circuit breaker location

The consensus here on the forum is to locate breakers between the TIU Outputs and the track connections.  An instant type is mostly preferred like the Airpax or PSX-AC.

Here is a link to another thread discussing this ogrforum: TIUs and Circuit Breakers

EDIT: Although if using postwar transformers, Circuit breakers on each output are also highly recommended.

@SteveH posted:

The consensus here on the forum is to locate breakers between the TIU Outputs and the track connections.

NO, sorry, that is incorrect.

Breakers and fuses should be nearest the power source- the transformer BEFORE the TIU.

PSX-AC specifically before the TIU not after because it could interfere with DCS signal.

Another reason is, the TIU itself is an electronic "load" and has TVS devices on each output that can fail shorted. And while the TIU has internal fuses on each channel, they are large 20A rating, so when/if a TVS is damaged and fails shorted- you don't want the TIU taking that brunt of the current until the 20A fuse blows.

Drawing originally by @CAPPilot but some details edited by me for fuses or breaker placement.

@Vernon Barry posted:

NO, sorry, that is incorrect.

Breakers and fuses should be nearest the power source- the transformer BEFORE the TIU.

PSX-AC specifically before the TIU not after because it could interfere with DCS signal.

Sorry Vernon but I disagree with your conclusion.

Breakers and fuses can be anywhere along a power feed, specifically because they monitor current through a wire, and thus the entire path, unlike voltage which is measured across two wires.

As a result anywhere along the path of the feed circuit between source and load is fine.  The breaker or fuse opens the entire length when it trips, regardless of its position.

The PSX-AC is a special case because it requires at 10VAC on its input in order to operate.  For this Vernon you're quite correct.

Mike

Fine to disagree but I stated the reason.

The breaker is there to protect as equipment much as possible. The way to do that is put at the source. Then it protects your wiring, the TIU, more wiring to the track, and so on. This is a basic electrical convention. Even modern recent autos are putting fuses right in the battery terminal these days for this reason.

And, given the cost of TIUs these days, you'd think you would want to provide as much protection against all faults as possible.

Would there be an advantage to installing a pair of instant breakers like the Airpax, one at the TIU Hot input (red) and another at the TIU common output (black)?

My thinking for the one at the input side is in line with protecting the TIU electronics as Vernon and Mike have mentioned.  The one at the output would be for a different scenario, not yet mentioned in this thread.  There could be an unexpected source of power somewhere on the layout that could back feed current into the common return to the TIU.  This second breaker on the TIU common output would provide protection in this case.

@SteveH posted:

Would there be an advantage to installing a pair of instant breakers like the Airpax, one at the TIU Hot input (red) and another at the TIU common output (black)?

My thinking for the one at the input side is in line with protecting the TIU electronics as Vernon and Mike have mentioned.  The one at the output would be for a different scenario, not yet mentioned in this thread.  There could be an unexpected source of power somewhere on the layout that could back feed current into the common return to the TIU.  This second breaker on the TIU common output would provide protection in this case.

I'm trying to play this scenario but IMO opening the common for any reason just invokes far too many problems. House wiring is similar- you never want an open neutral (AKA Common). In theory, an open Common causes just such differential voltages. Ideally, just have a breaker in every power source (example- aux power terminals).

Vernon, I agree that each power source's hot output should have its own breaker.

I also agree with not opening the common in house wiring where the neutral should be at or very close to earth ground potential, GFCI and AFCI protections rely on a solid neutral and /or ground connections and there are potentially dangerous 120 and 240 volt sources present.

Train circuits are supposed to be isolated from the house wiring via inductive/magnetic coupling in the transformers' secondaries.  House wiring should also only be done by a qualified electrician.  So, maybe house wiring is an apples and oranges comparison to train wiring.

As you know, many postwar transformers do have breakers that disconnect the common when tripped.  Although this isn't as good as individual breakers on the hot side outs, I'm not aware of tripped breakers on the common causing issues (EDIT: when supplemented by instant breakers on the Hot outputs), other then being too slow for modern electronics.

Since many model railroad enthusiasts don't have a firm grasp on signal flow, my thinking is that a belt and suspenders approach of breakers on the hot of every power source; and for an added measure of protection against the possibility of an unforeseen back fed power source, TIU outs/ common return breakers would be better than not having that common side protection for the TIU.  Especially considering the now scarcity of TIUs.

@SteveH posted:

As you know, many postwar transformers do have breakers that disconnect the common when tripped.  Although this isn't as good as individual breakers on the hot side outs, I'm not aware of tripped breakers on the common causing issues, other then being too slow for modern electronics.

Actually, a major shortcoming of the single breaker in the common for a multi-output transformer is the total lack of protection for a short between two of the outputs!  There can be unlimited current in that case with no circuit protection at all.

@gunrunnerjohn posted:

Actually, a major shortcoming of the single breaker in the common for a multi-output transformer is the total lack of protection for a short between two of the outputs!  There can be unlimited current in that case with no circuit protection at all.

John, where you've quoted me is taken out of context.  I've been advocating all along for installing instant breakers on every output of a postwar transformer.  This present discussion is about whether to also add a breaker to the TIU output common.  I've since edited the section you quoted to remove the ambiguity you've pointed out.

Well, I didn't see that in the reply, hence my clarification.

