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I am hooking up a system where there is 2 sources of power available. One source is the grid power, and the other is a generator. The grid provides me with 3-phase 380VAC with a neutral wire; and on the other hand, the generator provides me with 3-phase 380VAC BUT no neutral wire. I am using a SPDT relay on each phase, and all relays are controlled through a common signal coming from a micro-controller. So, all three relays are switched at the same time. The load is a battery charger that has a 3-phase input and does not accept a neutral wire, just three hot wires.

schematic

simulate this circuit – Schematic created using CircuitLab

Now, my main question is when I am looking for a relay suited for my need, should I look for relays that have a switching voltage rating of 220VAC or 380VAC?

The confusing part for me is that if I measure voltage between one phase and neutral the voltage is 220VAC, but 380VAC line to line. So, I am not sure whether I need a relay with a switching voltage rating of 220VAC or 380VAC!

Thanks

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    \$\begingroup\$ "I am using a SPDT relay on each phase ..." This is a really bad idea. Have you considered the possible failure modes? \$\endgroup\$ – Transistor Nov 30 '18 at 20:21
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    \$\begingroup\$ It leaves the possibility that one of the relays doesn't operate, leaving two of the lines connected to one of the supplies. You now have a singe phase supplying the charger - there's a good chance that it will have detection of this and will not operate, but why not use a three pole contactor? \$\endgroup\$ – Phil G Nov 30 '18 at 20:33
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    \$\begingroup\$ The right way to do this is with a pair of mechanically and electrically interlocked three pole contactors (With auxiliary contacts fitted to provide the electrical interlocking) such that it is physically impossible for one set of contacts to close until the others have fully opened. Depending on where you are located this thing may need to be certified by someone, in which case buying a " Three phase Automatic transfer switch" is the way to bet. \$\endgroup\$ – Dan Mills Nov 30 '18 at 20:33
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    \$\begingroup\$ @MarkWright, I would actually suggest 440VAC, unless your generator is synchronized with the grid. As drawn, there's nothing stopping Grid_P1 from being 180 degrees out of phase with Gen_P1 (and same for P2 and P3) for a 440V difference. \$\endgroup\$ – Nate Strickland Nov 30 '18 at 20:47
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    \$\begingroup\$ The proper equipment as described by Dan Mills would be rated for the purpose intended at 380 volts. \$\endgroup\$ – Charles Cowie Nov 30 '18 at 21:00
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The right way to do this is with a pair of mechanically and electrically interlocked three pole contactors (With auxiliary contacts fitted to provide the electrical interlocking) such that it is physically impossible for one set of contacts to close until the others have fully opened.

Depending on where you are located this thing may need to be certified by someone, in which case buying a "Three phase Automatic transfer switch" is the way to bet.

If building your own a "Three pole mechanically interlocked motor reversing contactor" (It is actually a pair of contactors with an interlock module) with an NC aux contact on each half can usually be easily modified to do what you need.

I would start by speaking to your generator vendor as they will probably be able to either supply a packaged solution it or tell you who to speak to, and can also provide useful things like automatic generator start controllers and generator starting battery float charge systems (And oil heaters if required).

This is totally standard data centre stuff. You may wish to take advice as well as to how often you need to run the generator on load to get best reliability (A machine that has been left standing for two years will very likely NOT start when you need it to).

Answer put together from a comment thread that ran long.

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