For starters, a common is whatever we define it to be, (by what we connect to it) and an internal power common can be (is) very different than a common rail common.  The #12 wire that we recommend being connected directly between multiple DCC boosters is NOT there for static and safety grounding. (That is a different subject) It is there to give a solid return path for the current flowing between boosters during the times that trains are moving between power districts. (either with or without auto reversing) Any time a locomotive with offset pickups crosses from one power district to another it is being supplied from two different boosters, and there must be a common return path between them.

The output of the typical DCC booster is driving the rails via an "H" bridge. I.e. internally the booster does not have a negative voltage supply, but just zero and positive. (Any booster that can be powered from an external single polarity DC supply must be constructed in this way.)

Given the above. The booster case is usually the internal DC power 0 volt side. (unless the booster is an opto-isolated version) If we tie that point to earth ground and also connect all our other boosters to it and to ground in the same manner, we may now call this connection our "Common Ground". ("Common" because we tied things together, and "Ground" because we tied it to the earth.) We have now created a situation where the highest voltage in the system (measured to our "Common") is the positive DC supply internal to the boosters. No voltage on any rail can ever exceed that voltage. On the other hand the lowest voltage in our system is the "Common" itself, and no voltage on any rail will ever drop below that point. In other words, if the DC power inside of a booster never exceeds that allowed on a decoder, then you can not ever exceed that voltage measured between any rails on a layout with any track configuration, even with reverse loops, derailments, etc. In fact if you will actually measure the voltages on the rails (to the booster case or "Common") you will find that they both go from zero to some positive voltage, but are out of phase with each other in order to develop our DCC power across the rails.

If you mix in a DC power supply for legacy operation, then the only way to guarentee we will still be safe is to tie the raw DC negative of that DC power supply (assuming that it has a safe open circuit voltage) to our "Common" prior to passing it through any reversing switches. This will mean that no voltage in the system anywhere could still ever exceed the DC supply voltage, (or DCC power supply voltage) nor ever drop below it.

Because of our grounded "Common" connections to the internals of the power supplies with the above setup, you will NOT be able to run with a grounded common rail, but will need double gaps at any block divisions between power districts. Any bridging of either (or both) of the gaps between DCC boosters that are out of phase will not be able to add together even momentarily, but instead will instantly cause a short through our internal "Common". This is desired (actually required) if you use auto reversing boosters, as it allows reliable auto reversing even with offset power pickups and dirty wheels etc. etc.

Any attempt to tie any rails to earth ground and then calling them common will result in the DCC voltages appearing on the cases and ground wires of your boosters. If you must do this for some reason, then be wary of accidentally grounding the boosters. You will also need the boosters control circuits to be optoisolated or transformer isolated to prevent grounding together boosters via that path, or you must guarentee that the phase of all boosters remains the same.

I have a AWG 12 "Common" ground on my layout for all my boosters, controllers, signal power, switch machines, and etc. but NO rail is ever connected to it. The following describes how I wire for power to my tracks. I connect my boosters to the heavy gauge wire in the same manner.

1. What I use is sold in the US as 12-2 With Ground. This is two insulated wires plus one bare copper one.

2. Remove the outer plastic covering and paper filler for a distance about 3 to 4 inches at each location where you want to make a connection.

3. Spread the wires apart about 3/4" from each other by grabbing each insulated wire near each end of the exposed section with a pliers and giving them a 45 degree twist, one one way, and the other the opposite way. Bend the bare ground wire in the same way but in a different direction from the other two.

4. Cut about 1/4" of the insulation from the black and white wires. Strip the black wire near one end of the opened up area, and the white wire at the other end. That way the connections can not easily short together, even if the wires accidentally get pushed together again.

5. Strip about 1/2" from the end of some #18 stranded wire, twist it around the little bare spots you made in step #4, and solder them in place. (Get a big tip on your soldering iron, as the #12 needs a lot of heat in a short time to do it right.) I cover the soldered area with liquid electrical tape, but that is optional.

6. Run the other end of the #18 stranded wires to a terminal block. This gives you a place to disconnect the track feeders for trouble shooting, and if you use 3 position terminal blocks it also gives you a place to connect your Block Occupancy Detectors.

7. Connect from the terminal blocks to the rails. Many folks use #20 wire for this short lead, but it shouldn't exceed a foot or so if you use that small a size, nor should the #20 wire be the only one connected to that section of track if the block is long enough to contain several engines at one time.

Dick :)