Technical Article

How Lightning and AC Faults Impact Your Choice of Conductor

Your Dairyland decoupler will come rated for both lightning and AC fault current. Both currents have the potential to damage your equipment without the protection from a Dairyland device. However, the significant difference between the behavior of AC fault current and lightning current creates specific needs in your choice of conductor length and diameter.

Lightning Current & Conductor Length

Lightning current is measured by peak magnitude and the rate of rise and decay in its waveform. The typical standard waveforms defined by the industry are 4µs x 10µs and 8µs x 20µs. With a peak magnitude of 100kA as an example, this means a lightning strike will only take 4 millionths of a second to reach that magnitude, and then decay to one half that peak value, or 50kA, within 10µs. This high rate of rise in the waveform can lead to a dangerously high voltage drop across even short conductors.

This voltage drop is due to the inductance of the conductor which increases with conductor length. Some applications are limited in how devices can be installed, but it is highly recommended to keep conductor lengths below 12 inches total. You can read more about the issues surrounding conductor length here.


AC Fault Current & Conductor  Diameter/Size 

Though the magnitude of AC fault current is typically much lower than that for lightning, AC faults can have dramatically larger amounts of energy than lightning events because of their duration which can be hundredths or even tenths of a second, instead of millionths of a second. The high levels of energy released in these events create a large risk to personnel and equipment.

AC fault current has much less bearing than lightning current on conductor length. What matters in ensuring your Dairyland device properly protects against high levels of AC fault current is conductor diameter or gauge. If the device is sized correctly for the anticipated AC fault, but is installed without the correct conductors, the large amounts of energy during a fault may cause the conductor to fail. Using the correct conductor gauge is important in ensuring the overall system performs to the required AC fault rating.

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  • In order to provide appropriate lightning over-voltage protection, use conductors that are less than 12 inches long total.
  • Sizing a conductor gauge that aligns with your device’s AC fault rating will protect both your personnel and equipment from high levels of fault current.

Both lightning and AC faults will affect your choice of conductor. The length and the diameter of your conductors  will change how the device is able to respond safely to fault events, so it is important that the conductors are sized and installed correctly. We have several mounting kits available to help with ease of installation and reduce over-voltage due to lightning and AC faults.

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