A lightning suppressor is similar to a mains-connected anti-surge device. However, the magnitude of the pulses it needs to quench are significantly greater.
These suppressors are rather like an insurance policy. It isn’t essential to have them, and each person will have differing ideas about the level of risk they are prepared to take. As I live on high ground with a clear view of the prevailing SW wind from the Atlantic, it’s a subject in which I have a keen interest!
A short tutorial:
Lightning is caused by electrostatic charge building up within clouds. The higher the cloud, the greater is the potential difference (voltage) between its base and its crest. The ground is positively charged in comparison with the clouds above us. For this reason, lightning tracks are normally measured as volts-per-metre (V/m). The higher the cloud, the greater is the voltage to earth.
Globally there are about 100 million lightning discharges each year, of which 25 million (25%) are ground-strikes. They are not evenly spread. High ground in Rwanda and the Black Forest will typically have several strikes each day. For this reason I like suppressors made by the German firm, Phoenix Contact. They are not only well engineered, but get lots of testing!
If you put metal objects on your roof, such as solar panels and wind turbines, they will attract lightning discharges because there’s a route to ground with lower resistance through metal cables.
A pair of lightning suppressors are connected to the +ve and -ve wires from the solar panel before they enter any other electronic equipment.
The other side of each suppressor connects to a ground stake via a 10mm² earth wire.
Do not attach this earth wire back to the earth in your consumer unit. To do so will re-route some of the lightning surge back into your house and to all your neighbours on the same phase!
What lightning “looks like”:
A cloud to ground strike is not a single event, but usually a series of 3 - 5 discharges with a few millisecond intervals between them. This is why we perceive lightning as flickering.
The following diagram is derived from work by V.A. Rakov ‡ and zooms in on just one of those discharges in the sequence:
The main Leader is preceded by a number of initial pulses, caused by the high voltage breaking down the air into ionised particles.
This plasma pathway channels through the Leader of the main strike which is rapidly followed by the Return Stroke as the ground re-establishes the equilibrium. Over the next 1-3mS there are a series of Pulse Peaks before the next Leader and Return Stroke pair in the sequence.
A lightning suppressor needs to act very fast.
Electromagnetic radiation in copper wire travels at two-thirds the speed of light. In the 5μS between the Leader and the Return Stroke, the electron wave-front will have traveled a kilometre along a cable!
The suppressor must also self-heal rapidly. There’s no point in it quenching the initial Leader (A) if it can’t be ready to fire again for the succession of Pulse Peaks (B) which follow.
It is this which underlies the significant difference in price between Surge Protection Devices. It is almost impossible for a consumer to know the resilience of an SPD. It’s important to check the standards it has been tested against rather than the headline specification.
Although a lightning discharge is of short duration, the current from a direct strike flows at up to 40,000 Amps. With a typical Potential Difference in the range of tens of megavolts, no amount of insulation around a wire is going to put up much resistance (pun intended!).
Wires connecting to a Suppressor need to be thick and kept short, especially the earth-wire to the ground-stake. Every screw terminal must be extremely tight to ensure that it really is the route through the Suppressor which offers the lowest resistance path to earth.
A lightning Suppressor will be marked with its maximum current and the breakdown voltage at which it “breaks down” to carry the current to ground. Most protection devices intended for solar panels are rated at 40kA and 1000v DC.
Sometimes, the Suppressors are built into the input of the PV Inverter. But, if not, then this should be preceded by a DC Connection/Combiner Box where the Suppressors can be fitted very close to the fuses and circuit-breakers for each PV string.
Make sure that Suppressors are connected to the solar-cable before the circuit breakers (shown here with coloured operating levers).
A DC circuit-breaker contains an arc-quenching “ladder”, which is needed to prevent the contacts arcing and welding themselves together when the circuit is switched off.
If the lightning discharge hits the circuit-breaker first, then the internal magnet will pull the pulse into the arc-quenching ladder and destroy the breaker.
Should that occur, there will be no route through to the lightning suppressor for the following Leader and Pulse-peaks. Consequently they will arc across wherever they can within the enclosure, probably resulting in a fire.
‡ V.A. Rakov and M.A. Uman, 2003: Lightning: Physics and Effects, Cambridge University Press