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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.

ac line protection

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  • \$\begingroup\$ you need choice bigger if have more regeneration. You can use basic value if absorbing all ignition. Did you build a internal filter ? But a lot system got bug because some power network got big harmonics(like 3.). Protection style Internal -->> External or External -->> Internal ? \$\endgroup\$ – dsgdfg Apr 11 '16 at 9:48
  • \$\begingroup\$ No internal filter is planned and only two transformer will be connected to the AC input: one toroidal (up to 450 VA) and one small PCB mounted (6 VA). The toroidal will be switched on or off with triac that is controlled with logic that is powered using small transformer. \$\endgroup\$ – prasimix Apr 11 '16 at 11:41
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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.

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  • \$\begingroup\$ Thanks Adam for your response and attached another Epcos note. There I finally can see mentioning of the MOV value when connected in series, i.e. for 230 VAC it is 250 VAC MOV. Something that is still not clear to me is the voltage when ignition will happen? Is that 250 VAC or higher? \$\endgroup\$ – prasimix Apr 11 '16 at 20:16
  • \$\begingroup\$ It will be the series sum of the MOV breakdown voltage and the arrester flashover voltage. \$\endgroup\$ – Adam Lawrence Apr 11 '16 at 20:58
  • \$\begingroup\$ Let's calculate that using DC values specified in datasheets. For the selected SAR (EF470X) spark-over voltage could be anywhere from 400 to 588 V and for mentioned MOV (S20K250) it's 300 V. That means that SAR will ignite when spikes larger then 700 to 888 VDC appears on the input. I don't understand how that could be just 10 or 20% over nominal voltage? For 230 VAC it should be 358 or 390 VDC. The question still remain: using suggested topology is it safe to goes with two 140 VAC MOV in series that will catch lower spikes and second route via SAR that should handle higher spikes/energy? \$\endgroup\$ – prasimix Apr 12 '16 at 12:03
  • \$\begingroup\$ MOVs and arresters are not intended for nominal increases in the line voltage. They are intended for gigantic spikes such as lightning strikes. The 20%-over-nominal is to ensure that under normal, non-lightning-strike conditions, the MOVs don't break down and nuisance-blow your input fuses. \$\endgroup\$ – Adam Lawrence Apr 12 '16 at 12:09
  • \$\begingroup\$ Ok, so we can say that with two MOVs of 250 V and one 470V SAR we have "solid" protection for 230VAC input in case of huge overvoltage/lightning strikes? \$\endgroup\$ – prasimix Apr 12 '16 at 12:26
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Like this :

Example of anti shock scheme :

enter image description here

Your MOV value is VC1/2. and can connect line side(Load side got a fuzzy voltage level(on-start/on-stop)).

Always use 2 pole switch for opening power line(LINE-IN).

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