com. The frequently asked questions below apply to Dairyland Electrical Industries (DEI).
How can the 3V threshold of a PCR or 10V threshold of the ISP be adequate when I have 50V AC induced onto my pipeline - won't the device instantly switch to the shorted mode?
In a word: "No." - The reason that the Dairyland device does not switch to the shorted mode is that the device is a low impedance to the flow of alternating current. The PCR AC impedance ranges from 0.005 to 0.010 ohm, while the ISP varies between 0.088 ohm and 0.265 ohm, depending on the device ratings. When the device is connected between a grounding system and a pipeline with induced AC voltage, this low AC impedance path instantly allows alternating current to flow and collapses the induced AC voltage to below the device threshold. See the application example regarding induced AC voltage.
What is the device threshold for DC voltage versus AC voltage or other signals?
Dairyland devices have a threshold that is an absolute voltage, which is the sum of DC plus peak AC voltage, if AC is present. There are not separate thresholds for DC versus AC. Normally, it is mainly DC voltage that contributes toward the threshold of the product, as a device such as the models PCR, PCRH, or ISP collapse any induced AC voltage to a small value, which is the below the threshold (see question above). If the normal DC voltage, measured across the two points to which the Dairyland product will connect, is below the product threshold, the user will normally not need to consider the small AC voltage contribution. The only exception is the Dairyland model OVP, which blocks both AC and DC voltage/current, and can be put into conduction if AC voltage is present. See both the application information in the above question, and data on the maximum DC voltage allowed in the application notes.
Do Dairyland devices need to be put in series to achieve a higher threshold?
No. Unlike liquid-filled polarization cells, which have a low threshold of around 1.3V, the Dairyland model PCR has a 3V threshold, eliminating the need to place devices in series. If a higher threshold is needed, consider either the model ISP, which blocks to 10V or 17V, or contact Farwest for other options. Most applications can be addressed with the standard threshold values.
Are lightning and AC fault current ratings equal?
No. Lightning has a unique waveform that features a certain rise time to the peak value, and then decays down over time. This waveform is defined for various voltage and current scenarios, in documents such as ANSI C62.11. The most commonly stated lightning current waveform is an 8x20 microsecond characteristic. Dairyland devices have a stated peak value of 8x20 microsecond current capability. AC fault current has entirely different, and consistent, properties compared to lightning. The repetitive, sinusoidal waveform of an AC fault has predictable energy values that can be used to calculate fault current ratings of products. Most AC faults last approximately 3 to 5 cycles, until a power breaker interrupts the fault current. The energy involved in an AC fault is usually much higher than a typical lightning event, in terms of how protective products should be rated. For these reasons, Dairyland products have separate lightning and AC fault ratings. For information on AC fault ratings needed for various applications, see the AC fault application data.
