Solution for Power Supply Switch Circuit

Well, that's pretty safe, albeit rather passive. A 30A 30VDC SPDT switch perhaps?

I don't get it. You have two isolated DC power supplies that must maintain isolation from the load but yet power the load. This sounds like a classic open circuit to me or I just don't understand your question. Oh since when has a supply or a load had just one wire? This is looking more like a ladder logic diagram than a schematic.

I will try to render this with some simple typography:

Supply1 •---,• Load

Supply2 •'

The horizontal hyphens in line 1 show the SPDT switch (also known as a 3-way switch) making contact between supply 1 and the load.

Alternatively, the switch can connect the apostrophe in line 2 to the comma in line 1, making contact between supply 2 and the load.

It makes no sense to isolate the supplies from the load, but this scheme does isolate the supplies from each other, which may or may not be necessary.

Ok so why not just power diode OR the load side. If your sources are unregulated and each independently vary from 18 to 30 VDC but your load requires 24VDC regulated, then you will need at least one DC/DC switching regulator that will accept this input range. A 720 Watt DC/DC converter is not a simple circuit so purchasing one will clearly be better than trying to design one from a blog.

Yes. If we want to design such DC-DC, are there some refer solution?

Ok, here is an excellent application note on utilizing just two similar family of chips in making a much smaller DC/DC converter. One can scale up from here to eventually 30 amperes using this family of parts but this is a good place to start. Now my question for you, did you understand the math of page five or did you just delve into trying to build something that worked?

Solution: Off-Line DC to DC converter. http://www.smps.us

The op has to my mind described what he needs very, very badly. Therefore its difficult to give an accurate answer.

What I cannot believe is that we are not allowed to connect a supply to a load, but if that is the case, then AC is a possible way to go......via transformers of course....that will give full isolation.

Or a rotary converter is a further possible method but with some losses of course.....

If he just meant that one supply cannot connect ever to the other supply, the problem is simple and has already been fixed by other posters.....

Seems to me that he's trying to couple two circuits without connecting them in a hard-wire set-up, so the easiest thing is to use a properly sized 1:1 isolation transformer.

With DC voltages? That's why I recommended the DC/DC switching supply. Yes a careful design can put an isolating transformer inside the supply and control loop. But a transformer only gets warm with a DC supply.