Technical Article

The Importance of Verified AC Fault Current Ratings When Choosing a Decoupler

Wherever pipelines or other metal structures are in close proximity with high voltage AC transmission lines, there exists the potential for the structure to experience AC faults. An AC fault occurs when there is a short between one of the phase wires and the tower or ground. The ground fault current flows through the earth surrounding the tower and possibly onto nearby buried pipelines, storage tanks or other structures. Any decoupler or over-voltage protector connected to these structures must be able to reliably carry the fault current that can be experienced at that location without failure.

To address AC fault concerns, Dairyland decouplers and over-voltage protectors are designed, tested and third-party certified to ensure reliable, repeatable operation during AC faults. Figure 1 shows the results of an AC fault test performed on an SSD-2/2-5.0-100.

Fault Current Ratings Graph

Third-party verified tests ensure product performance meets the stated manufacturer claims, and are valuable confirmation to users. Products that have not been third-party certified frequently do not perform to their stated values and often experience failures leading to high replacement costs and safety issues.

In addition to verified fault current ratings, Dairyland products are certified “fail-safe” devices, providing an important safety redundancy. Our verified fault current ratings lead to rugged product performance and very few failures, but in the unlikely event a product does fail, it always fails closed. This failure mode is an important safety feature when considering personnel protection. Decoupler users throughout the industry should look for both certified fault current ratings and fail-safe operation in order to ensure product performance, long life and safety. Existing products on the market with inadequate characteristics and uncertified designs have been observed to arc and fail open during AC fault exposure – a totally unacceptable condition with severe risks to industry personnel and equipment.

Sizing for Fault Current

Dairyland products are available in a range of AC fault current ratings, as seen, for example, in Figure 2 for the SSD product family.

AC Fault Current Ratings for an SSD

When selecting the model for an application, users should take into account the available fault current exposure their structure might see during a fault event. Note that the fault current capability for each device is conservatively rated at 30 cycles (i.e., 0.5 seconds for 60 Hz and 0.6 seconds for 50 Hz).


Learn more about sizing for fault current: Determining AC Fault Current

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