Surge Protector - how they work?

A voltage-dependent resistor, usually.

Lo

High voltage causes a MOV (metal oxide varistor) to close. Both the input and output points become the same voltage, preventing current flow through the circuit.

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.... Phil

A common surge protection device is a Voltage-Dependent Resistor (VDR), also called a Metal Oxide Varistor (MOV) or simply a varistor. It acts rather like two zener diodes back to back. It's placed across the power line and does nothing when the voltage is within normal limits, passing only a small leakage current. When the line voltage exceeds a certain limit, the varistor breaks down and acts as a shunt. It can absorb large amounts of energy from momentary spikes, but when the high voltage is sustained, the varistor breaks down permanently. It may burn up or even explode. Even then, it may have done a useful job by blowing a fuse.

However, it's a good idea to keep in mind that few, if any, protection schemes are 100% foolproof. For more details, you may want to read up on MOVs/VDRs on the net.

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Kari Laine expressed precisely :

Adding to pimpom. You asked what was in those devices. You will have to open up one from your locale. Here in California, we have Radio Shack stores around the corner and my friend an I were trying to do something similar so we went to Radio Shack and opened one up. Inside we found the MOV and a toroid. The toroid may have been wound a a balun or simply as an inductor. We did not want to cut into it to see. This also helps reduces spikes coming in. In our case we were trying to stops spikes going out of an appliance so we added some 500V ceramic caps (essentially high frequency filters across the line) to further reduce that noise spectrum that might interfere with X10 devices we have plugged in around the house. We are in test phase now.

balun?? probably as a common-mode choke (both windings in parallel)

You should use X capacitors for cross-line applications

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