In my work managing power systems and lightning protection at Helios Power Solutions, a conversation that frequently comes up with facility managers and electrical engineers across New Zealand is how to properly interpret surge protection data.
We all know lightning poses a massive threat to our critical infrastructure. A discharge several hundred meters away from a power transmission line can couple enough energy onto adjacent conductors to disrupt operations and destroy sensitive equipment. But how does that energy actually reach our systems? Transients typically couple onto power and communication circuits via three primary mechanisms:
Galvanic Coupling: A direct electrical connection to the transient source, often through a shared earthing system.
Magnetic Induction: The rapidly changing magnetic field from a high-current discharge induces a destructive current onto adjacent conductors. Because of these magnetic fields, simply burying power cables does not provide adequate protection on its own.
Capacitive Coupling: Transient voltage is coupled through the inherent capacitance between two circuits. Nearby power circuits sharing cable trays with communication lines often become sources of capacitively coupled transients.
Direct vs. Induced Surges
Understanding these coupling methods allows us to categorize surges into two main types, each requiring a different approach to protection:
Direct Surges: These occur when lightning makes a direct strike on a power line, a building's lightning protection system (LPS), or the grounding system.
Characteristics: These strikes involve massive amounts of energy and extremely high current amplitudes. The transient itself lasts for a significantly longer duration. This is essentially the Galvanic Coupling shown in the illustrations.
Simulation Waveform: To properly test Surge Protective Devices (SPDs) installed at a service entrance (Type 1 primary protection) to withstand these direct strikes, the 10/350µs current impulse waveform is used. This longer, higher-energy waveform accurately simulates the destructive potential of a direct lightning discharge.
Induced Surges: Far more common than a direct strike, this happens when lightning strikes nearby—perhaps a nearby tree, ground, or a separate power/communication line.
Characteristics: Rapidly changing electromagnetic fields induce currents and voltages in adjacent conductors via Magnetic Induction and Capacitive Coupling. While still dangerous, these surges generally have much lower energy and peak current compared to a direct strike.
Simulation Waveform: For testing secondary SPDs (Type 2, branch panel or equipment protection), the 8/20µs current impulse waveform is employed, representing the faster rise and much shorter duration typical of these coupled transients.
Standard Waveforms and the 8/20µs Defined
Because transient disturbances are unpredictable, the industry relies on standardized testing to evaluate SPDs. International standard bodies, such as ANSI/IEEE C62.41, define typical location Categories (A, B, and C) to help classify these environments.
When evaluating secondary and downstream SPDs, you will almost always see the 8/20µs waveform. But what do those numbers actually mean?
Put simply, it represents the short-circuit current from a combination wave generator. Here exactly what those parameters specify.
The "8" specifies the current rise time: The current climbs from 10% to 90% of its peak value in exactly 8 microseconds.
The "20" specifies the current decay time: The current then decays to 50% of its peak value at the 20-microsecond mark. Crucially, this duration is measured from the initial 10% rise point, not from the peak current
Interactive 8/20µs Waveform
Drag the slider to adjust the peak current and observe the constant 8µs rise and 20µs decay timing.
The Role of AS/NZS 1768 and Compliance
Locally, the AS/NZS 1768 standard provides the essential framework for lightning protection and risk management in New Zealand.
AS/NZS 1768 integrates these different waveforms and protection strategies to define appropriate protection levels for various scenarios:
It guides how to assess the specific risk of both direct and induced surges based on building structure, location, and power system configuration.
It helps classify locations within a facility into specific Lightning Protection Zones (LPZ), ensuring appropriate SPDs are specified for the exposure risk of that zone.
Whether we are aligning with global IEEE tests or ensuring strict compliance with the latest AS/NZS 1768 updates locally, understanding the specific type of transient your facility is exposed to is step one in specifying the correct protection.
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