A good surge protection system works by totally protecting the system from small to medium lightning storm effects, and by minimizing the effect of large lightning storms.
The minimum recommended level of protection is to have one decoder grounded at the end of each wire path and one decoder grounded every 1,000 ft/300 m or 12th decoder. For higher levels of protection, ground the decoders more frequently. There is no limit on the number of ground connections in a decoder system.
It is important that both the controller and the decoders are grounded to ground rods or plates with less than 10 Ohms resistance. The ground should always be measured with a ground resistance meter. A "clamp on meter" cannot be used for ground measurement, since this is an isolated system. Ground resistance measurements should be performed with a "fall of potential" type meter in decoder systems. The ground should be tested regularly for resistance.
Surge protection inside the decoder can wear out, and a decoder should be replaced when it might have been damaged by a lightning strike in the immediate vicinity. The decoder is a complex electronic part and it is not possible to fully test whether it is working. Replace the decoder if there is any visible damage to the device, or if nearby decoders or controllers have been damaged.