I am not at all sure this thing is going to work. Once you turn it on and push SW1, the relay will pull in and connect output to input. In theory, the darling pair will provide a bucking current to turn the SCR and relay off when a surge comes along. The first problem is that this is going to be so slow that whatever is connected to the output terminals will probably be fried before the relay can drop. The second problem is that I don’t really see the path for the shutoff current nor do I see an absolute reference for it to reference. That means it will only work if there is a quick change in voltage, but the nature of the components won’t allow it to respond in time to do any good.