AC circuit powered from 2 sources

[michaelmrc]

good day everyone hoping this is the right forum for this post. what im wanting to do here is to build a sort of bypass system for my rv. im installing an inverter but only want to energize certain circuits ie tv stereo computer  type of thing. i dont want to install dedicated plugs so i am planning on putting in a dpdt 40amp power relay. i know how to wire it as a transfer switch breaking the hot and neutral but this way i would need 1 relay for each circuit. is there a way i can just break the hots and feed the neutral from the inverter into the rv panel. i would need a way to prevent feedback to the inverter and this is where im stuck. any thoughts or suggestions on how to do this efficiently would be greatly appreciated thanks.

[ajb]

The way this is typically done in homes with backup generators is you move all of the circuits you want to provide with backup power to a subpanel.  This is fed from a transfer switch with one input being from the generator and the other from a suitably rated breaker installed in the main panel.  That would be the easiest and safest option in your case as well, as long as you have room to install the sub panel.  The other option would be to see if you can fit all of those loads onto one branch circuit (TVs, computers, and stereos generally don't draw that much current) and then just switch that circuit over.

[Pillager]

I drew up a quick circuit that basically does what ajb wrote.

You can swap the inverter and mains connections, depending on your preference, shouldn't make any difference (unless you figure in the relay's current draw from the inverter).

The circuit breaker (if it can handle inverter input) could be placed between this circuit and your load, protecting both inverter and mains source from failures. If the inverter has its own protection, just put the breaker on the mains side.

[kripton2035]

I asked this question some times ago
a quite nice answer for it was : get an inverter that can handle all the power
power everything with it
and charge the inverter with the pv panels and the mains
with that you have the inertia of the inverter, and you use the amount of battery you can have
when the batteries are over, you use the mains to charge the batteries again or the pv if it has sun on it.

[johansen]

depending on the inverter hot and neutral are either isolated relative to ground, or one of them is tied to ground, or both of them are tied to ground through the battery (-) post, and you will have about 60 vac on both "hot" and "neutral" 

[ajb]

I drew up a quick circuit that basically does what ajb wrote.

You can swap the inverter and mains connections, depending on your preference, shouldn't make any difference (unless you figure in the relay's current draw from the inverter).

The circuit breaker (if it can handle inverter input) could be placed between this circuit and your load, protecting both inverter and mains source from failures. If the inverter has its own protection, just put the breaker on the mains side.

While the circuit above will basically work as a rudimentary means of automatic transfer between a primary and backup power source, it leaves a lot to be desired.  Power relays may have a pull-in voltage of 85% of nominal coil voltage, but the dropout voltage is often unspecified, so it's hard to tell at what point the circuit will cut over to the backup source as the primary input falls, so you could very well end up with brownouts at, say, 50% of nominal line voltage passing right through into your load.  Whether or not that's a real problem depends on the nature and importance of your load.  You'd want something to provide a better-defined changeover point.  I'm not sure if there's an easy off-the shelf solution to that, other than just buying a proper automatic transfer switch.

There's also nothing to prevent chatter when the supply voltage is marginal or fluctuating, nor is there a manual override mechanism.  This can be overcome by adding a control relay set up to latch the transfer switch in the backup position, which will require a manual reset to revert the system to the primary supply, but allows for manual override or an auxiliary control input as well.  You could also use a simple timer that enforces a minimum time the switch must remain on backup power before restoring to the primary supply to keep the system from bouncing back and forth. 

Anyway, the OP hasn't actually specified if they want something to automatically transfer power from one source to the other or if this is meant to be a purely manual system, in which case it's all moot for the task in question. 

[G7PSK]

You need a double pole double throw switch or relay, load on the moving contacts and a supply on each set of fixed contacts, if you use a relay with mains voltage coil the coil can be energised from either the mains or inverter, the mains is best if your inverter is self starting so that whenever the mains is pledged in the inverter is switched out automatically. I have use this arrangement many times with generators.