The KISS Transfer Switch

There are likely countless DIYers who have wired up panels that are dual feed, (maybe Inverter or Generator power, and they rely on their memory to throw breakers or switches in the right order. Problem is, even the best memories fade after a few years, and add to that, the need to remember to do it right in darkness, and at the worst time one can imagine, that's when it's usually required to perform the transfer :-)

In a lot of power plants, and power distribution systems, there are transfer switches with simple interlocks, and you MUST open one breaker before another can be closed. This makes it mighty unlikely you can tie the commercial power and your generator or AC inverter together and ruin expensive equipment. It also means that others can't smoke you expensive equipment should they make a mistake in your absence. 

In my case, I'm finishing up my basic off grid power system in Easton, and most all loads are handled by the outback Inverter making 60 HZ AC. Our needs are modest here, so I don't see a lot of need to run the Lister 6/1 generator to charge batteries we have decent solar gain here, and I expect the PVs will do most of the work. If there is a need for the Generator, the outback has a built in charger and AC transfer, and when it sees stable AC power from the generator, it auto transfers all it's loads to the Generator. Now I think this is really a nice feature, and Inverters have come a long ways in the last several years.

BUT

But what if we have an inverter failure? Well naturally, if it does happen, it'll be dark as coal, the wind will be blowing, and God knows what else.

IF

If you designed your system properly, you can effectively (BYPASS) the bad inverter and use the diesel generator to power all of your loads, or at least the most important ones until a repair of the inverter can be made. If you look at the OUTBACK WEBsite, there are examples of these (BYPASS) arrangements in their wiring diagrams.

The problem I had was finding something KISS, and part of KISS as far as I am concerned is easy to find replacement parts. There are all kinds of Transfer switches, some are downright expensive, and never have I found a really inexpensive one.

What I finally decided on was the 70Amp breaker box often used by the Solar Industry for a number of purposes, and they're at HOME DEPOT and other places for as little as $12.95 with all sizes of breakers available cheap. I have seen transfer switches in home use, and in Commercial apps that used breakers as the switch, BUT in some jurisdictions, I'm told that a breaker can NOT be used as an isolation switch because it's possible it can fail closed. an example of this concern might be where you will open a breaker to assure there is no back feed from your home Generator into the commercial grid, in this case, some Jurisdictions apparently want an approved switch, and do not allow a breaker to be used for that switch. You'll need to check your regulations to know for certain.

Above: The Allmand made DIYer interlock is just a piece of metal stock with some slots cut, and a notch to allow one (OR) the other breaker to be turned ON. You can see that the left hand BREAKER would need to be turned OFF BEFORE you could slide the bar to the right, and align it with the right hand breaker allowing it to be turn on. Certainly you can make your own, or for a few dollars, you can buy the bar, drill a few holes, and with a few machine screws, and nuts, quickly assemble the DIYer INTERLOCK TRANSFER SWITCH.

Now in the case of my Easton Off Grid  Power System there are several Square D 70Amp boxes on the shed wall, and two of them need these interlocks before I can finish the wiring.  One is the power to the Power Shed inside wall plugs, lights, and outside plugs, another breaker box controls the power to our small cabin 200 feet away, and a small storage shed and shower room. With the transfers installed, I can choose to feed the boxes by either the inverter, (NORMAL), or BYPASS the inverter, and power these loads directly by the Lister 6/1 Generator. In the case of an inverter failure, I can also open the AC Power IN (from the generator) to the the inverter for complete Isolation, and then remove the Inverter for repair If necessary.

 Above: Boxes on the left will receive the interlock covers, and I will Permanently and clearly mark all switches next trip up there.. The right most box is used to isolate the PV panels from the Outback MPPT (right most), and the output of the MPPT into the batteries.

Above: Power Shed, lots of work left, but the pine floors are in, and oiled.

Above: Power Shed Summer Construction, this shows the access door to place the Generator, the door blends with the board an batton.

Above: Here's the cabin about 200 feet away, that ice chest is a thing of the past, we now have enough juice to run a small electric fridge! Four gauge copper between the power shed, and the cabin keep the voltage drop low, and plugging  in an appliance in the cabin or shed behind the cabin gives us the same results as at home.

 More to come

All the best,

George