Hi.
Victron sells this single-phase automatic transfer switch. If you look at the second photo, the ATS is implemented by using a four-pole 2NO+2NC contactor (labeled K1 in the photo).
Poles 1-2 and 7-8 are NO. Poles R3-R4 and R5-R6 are NC. Victron connects line and neutral from the first power source (L1 and N1) to R3 and R5 and line and neutral from the second power source (L2 and N2) to 1 and 7. Then they obtain the output line and phase by connecting 2 with R4 and 8 with R6, respectively.
As long as K1's coil is powered the connected source is the first one. As soon as the coil loses power the second power source is connected. Pretty straightforward.
However, the most important characteristic of an ATS is that the two sources are NEVER paralleled, not even when the ATS fails. One of the possible failures of a contactor is when the two contacts of a pole meld together and can't open anymore.
What happen if, say, contacts 1 and 2 meld together? Under this kind of failure does K1 guarantee that its two NC contacts (R3-R4 and R5-R6) will stay open? (because otherwise the two sources would be paralleled).
One would think that the moving contacts inside K1 are all mounted on the same structure moved by the coil. This is certainly true when a contactor has all NO poles. However, when a contactor has a mix of NO and NC poles I'm not sure that that is necessarily true and it might depend on the implementation of the NC contacts.
I'm not aware (and maybe it's my lack of knowledge) of any standard prescribing that all the moving contacts have to be on the same moving structure also when there a mix of NO and NC contacts in the contactor.
Is anyone aware of any such standard? Because otherwise the only way this ATS is completely safe is if the contactor maker (Schrack in this case) guarantees the above property for the moving contacts.
