What everyone knows. To protect low-voltage installations against power surges caused by lightning surge protection devices (SPDs) should be installed as far upstream as possible. Why? To drive the awesome power from lightning strikes to the ground and stop it from spreading down through the installation.
What everybody doesn’t know is that such protection may not be enough to protect sensitive electrical and electronic equipment – especially if it’s some distance away from the primary upstream surge protector.
How do long cables affect voltage in equipment?
If the length of the cable between the primary surge protection (in the incoming switchboard) and the electrical equipment you want to protect is too long, oscillations and wave reflections may lead to sharp voltage rises in the equipment. In fact, such rises may exceed the SPD’s voltage protection level (Up) and rise to levels that are twice as high as Up.
The figure below shows how the maximum voltage curve at the end of a cable is determined by cable length for an incident wave-front voltage of 4kV/µs.
Why does voltage double?
Lightning emits electromagnetic waves in frequencies measured in hundreds of kHz and MHz. The voltage variations are so fast there is no way you can assume voltage is the same at any point in the cable. And the incident surge front may further excite oscillations caused by the cable’s own resonant frequencies (e.g., parasitic capacitances or filtering).
What is taking place is known as total internal reflection. This is when a wave traveling through a medium hits the medium’s boundary at an angle that is greater than its particular critical angle.
A useful way to picture total internal reflection is a wave that crashes against a wall. The wave represents the voltage wave and its height the wave’s magnitude. When it crashes against the wall, it doubles in height and flows back whence it came.
What is the best solution for failsafe lightning protection?
When reflection occurs over cable lengths shorter than 10 meters, voltage surge fronts in buildings may be disregarded.
But as recommended by the standards (IEC 61643-12 and IEC 60364-5-53), when the loads are located more than 10 meters away from the incoming-end protection device, it is necessary to add a second SPD for additional fine protection as close as possible to sensitive loads.
The second device doesn’t need to have as high a discharge capacity as the primary SPD. Imax <8kA (8 / 20) is enough. That’s because the primary surge protection device has already diverted the lightning current to the ground.
Visit our SPD site to learn more about solutions to better protect your power.