When making more or less general purpose systems to be installed into customer premises by people with very varying level of experience, you have to make sure that they can get the system to work on their own. The product must be designed properly and the installation instruction be clear and simple that works in all situation. It is not cost effective to travel to the other side of the world to solve "mysterious" communication failures or burnt transceivers.
It would be too much to expect a local instrumentation technician to be able to figure out if the customer wiring system is
- TN-S, in which case the grounding at both ends could be justified
- TN-C, in which case only a single point grounding should be used
- TN-C-S, in which case you would have to check that all current and possibly later added devices are in the _same_ TN-S branch now and that this will remain in the future
- IT (floating mains), in which case there should not be too much problems, even if grounded at all places
- TT (N grounded at transformer only, PE separately in each building) which might have some consequences in fault and thunderstorm conditions
Even if the technician would be knowledgable about these issues, getting reliable and up to date information about the wiring in an existing building can be a problem, especially if the wiring has been done during different periods.
In order to keep the instructions clear I prefer to recommend methods that work well in most cases and firmly grounding the shield at both end is certainly _not_ one of those methods, even if it does not cause harm in certain special cases. Paul
Assuming these cable splices are in poles at least some distance (say
100 m) from any buildings or transformers and their grounding electrodes.
What do you expect that the grounding resistance from a single copper rod driven into the ground to be ?
1) It could be several ohms depending on the ground conditions. This is sufficiently to bleed any static discharge accumulated on to the cable shield due to atmospheric conditions. Even if there is a lighting (10-100 kA) parallel to a section of cable, quite a high current would be induced into the shield, but the total _ground_ resistance between the grounding electrodes 1-2 km from each other would still limit the current to manageable levels.
2) Assume for a moment that a stupid electric distribution system is used in some country with a single overhead wire at say 10 kV AC and the ground serving as the return conductor, i.e. each building would have a step down transformer between the overhead wire and the grounding electrode at the house. When walking without shoes from the house towards the power source, there would be a small voltage difference between your feet at every step due to the ground resistance.
Assume that there is a telephone cable section with one grounding electrode close to the power source and an other close to the house. Part of the power system load current would go through the grounding electrode close to the house up to the pole, through the telephone cable shield with a very low resistance to the other grounding electrode close to the power source, down the pole and into the ground and then back to the power source. Thus, the telephone cable shield would carry most of the power system return current. What does this have to do with RS-485 grounding you might ask ?
Situation 2) closely resembles the TN-C case, in which you have a common PEN connector and the electric system load will cause a voltage gradient along the PEN line, thus stations A and B will have a different frame potential. Connecting a data cable shield between A and B will cause some of the PEN wire current to be diverted through your data cable shield.
In situation 1), the two telephone line grounding electrodes are nearly (if not exactly) the same potential and no current would flow, thus this corresponds to a shielded cable between two nodes in a TN-S system with completely separate N and PE wirings.
Even if there would be a small voltage difference between the ground potential around stations A and B, the current would still be limited by the grounding resistance at each electrode. This resembles the common grounding practice which used in quite few RS-422/485 systems, in which the shield is not connected directly to the frame ground, but instead a 100 ohm resistor is used to limit the "ground loop" current.
While your claims about grounding the telephone cable at every 1-2 km is believable, it does not justify your claim that RS-485 cables can be directly grounded (without current limiting resistors) at both ends in all cases.
If we want the devices to survive after a ground fault (without any residual current breakers), it should be noted that even with a TN-S system with separate N and PE conductors and assuming the same cross section as the L wire, a ground fault between L and PE would cause an identical voltage drop in both wires. With 230 V mains, the ground fault potential would go up to 115 V before the fuse is blown. Any equipment connected directly to the PE close to the ground fault point would get that potential and would most likely cause some permanent damage to the RS-485 system.
Even with the often used 100 ohm resistors between the cable shield and frame ground would limit the current to about 1 A (or 0.5 A if all stations use the 100 ohm resistors), which would perhaps increase the survivability of the data communication equipment.
It should be noted that a simple short circuit (eg. at the filament of a dying incandescent lamp) would cause in the TN-C system a high current peek in the common PEN conductor and some equipment connected directly to the PEN connector would jump to 115 V (at 230 V mains).
So in practice, if high reliability is a requirement, use optical isolation or at least keep the RS-422/485 signal grounds well away from the PE or PEN conductor potentials.
While my namesake had an example about 80 volt common mode difference, I demonstrated in an other post, how you momentarily can have a 115 V (for 230 V mains) or 55 V (for 110 V mains) common mode difference in various ground fault or short circuit situations.
