I have an inverter/charger unit, with features I want, that is not typically used in an RV. It (unfortunately) does not automatically make/break a ground to neutral bond. I would like to automatically do this anytime shore power is connected to the RV. (As required by electrical codes).
I experimented with a 2 NO 2NC electro-mechanical relay with the coil connected to incoming 120V shore power. For the test, I ran a ground to neutral bond through one of the NC contacts, so that when shore power was not present, the bond was made. This appeared to work. When energized, the bond was broken, when de-energized the bond was restored.
But the coil could fail someday, and the manufacturer said it should fail in its normal state, not its energized state. I understand that is not a certainty, and therefore will deal with that issue separately. But for those reasons I also tested having shore power run through one of the NO contacts. That way if it the device fails and the ground to neutral bond is not broken, power is also not passed through to house loads.
When testing this, the coil began bouncing and no power was passed through the contacts. I called the manufacturer who stated that you can not power an EM coil with the same source that you're running through the contacts. The coil and and contacts are normally isolated, but by using the same source for both, you've now effectively created a jumper between them, which results in constant vibration/bounce, much like old AC doorbells use.
That being the case, in a chassis fault scenario where ground becomes hot, the coil would again be connected to the same source as one of the contacts, so it appears a contactor like this can not be used for this application.
I am therefore exploring alternatives. One option may be solid state relays, but does anyone know for certain if they will have this same issue? I see many for sale on Amazon boasting "switch the fan on when you turn the lights off," and other similar statements suggesting the SSR input and output can be connected to the same source with no issues. Is this correct?
Please note, I appreciate words of caution, and I am being careful. I am testing everything with full protective gear on a bench.
Here is the system's wiring diagram, but I had not yet drawn a diagram for the contactor that I was planning to use to make/break the ground to neutral bond It's the large yellow box with text on the lower right. (update: separate drawing is now included). I originally tested with the coil powered from 1 leg of the 120V utility hot (the one that will always be powered when using either single phase 120V hookups or when using split phase 120/240V hookups), and that same leg went through the contactors NO contact and then connected to the AC breakers in the diagram. That way if the coil's NC contact failed to break the ground to neutral bond when on shore power, no power would even reach the breakers, and I'd know to replace the contactor. But this didn't work in bench tests and manufacturer says it won't ever.
Update: I've added a drawing. Please understand my diagram skills are still limited, and I was trying to draw the device using the only available objects in EasyEDA's library. I also wanted the orientation to match the Contactor's diagram and my wiring. EasyEDA does not indicate when a wire jumps over another, so near R2 you'll see I tried to make the typical jump shape, but if there is no red node, then it means there is no connection. Please also note the Vehicle Chassis Ground to Neutral connection via R1 and R2 is mandatory code for RV power when not running on shore/main utility power. There is no debate over this. Where I have a circuit breaker and load drawn below, that is where the incoming power arrives at the AC Switchgear pictured above, then out to loads. But I included a simple CB and load below to complete the drawing.
Here is also a drawing of the front of the Contactor. You'll note its 4 pole, but I couldn't find a 4 pole in EasyEDA, so I used the next best thing.