And with that, IMO "the rules":

#1 Ideally place breakers/fuses nearest the source (transformer or power supply) in the "hot" or red terminals output

#2 Place or ensure the source has breakers or fuses in each output (examples- each handle output, or accessory outputs)

#3 Do not place breakers or fuses in Common (black post- sometimes U) wiring- since most transformers share common and many folks wire layouts to depend on common for different voltages and activation of accessories. Ideally common should be your most robust wiring since it is shared and may be carrying return currents.

@Vernon Barry posted:

Fine to disagree but I stated the reason.

The breaker is there to protect as equipment much as possible. The way to do that is put at the source.

Vernon,

Your conclusion is correct for clamping voltage spikes, but not for stopping excessive current flow.  Why?  The current is interrupted through the entire path at essentially the same time (given the speed of electrons).  It's not interrupted near the source any quicker than the at the far end.

Therefore a traditional breaker or fuse can go anywhere along the path.  Everything in that path is protected simultaneously and to the same degree.

(Electronic breakers are a different story because they need to be powered in order to do their job.  Where that power comes from is important, but not with traditional fuses and breakers which need no external power to determine that excessive current is flowing.)

Mike

Just to clarify a minor point. The PSX-AC is a special case where it has both terminals but generally does adhere to the above 3 rules. The common through a PSX-AC is not broken, the "hot" is the one that breaks or opens, and thus it follows rule #3. Again, the black common lines straight through clear to the terminal blocks are not broken in the even of a current trip event.

Another great diagram originally by @CAPPilot and further edited by me.

@Mellow Hudson Mike posted:

Why?  The current is interrupted through the entire path at essentially the same time (given the speed of electrons).  It's not interrupted near the source any quicker than the at the far end.

Therefore a traditional breaker or fuse can go anywhere along the path.  Everything in that path is protected simultaneously and to the same degree.

(Electronic breakers are a different story because they need to be powered in order to do their job.  Where that power comes from is important, but not with traditional fuses and breakers which need no external power to determine that excessive current is flowing.)

Mike

The TIU is a 2 terminal device ACROSS the hot and common. It can short and fail internally as a short (most times the internal TVS shorting). A breaker/fuse after this will never detect that short.

I understand your thought process, but again, just like a switch- everything before the breaker/fuse is still connected- still live, still "hot". If that device has a return path and internal circuitry that can fail- the short bypasses the breaker completely.

And TIUs DO short on the outputs/inputs as a fault.

I am have an old TIU with no internal fuses. A Z1000 transformer,a 15amp circuit breaker from Scotts-Odds-n-Ends.I have not used DCS for 18 years, forgot alot. I bought a home 22 years ago with a big basement for my Trains. Finally starting my dream layout .Taking out my MTH emgines and all batterys  are dead except for a 765 Proto 1 I did a upgrade on.Made a big loop in my living room, hardwood central. trying to remember how to run DCS. Loving Retirement.

Vernon,

In the scenario where the 3rd-party breakers are between the TIU and track, and if a TVS in the TIU shorts, aren't the internal thermal breakers within the Z4000 sufficient to protect itself?

Ok, here's a real world possible scenario demonstrating what I mean:

Let's say the TIU's 2 Active channels are each being powered by an output from 2 different phased post war transformers, each capable of delivering >10 Amps.  Following best practices, the transformers' Commons are connected together and each transformer Hot output passes through an instant type breaker on its way to the TIU input.  So far so good, if either circuit's current exceeds 10 Amps its breaker trips.

Everything works fine until one day the (grandson/daughter, dog, cat or rodent) accidentally disconnects, chews through or otherwise interrupts one of the Common connections returning to only one of the TIU's outputs.  The next time the layout is powered up and significantly more than 10 Amps is drawn by the trains, puff magic smoke from the TIU.

~ 20 Amps of current would then be flowing through the one remaining TIU channel Common that's still connected.  If the TIU output Commons were also individually protected by 10 amps max instant breakers, this scenario could be prevented.

If the TIU's Common circuitry path can handle more than 10 amps (unclear), then all of this is a mute point.

@Vernon Barry posted:

The TIU is a 2 terminal device ACROSS the hot and common. It can short and fail internally as a short (most times the internal TVS shorting). A breaker/fuse after this will never detect that short.

I understand your thought process, but again, just like a switch- everything before the breaker/fuse is still connected- still live, still "hot". If that device has a return path and internal circuitry that can fail- the short bypasses the breaker completely.

And TIUs DO short on the outputs/inputs as a fault.

Agreed, but this is a unique situation because the TIU is not a simple electrical device.  There isn't a single path for current to flow through it.  There are two.  One through the TIU and the wires out to the train and back to the power source, and a separate one through the TIU's internal circuitry back to the power source.  I've been speaking about the particular path through the tracks and the train and not the other one.

If you want to protect against a short within the TIU put the breaker or fuse between the source and the TIU.  If only the train then between the TIU and the track is fine.

Mike

"As a result anywhere along the path of the feed circuit between source and load is fine.  The breaker or fuse opens the entire length when it trips, regardless of its position."

Nope. An open circuit breaker or fuse opens the circuit towards the load (downstream). It cannot open a wire behind it (upstream). The upstream feed wire is still energized.  This simple concept may not be applicable to the huge discussion above, but it's a fact of physics.