I don't know anything about telephone cables (I *do* know something about RS-485, but I'm keeping quiet), but I've noticed one thing about this argument between Floyd and Paul - you both claim to have examined telephone cables, you both claim to know the regulations, and you both claim the other hasn't a clue and has never seen a real telephone cable. Since one of you (Floyd) appears to be in the USA, and the other (Paul) appears to be in the UK, is it possible that there are differences in the standards, regulations and common practice on the two sides? If this thread is to remain of interest to others, it might be worth establishing this.
I merely worked in the industry for 34 years before retiring. I've seen more at least twice as many cables! :-)
There isn't any significant difference in cables as used around the world. Other regulations do differ significantly, though generally there are two basic patterns an most countries follow one or the other. Such things as loop current specifications, layout of the digital heirarchy (E1's vs. T1's etc), and the type of codecs used (Mu-Law vs. A-Law), are the most significant differences.
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Floyd L. Davidson
Ukpeagvik (Barrow, Alaska) floyd@barrow.com
Only one of them produced no serious damage (it was a reversed two wire 120VAC drop in a bush village, and while it had the potential to kill someone, there were no unusual voltages applied to equipment).
The others all caused serious equipment damage to many things, each costing thousands of dollars. I doubt that many RS-485 systems have ever been engineered to survive such a condition, and further that there would be little point in such and expense.
90% of the equipment the RS-485 was associated with would be destroyed.
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Floyd L. Davidson
Ukpeagvik (Barrow, Alaska) floyd@barrow.com
Floyd, you're being unnecessarily insulting. It's a little unseemly.
You have demonstrated time and time again that your knowledge of RS-485 is sketchy at best. You've taken us on a long-winded trip along armour grounding in telephony, which, while entertaining in a way, really has nothing to do with the point under discussion (RS-485 common-mode voltage range).
Please go away and actually *read* the RS-485 spec [1]. Then perhaps we can have a grown-up discussion without resorting to petty insults.
All we have seen is a lot of hand waving. I have, at least put numbers to the, admittedly, serious case but I think it holds for quite a number of situations (even if it isn't all). Would you care to put numbers to your scheme and state clearly the assumptions, deal with the assumptions we have made about different earth potentials (or put real numbers to why it does not happen over such long distances). Then you might get a bit more respect for your position. Steve and myself have stated that we seen fried cables because people disobeyed the rulings about screen connectivity.
We are still waiting some serious attempt to convince us otherwise.
I never claimed it to be standard practice everywhere but certainly is in situations that I have come across. I have, as I have stated, also seen the other side in the ship wiring side of things.
My numbers were not off the wall as I have the readings from my log book to back up those I used in the example. We still haven't seen you put numbers to your diagram so I wait.
--
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Paul E. Bennett ....................
Forth based HIDECS Consultancy .....
Mob: +44 (0)7811-639972
Tel: +44 (0)1235-811095
Going Forth Safely ....EBA. http://www.electric-boat-association.org.uk/********************************************************************
I did in fact check with someone who spent a few years on an aircraft carrier. The above is a little too much imagination.
Aircraft carriers are up to 1,100+ feet in length. He figured the longest loop on the ship would be the "sound powered phone", but can't imagine that having much more total cable than maybe twice the length of the ship (I suggest three times, and he shrugged).
The longest actual loop would be a looped back data circuit between the stern located approach radars and the bow based command center. His flat statement was "twice the length of the ship". No weaving through many ups and down on the way either, a pretty much straight shot stem to stern.
When I told him the discussion was about the possibility of a 7 km loop on a ship, he looked at me like I was nuts.
So we talked about ground differentials, and that added another interesting perspective. His eyes lit up a the mention, and he said most people have no idea how big a battery that steel hull is, or how much voltage it generates just sitting in salt water. As to ground differentials, "*Lots* of voltage!" So I asked how much, 10's or 100 Volts, or what.
He said, "Oh, I don't know exactly, maybe 30 volts!"
Of course he thought the whole discussion of RS-485 was a useless endeavor, and just dumped any reference to it in favor of fiber optics.
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Floyd L. Davidson
Ukpeagvik (Barrow, Alaska) floyd@barrow.com
What is insulting about that? A little bit of "I told you so, right at the start of this lengthy conversation" is not inaccurate at this point, nor inappropriate.
What you have posted, right from the start, was *far* more insulting!
And better grounded that yours is, so what's the point of trying again to insult me. It won't help you understand how it works.
I was *asked* about armor, and was *not* the one who brought it up. Of course, you obviously still don't have any idea how this applies to RS-485, which is *exactly* the problem! You don't seem to know what the difference between armor and a shield is, and clearly don't understand the effects of a shield on twisted pair cables.
So stoop to posting articles that are *nothing* but insults. You can't argue technical points, and are confused by basics, so it's down to insults...
I don't expect that you could. Not an accurate one.
