gaps

[Anonymous]

When gapping the rails for different blocks, should you gap both rails, or gap just one and leave a "common rail"? HO code 83 if it helps
Thanks

[rrinker]

I'll say it first and take the hit...

Gap BOTH rails. Even for DC. If you go DCC in the future, you won't have to rewire anything. If you're after technical answers, there's any number of websites, like http://www.wiringfordcc that explain WHY common rail is not good.

Even on my previous layout which was a small shelf switching layout, i never even considered common rail wiring. Probably because even back when i was a kid and my Dad built the 4x8 (+ a little extra) we used to set up in the family room, he ALWAYS gapped both rails. It ALWAYS worked, too - as long as I was the one in charge of throwing the switches, he built the thing but could never run more than 5 minutes without derailing something.

--Randy

[Seamonster]

I use common rail and I gapped both rails. Better isolation, no problems with different kinds of turnouts, and if I ever change my mind, it's not much re-wiring.

[selector]

Darn. I guess I'll have to ask. Randy, what is 'common rail"?

[ukguy]

From what I understand Crandall, and this isnt gospel.... common rail is the rail which has power fed to it continuously, this rail is always 'live'. Isolated areas are obtained by controling power through the other rail, as both rails need to have current to complete the circuit to the loco or whatever, simply switching one of the rails 'off' breaks that circuit.
I kind of have a common rail, just to cut down on the spaghetti bowl of wiring that seems to be under my layout, however it was a 'female dog' to get it to do what I wanted, although I am/was wiring illiterate, I have improved somewhat and looking at it now it is fairly straightforward, the problem was that I didnt know where to start/go but some serious thinking got it how I want it.............. almost ready to start again..... counting down the projects for completion till the next layout is born..... fun,fun,fun.

Have fun & be safe,
Karl.

[selector]

Thanks, Karl. However, the newbie (I still am, even though I'm running a Digitrax SEB) will see the sentence in all of the on-line how-to's that says, "Wiring for DCC is like wiring for DC. If trains run on your track in DC, you probably won't have to change anything." So this 'common rail' thing still confuses me. Do I have to manufacture gaps on one of my rails, and if so, which rail, and how would I go about figuring which is the one to gap?

AAARRRGGHHHHH!!!

-Crandell

[IRONROOSTER]

Common rail wiring is where only one rail has electrical gaps in it - usually insulated rail joiners, but there are other methods. This divides that rail into electrically isolated blocks. Each block has an electric switch so that it can be switched between two (or more) power packs (or throttles) The rail without the gaps has no switches and is connected to both power packs. This allows you to use single pole switches. The other method is to gap both rails and use double pole switches. The advantage with common rail is less wiring is required for the common rail and single pole electrical switches are cheaper than double pole. For reversing tracks the common rail still needs to gap both rails to avoid a short. Chief dis-advantage is that it's more prone to electrical problems which are harder to isolate and fix.

This page http://www.nmra.org/beginner/wiring.html has a longer write up with some drawings to help you understand it.

Enjoy
Paul

[rrinker]

OK, let's try this:

There are, two rails. With DC, to be able to run more than one train, you must divide the track into blocks. To do this, you naturally put gaps in the rails (or use insulated joiners). If you only put gaps in one rail, the other one has no gaps - this is the 'common rail' because it is 'common' to ALL blocks.
With DCC, you don't need blocks for the same purpose. However, you might have a large railroad and need more than one booster for power, or you might want to use an electronic circuit breaker to divide the layout into 'power districts'. That's just another term for what are, in fact, blocks. However, a power district is typically larger than a DC control block. You can do exactly the same as DC - gap only ONE rail, leave the other as a common rail. The problem here, especially when more than one booster is involved, has to do with the square wave DCC signal. If the two boosters are 'in phase' that means when one is at the positive half of the wave, the other one will be too. If they are 'out of phase' the one will be positive and the other negative. It is therefore possible that if you measured the voltage across that one rail gap, you could see DOUBLE the voltage. This won't do your locos or decoders any good at all.
Now I know some people will say "I have 4 boosters and common rail, and it's been like that for 6 years and never had a problem" Entirely true - but why take the chance, all you have to do is gap the other rail, because you still need frequent feeder drops fromt he rail to the bus wire - you don't run one bus wire around the layout and then connect just one feeder to the common rail and expect the rail joiners on that side to handle everything.
The alternative to common rail is to 'double gap' all track - that means cutting a gap or putting an insulated joiner in BOTH rails, not just one. Short of using slightly less wire for a DC installation,t here really is NO reason to wire common rail. If all sections of track have two wires goign to them, it's a lot easier to troubleshoot.
If you are building a new layout - there's no excuse to use common rail. For a large westablished layout, it's possible that it might be far too time consuming to rewire everything to eliminate the common rail.
Wayne Roderick has a good technical bit about the pitfalls of common rail on his web site. Wiring for DCC also has some more technical information on common rail. Mark Gurries has a PDF file from a clinic presentation he did which I believe also covers the issue.

--Randy

[selector]

Wow, it finally falls into place, and I thank you both immensely!!!!

Randy, you thought I was pulling your leg in the other topic when I said you have to begin with, "Take the red lead in your left hand..." [:D]

Okay, for proof of my understanding- if I were to gap both rails in my 'one-district-on -the entire-layout' layout (I have no districts, just one monolithic rail with bus***feeders every five feet), and gapped both rails somewhere near the centre of the distance between each feeder, I would have no comon rail, and satisfy Randy's dictum that DCC should have those gaps on both rails for safety in the event I ever increase the number of booster from the current one?

[gmpullman]

Here's a handy hint while you're doing your wiring and it's helped me immensely. I have kind of a "folded dog bone" shaped layout and it can be pretty confusing at times to remember which is the "north" rail and "South" rail (or rail A and B) so I got a pack of Dennison colored dots and followed each rail around the layout and placed the peel & stick dots (the small 1/4" size) pick any color. I used red & yellow. This makes it easier to identify which rail gets which feeder. I put the dots about 4 foot intervals so I always know which rail is which. You can peel them off when you're ready to place your ballast...
Ed

[rrinker]

QUOTE: Originally posted by selector

Wow, it finally falls into place, and I thank you both immensely!!!!

Randy, you thought I was pulling your leg in the other topic when I said you have to begin with, "Take the red lead in your left hand..." [:D]

Okay, for proof of my understanding- if I were to gap both rails in my 'one-district-on -the entire-layout' layout (I have no districts, just one monolithic rail with bus***feeders every five feet), and gapped both rails somewhere near the centre of the distance between each feeder, I would have no comon rail, and satisfy Randy's dictum that DCC should have those gaps on both rails for safety in the event I ever increase the number of booster from the current one?

That would definitely work, however you might want to work out a logical division for what might become seperate power districts in the future. For reference see my track plan, so you have a picture to go with my words. There's two mainline loops, and an inner section with a yard lead, yard, and some sidings. The only palce I have gaps right now is at each crossover between the inner and outer main, and at the crossovers connecting the inner main to the yard lead and the attached trackage. These are my three potential power districts, but for now they are simply all connected together.
The nice thing is that if your power bus follows the track, you can ALWAYS go back and easily seperate things - cut gaps in the rails with a Dremel cutoff wheel, and cut the bus at the same point.
It may seem a bit of overkill for my relatively small layout, but plans are and have always been in the works that make this just the starting point of a far larger layout. No doubt some of my planning will end up not fitting the final result and I'll still have to whip out the Dremel and wire cutters, but it's a start.

--Randy

[selector]

Got it, Randy. Thanks.

Ed, that's a good idea. Thanks.