Need 110/220v relay (switch)

Don't write off Frequency as a source of problem so quickly. I dealt with a new MTU at start up where the Lube Oil Priming pump would work on shore power but not on Genset.

The shore power which was being converted locally was giving us approx 58 Hz and the Genset 60 Hz, upon deeper investigation it was discovered even though the pump system itself had been ordered to run on 60 Hz the control gear was only rated for 50 Hz service.

Why was that switch selected? It looks like the wrong application for it.

I would use a DPDT NC* 110VAC coil relay or contactor sized to handle the current consumed by the battery charger.

When on shore power the NC contacts power the charger and when the generator starts, 110VAC energizes the coil to open the shore power contacts then close the generator contacts to power the charger.

* NC or NO depending on which source is used the most and what you prefer as the fail safe power source. If you don't like mechanical relays, it is easy to do with solid state relays (SSRs) but a possible failure mode for them is to fail closed. An SCR fuse on each leg will take the worry out of that kind of failure though.

Electricity 101

Relays (and similar devices) have two sides
- the control side: apply signal here to switch load side
- the load side: connect whatever gets switched here

An quite obvious and very fundamental difference.

Both sides have different ratings!
- control side: The voltage, frequency and the current the relay draws when energized. - This can be a range.
- load side: the voltage, frequency and the current that can be switched. If there are multiple contacts they may have different ratings. Further differences that can usually be found as markings are: Switching AC vs DC and the type of load. (resistive vs inductive vs capacitive)

If in doubt always refer to the datasheet! Which is very easy today as long as you have internet access and the type of the device.

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The device pictured is 230V only and internally more complex, it uses 2 relays and adds a delay to the crossover time. A different version also adds voltage basic monitoring.