Electronic – Is a varistor basically two diodes in series, with opposite polarities

One of the main differences between a TVS and a Varistor is that the voltage tolerance of a TVS is much tighter than a Varistor. They also don't appear to have the "wear" factor that Varistors have.

Varistors that are repeatedly subjected to transients appear to have their clamping voltage lowered as a result of the stress. Eventually, they become conductive at the operating voltage and then fail catastrophically. That is: they get very hot, then turn black, then eventually catch fire.

Many of the better AC power bars that have Varistor clamps within also have a thermal cutoff device mounted between two Varistors. The thermal cutoff device disables the power bar when a Varistor fails.

In terms of what happens to a transient suppression device when it is exposed to voltages in excess of their rated clamp voltage: they get hot. The will, in fact, get hot enough to fail.

When a Varistor fails because of over-temperature, it generally becomes restively-conductive. That is, it turns black, then catches fire. Your over-current device may or may not operate, depending on how low the resistance of the Varistor has dropped.

When a Tranzorb fails because of over-temperature, it generally becomes a short-circuit. This causes sufficient current to flow to cause your over-current device to open.

The GDT has effectively no leakage current, up to the voltage it's specified to, and very low capacitance. That's why it is used next to the line.

Unfortunately when a GDT fires, the arc has very low resistance, so is not good at dissipating a large amount of energy. This is where the varistor comes in. The varistor will tend to be able to sustain a higher voltage while conducting, allowing the transient on the line to be absorbed, rather than reflected which is what a GDT would do by itself.

A varistor tends have a higher capacitance than a GDT, so if the line is sensitive to capacitive loading, carrying high speed data for instance, it cannot be connected directly to the line.

The capacitor is to protect the varistor from very large, very short transients. If you check the specifications of the individual components, you will probably find that the GDT can safely switch much more current than the varistor can absorb. Although it won't handle all eventualities, the capacitor 'hardens' the circuit, it will survive a greater range of fault conditions.

The resistors are to discharge the capacitor, the varistor may not do this fully. Two resistors are used in series to allow lower voltage rating (cheaper) resistors to be used.