loop-start

TIP = white/blue, white/orange, white/green, etc RING = blue/white, orange/white, etc

Scotty

You missed my point.... I know the answer... But we need a mnemonic, hopefully one that rhymes...

Ahem, I don't think so...

It just has a different definition of "ground" than a stake pounded into the dirt next to your service entrance.

But it *is* at ground potential. If it isn't, the whole switch would be excessively noisy. (I've seen it happen.)

Sure. Aboard ships.

Sorry for the pun, but could not resist! But literally true too...

Don Miller

Something that might help understand this is to change the name of "ground" to something else. What will work best depends on you... but maybe "system ______ common voltage point" or something to that effect, or perhaps even better would be totally non-specific, like "level Z". Give it a label though, and then define what it is, and is not.

The term "ground" is being overloaded, or multi-tasked, or used for too many different things, which then become confused with each other simply because they each have the word "ground" in the name.

A ground-free design is a different ground than the ground-potential referred to in a communications system design.

I would expect it to be included, though not because of the voltage levels. (I am absolutely ignorant of ship designs, Navy or otherwies, so this is wild speculation.) I'd think that they want the common point for the telecom system to be tied to the common point for the electrical power system, and as a result it would perhaps be isolated from the metal hull.

It is indeed! And almost no matter what area of telecommuncations you work in there is some part of this that needs to be considered.

The need, for the telecom system, is to have one common point for all voltages and all signals. The "one" part means there cannot be multiple paths. That's how "current loops" exit and cause problems, so that is to be avoided.

For a land based telco there is likely to be one such ground point for each floor in a building, and then one such point for each building. Then, to make them all common, each building is connected to a ground rod of some kind. Note that the power system does the same thing, except it is separate from the telecom systems ground.

Also you can extend the concept a little, and consider that each junction box or pole along a cable is essentially the same as a building. One common point, connected to a single connection to earth to make it common to all of the other systems. (On a ship that "earth" might be a specific conductor bus that runs the length of the ship, and is at no point connected to the hull???)

(Maybe somebody who knows more about this than I do can correct me and explain it with more detail.)

Is that 60 VAC to each other, or 120 VAC to each other and 60 VAC to ground or neutral?

Floyd:

Thinking more carefully about it, for most ships you're right - ground is established by paddles in salt water in the ocean - can't get much better than that for "earth". A ground-start facility from shore would even work fine.

But I was thinking of the "ground-free" design one sees on Navy ships, where although the metal parts of the ship are obviously grounded (at least in salt water), as I recall the design rules require that no circuit be at any potential to that - IOW, you can stand on the wet metal deck and grab any wire, and there will be no current flow. Common power receptacles are each wired with a transformer circuit back to line, to simulate ground for devices that rely on it. I believe that spec applies to low-voltage communications circuits as well.

In that kind of isolated environment, a comm circuit's potentials would exist in a vacuum, right? And could in theory have any relative potentials, irrespective of earth ground? I am no maritime engineer, but it does at any rate seem possible, and with proper isolation, it should be quiet - no connection to Earth, no ground hum for sure.

And of course it begs the question of a PBX aboard an aircraft... :)

Thinking way too much about this... but great thought exercise!

Taima,

Don Miller

Related to this: many naval ship power circuits use a "double hot" outlet. It looks like a standard (USA) 15 amp 120v unit, but is wired differnetly.

Standard US code is that one blade is at 120VAC (in reference to ground), the other is neutral (and the third a safety ground).

In some ships both blades are at 60VAC in reference to each other.

This works ok for the vast majority of standard electrical appliances.

oops.sorry about that fat and fast finger. it's 120vac in reference to each other, wth 60v compared to ground..

And again, this means that the blade that, in standard land based wiring, is neutral and cold (that is, you can stick your screwdriver into it without too mcuh fear...) is, in fact hot compared to ground.

The reason for this is withstanding battle damage. You can short either side of the outlet to ground^H^H^H ship, and everything still works. It's only when both legs get ..shipped.. that the breaker trips.

I suspect the 60-0-60 is really via a set of voltage dividing resistors that draw quite limited current. There are good reasons to not want the grid to totally float vs the ship.

Yep - that sums it up concisely, and is exactly right, on an aircraft carrier, a space station, or in your living room. If you don't, you will get noises, and software-controlled systems may have "ghosts" (transient inexplicable malfunctions).

(Reminds me of an old Comdial Executech I used to maintain, old wire in a metal-framed building - it was _haunted_! )

Don Miller

Yes, that is the reasoning, as I understand it. Also, to avoid the risks of arcs to "ship" in damaged circuits. (Which is a concept sci-fi shows never get right! I'd have thought at least the Enterprise D would have had something similar - but the "fireworks" must make for better visuals!)

They get even more fanatic: their specs require each receptacle circuit to be isolated from the main systems by a single-phase 120v/120v transformer. The "ground" leg created by the resistors (or similar) exists only on the outlet side. The mains have none.

But for telecom circuits, that begs the question: Does a shipboard PBX, for example, have its one "ground" that is consistent throughout the loops ship-wide, or do they follow some kind of local isolation model there too? It would seem like it'd have to be the former, but I don't know.

Interesting...

Don Miller

The ones we installed were grounded to the deck. Power came from a battery locker, but I'm not sure how they were wired as certified shipboard electricians did all that work.

And if you ever get near a old Navy Electrician; RUN!!

A friend bought a house a retired Chief had rewired. He had decided that white was hot and black was neutral...

In the past 60-0-60 was an electrical code requirement for some industrial "wet room" situations. I think the intention was to reduce the peak voltage to ground in case anyone ever came in contact with a housing shorted to a hot, before the era of three-prong cords. And I cannot imagine it done any way except for a center-tapped transformer.

Of course lots of household appliances are 120-0-120 (sometimes without a ground, just a neutral, due to some really anachronistic codes) today.

Tim.

But that would defeat the intent explained to me: shorting either leg to ..ship.. was not detrimental.

Note ships have "battle-shorts" -- hard jumpers added across fuses that are used on vital circuits. The theory being the wiring may catch fire but that's less important than losing turret movement while shooting at the Bismarck, etc.

NASA's master control room had a similar feature during uber-critical parts of the Apollo landings.

My stove wiring is even worse; there's a fuse in the neutral!

I remember those. It is amazing how many times a line would be reversed and the darn thing wouldn't be able to dial.

Modern items that care about polarity are VOIP fxo/fxs gateways (like the Sipura SPA-3000). They will optionally test for polarity reversal to indicate remote-end hangup. Getting the polarity wrong probably makes for really short incoming calls. ;-)

-wolfgang

(Some snippage)

Well, Clarence that's not so far-fetched an idea! Usually antennas are either horizontally or vertically polarized. Based on what you say, I'll guess your daughter is sitting or standing when she calls you - her antenna is probably vertically polarized. Ask her to lie down the next time she calls you - horizontal polarization! (just a random thought...)

wrote:

Nah. The charger.

Take care, Rich

God bless the USA