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 spreading down through the installation.
What everybody doesn’t know. 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 arrester (in the incoming switchboard) and the 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 arrester’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 travelling 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 metres, voltage surge fronts in buildings may be disregarded. But at anything over 30 metres there is a serious risk that the voltage wave magnitude will double.
The solution is to add a second surge protector (or arrester) as close as possible to the equipment you want to protect.
The second device doesn’t need to have as high a discharge capacity as the primary arrester. Imax <8kA (8 / 20) is enough. That’s because the primary surge protection device has already diverted the lightning current to ground.