Improving the earthing connection
In theory, once exposed conductive parts have a correct equipotential bonding with each other, a SPD can be installed in spite of a poor earting connection. However, our experience has shown us that it is easier and more affordable to improve this earthing connection to obtain a reliable LPS (not to mention the positive impact on people's safety in case of defective ground fault interrupters).
A reliable earthing connection, when the SPD is installed as closely as possible, is the best solution to evacuate overvoltage surges. but sometimes the soil does not favour such configuration or the SPD is too far ...
What is a good earth resistivity?
With a good earth resistivity (10 Ω or less), an SPD can evacuate overvoltage surges with no risk of generating secondary surges. Electronic devices remain protected up to 30 Ω but isolating materials remain exposed to breakdowns.