Note that the others in this discussion tend to keep the discussion technical, which you seem to try avoiding.
I won't respond to any more of your articles if they do not contain technical discussion.
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Floyd L. Davidson
Ukpeagvik (Barrow, Alaska) floyd@barrow.com
I suggest that's because you've totally misunderstood what we're talking about - RS-485. For looooong distances, sure, use something else. For what it was designed for (high-integrity short- to medium-haul datacomms), it's the mutt's nuts.
Why make claims like that. It isn't true, and you *know* it. I'm just astounded at how thick skulled some people can be.
In addition to detailed discussion of the theory, I've previously posted references that make it clear enough that the above is a fact. Outside plant communications cables are installed with a ground at every section splice. At each end where they enter a building they are grounded to the building ground.
Here they are *again*:
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Or, you can skip all of that again, and just respond to this one quote (emphasis added):
3.2.4 Grounding of Cable Shields/Support Strands (Messengers): Normal construction practices and NESC provisions require that cable *shields*, *messengers*, and other *non-current carrying metallic hardware* be effectively grounded. It is especially important to effectively ground cable *shields*, *messengers* and *non-current-carrying* metallic hardware at *dead**ends* and other *junction**points* for noise mitigation, personnel protection, and/or power contact protection. RUS Bulletin 1751F-815
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It also says that cable shield should be bonded to messenger cable or other support at least 4 times per mile if that can be done without opening the sheathing otherwise (at points where splices exist anyway), but for long runs with no such convenient points the cable sheath should be broken every 1 mile to provide such bonding. The messenger cable or support, of course, is required to be grounded.
If you are not aware of what RUS is, I would suggest reviewing their homepage at
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because they are a major part of the interface between the Federal government and the US telecommunication industry.
Rather than doing your own hand waving, you need to *read* the cites that I provided. There have been three people involved in this discussion doing a lot of hand waving. They either cite
*nothing*, or use examples that might best be described as imaginative. Sometimes those are indeed accurate, but none of them are common enough that typical engineering requirements would take them into account.
(I've seen locations where the outside air temperature can be
-40F, and the inside heat has been known to fail, therefore
*you* should only design equipment that can operate at -40C??? That is no different than claiming there are places where a ground fault in the power system has melted a cable. In one case the problem was the *heater*, not the equipment that didn't work at 40 below. In the other case the problem is the power system, not the RS-485 ground.)
A lot of people attribute any number of things to odd causes that often have no significance. The fact that you've seen cables fried by ground faults should tell you something about ground faults.
As I've demonstrated *very* well, communications cable is grounded at regular intervals, including both ends. I've discussed the theory involved to show why that is done.
Yet you want to say it shouldn't be...
I can't get past people who tell me there are 7km loops on ships. I probably can't convince you that there is no Santa Claus at the North Pole either.
That doesn't mean that I haven't provided a very clear and very logical proof of the above statement.
*I* said it was standard practice. *You* said it isn't. I showed where it is used, explained why it is used, and you say I haven't convinced you.
Here's the water trough... but *you* are the one who drinks or not.
Yeah, sure. 7 km loops...
Your 7 km loop is off the wall.
What do you mean I haven't put numbers to my diagram? I posted several diagrams, and have listed more numbers for a larger variety of things than you seem to be able to comprehend, much less remember. I provided *detailed* job specifications describing how it is done too! Right down to the minimum size of the ground wires.
You can't come up with anything that refutes what I've shown, and the references I've cited... why not just admit that it was correct from the beginning. Like I said, it really *is* useful information to understand.
One of the most notable characteristics of this exchange is how none of you want to discuss the details I provide. I post cites, nobody mentions them and you all drop whatever angle the cites were posted to counter, and instead you try some other equally inaccurate approach.
But the few cites that others have made *do* get a response from me. I show *in detail* why the cite does not support a point or is not correct. Nobody has answered that. It, again, was dropped immediately... because it is *obviously* irrefutable.
So, tell me again who is doing the hand waving here? And then tell me about 7 km loops on a ship again too. And all about Santa while you are at it.
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Floyd L. Davidson
Ukpeagvik (Barrow, Alaska) floyd@barrow.com
A gentle reminder: two wrongs don't make a right. Our behavior should be controlled by our own values, not by those of the person we are replying to.
You should also keep in mind that it is a common occurance to see one person criticized for being insulting while the person who insulted him did far worse. This is caused by the reader concluding that one person is a reasonable fellow who sometimes gets carried away and starts posting insults that are beneath him, and that the other fellow is a lost cause - a total loser who will never respond to any plea to moderate his bad behavior.
It is my considered opinion that you are one of those reasonable fellows I just described, and that phrases like "many lines of imaginative fiction" and so forth are inappropriate. I think you are better than that.
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