A standard 3-prong surge protector is plugged into a US 15 amp 3-prong wall outlet. I want to protect a Christmas light string with a "computer" controller box. The light string only has a 2-prong plug to plug to the surge protector. Will the light string controller be protected? Bottom line - is it useless to expect surge protection for a device with only a 2-prong plug?
The surge protector will dump high voltage spikes from hot to the ground and many will drop from hot to neutral, so it would depend on the model, if internally the mov’s or metal oxide varistors are wired hot to neutral you would still have spike protection. If only wired got to ground it will do no good as their is no connection.