I have a doubt about proper values of MOV and SAR when they are connected in series as suggested in Epcos app notes (see below Fig. 12). For the 230 VAC mains they recommends SAR of 470 or 600 V, but MOV specification is missing. In my current design I'm using 275 VAC MOVs that doesn't makes any problem but possibly also cannot protect the circuit in a proper way. It seems that MOVs voltage has to be cut in half (e.g. 140 VAC) to start protecting when input voltage goes 20% beyond nominal value. There is many guides how to select proper single MOV but without mentioning such connection in series. Any suggestion? I'm also wondering why SAR could be 470 or 600 V in this configuration - just to provide different threshold for ignition? Thanks.
The combination of two MOVs in series with a surge arrester to earth offers you both common-mode and differential-mode surge suppression capabilities. A surge from line to neutral will be clamped by the series combination of the MOVs. A surge from either line or neutral to earth will be clamped by a single suppressor and the surge arrester to earth. The series MOV will also act to limit the current available to the arrester.
A single MOV (or two MOVs in series) works to clamp line-to-neutral surges but doesn't help you much for a line/neutral-to-earth surge. Splitting the MOV into two series elements with the arrester at the midpoint going to earth gives you the 'most' protection (at the expense of two extra parts.)
The series combination should be sized to provide the needed hold-off voltage (as you speculated, each could be half of the value of a single MOV) - then appropriately size the surge arrester (based on the series combination of one MOV + the arrester) in much the same way.
Citing an EPCOS surge arrester note:
Surge arresters must not be operated directly in power supply networks. Because of the extremely low internal resistance of these networks, an excessive current which as a rule exceeds the permissible follow current would flow through the ignited arrester. The arrester no longer extinguishes and can reach very high temperatures.
Varistors connected in series with the arrester are well suited for limiting the follow current ... To stop the arrester from responding during normal operation, a permissible tolerance of the line voltage of +10% and a possible derating of the arrester of –20% were taken into account.