You asked:
Why doesn’t plugging surge protectors in series provide more protection? — Don, Greensboro, N.C.

Marshall Brain Answered:
The basic idea behind surge protectors is pretty simple. Inside the surge protector there are components called varistors that bridge between the power line and ground. These devices sense high voltage and, when they see it, they shunt it to ground. Therefore, high voltage surges never reach electronic devices plugged into the surge suppressor.

If you chain two working surge protectors together, the first one is doing all the work. It is catching and eliminating surges. The second one doesn’t have anything to do in most cases. However, if an especially large surge were to come along that the first protector could not completely handle, the second one would have something to do. Also, varistors wear out. So as the first surge protector degraded, the second one would be called on more frequently.

For details see: How surge protectors work and this page.

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