short out question

[chiefred]

While testing cables--my partner asked me to short the pair--I did not know what he was talking about---therefore he did it and when I asked him the reason, he said it was just another way of testing---however, I did not get satisfied and need a better explanation---
is this common? how is it done? why is it done? what practical applications---is it used for?


Thanks to anyone that can help me learn

 

[Jtodd]

We used this technique in the past to test non-category rated links for continuity. Partially because it can be done very quickly. It's also good for those clients that are too cheap to pay for real testing.

Justin T. Clausen
Physical Layer Implementation
California State University, Monterey Bay

 

[chiefred]

Thanks--but do you know how to do it--why its done and in what applications?

Thanks

 

[franklin97355]

How it's done: Wire comes in pairs, take both wires in a pair and touch them together, measure resistance.

Why it's done: to make sure the pair is not bad.

What application: Any non-category rated cable testing.

 

[chiefred]

Sorry Franklin---although I have been cabling for a few years, that just didn't help.

How it's done: Wire comes in pairs, take both wires in a pair and touch them together, measure resistance.

How do I measure resistance? Do I have a device on one end? The way I was quickly shown was with snips, but with no explanation. Would this be helpfull when I hear tone coming from all four pair (bleed-over)?

Why it's done: to make sure the pair is not bad. Can I do this on any cable? Coax too?

What application: Any non-category rated cable testing.

 

[RSKing]

example:

You have a four pair 24AWG communications cable, the pairs are color coded blue/orange/green/brown. It measures 100-feet in length. You are going to use the blue pair for a telephone and want to just test that pair.

connect both of the blue pair together at one end (directly, or via a conductor such as scissors).

24AWG UTP communications cable has a resistance of 100 Ohms per 1000-feet. The cable is 100-feet long, but you shorted one end which makes the circuit length now 200-feet when measured from the end not shorted.

Attach a multi-meter to the two wires at the end not shorted (set the meter to measure resistance in Ohms). You should see approximately 20 Ohms plus/minus.

1000-feet = 100 Ohms
divided by 10 = 10 Ohms per 100-feet
circuit is 200-feet
10 Ohms x 2(100 ft units) = 20 Ohms

A radical variation in this number would indicate a problem with the pair being tested (ground, short, etc.)
Hope this helps.

Richard S. King, RCDD/LAN/OSP
TKG Consulting Engineers, Inc.

 

[wires]

"24AWG UTP communications cable has a resistance of 100 Ohms per 1000-feet"

I just found a new source for low value resistors...

The impedence of UTP is 100 ohms. The resistance is more like 28 ohms per 1000 feet.

Shorting pairs is commonly used as a "sanity check" when locating cables using a toner (or TDR). The tone probe is many times outfitted with a metal tip to allow easy shorting of the pair which will result in the continuity light going on the end with the toner.

Simply put shorting (and opening) and pair of conductors acts like a switch. This allows the person on the other end of the wire to verify that both of you are looking at the same pair of wires. The person on the non-shorted end can be using any device that can "show" continuity, like a battery powered testlight, multimeter, pairscanner or TDR.

 

[chiefred]

Thanks Richard

It is beginning to be clearer to me, when my partner asked me to shortout the pair--he had his toner on the other end. I recall the tone stopped when he shorted the pair--so that must have meant there were no cuts in either wire, right? (He did not have a multi-meter.) Is this a common practice or something that one or two might use-----Is there a better way, without a multi-meter I mean?

 

[TouchToneTommy]

When two technicians are testing outside plant, one will ask the other to short the pair at his end. Then with an ohm meter, you can read the total loop resistance, and knowing the expected distance, and the ohms per foot for the type of cable, calculate what you _should_ see on the ohm meter. If it is wildly out of range, there may be a "dirty" splice along the way, adding series resistance to the loop which will result in imbalance and static. In this situation, it is time to break out the TDR to find out where in the loop the problem lies.

When testing in house cable, connect your toner to one end of a pair, and when you find the tone at the other end, put a short on it. If the tone stops when it is shorted, the pair has continuity all the way through. If the tone continues, one side of the pair is open.

A couple of days ago, I had an electrician run a Black, Filled, underground cable from the 2nd floor suite to the Telephone Equipment Room. When he punched down the end in the suite, he lost the twist of the pairs. On filled underground cable, there is no tracer on the wires. There are 5 white wires, and there are 5 blue wires. If you lose the twist, you don't know which Tip goes with which Ring. I punched down the Telephone Room end, (properly, of course), and put a short on EACH pair. Then using the continuity function of my toner, I was able to sort out which Tips and Rings made up the pairs, and repunch his work.

 

[chiefred]

Bingo---thanks TTTommy! That made sense. I am not much on a multi meter or no ohms from ahms but the shorting out procedure you described was the dexcription I was looking for. Sorry to the rest that tried to explain it to this Knuclehead. Now I feel confident that I can teach it to someone else so it becomes another usefull tool!

 

[Servamatic]

TTT

Did I understand you right, you terminated OSP cable that went from a 2nd floor suite to a telcomm room? Did it come in from outside?

Richard S. Anderson, RCDD

http://www.smartbuilthomes.com

 

[TouchToneTommy]

Yes I did terminate it, NO I did not install it. The (idiot) electrician ran it, and I disavow any responsibility for it.

 

[AvayaNovice]

Look at Servamatic, ready to jump on anyone with the word OSP cable in ISP.

Sigh.

 

[daronwilson]

wires thanks for pointing out the impedance issue, I go through this often as well. Just because impedance and resistance are both measured in ohms doesn't mean they are interchangeable.

AvayaNovice I suspect Richard's concern with OSP is the fact that there are limitations on what you do with underground PE rated cable inside the building. It's clearly a NEC violation to have that stuff running more than 50 feet inside the building unless its in metal conduit. It just isn't a rated cable for inside, I think that was the issue.

Shorting pairs - we do this quite a bit in the field, and especially in alarm work where we just need a closed loop to make it work. Most of my toners will light the LED when the pair is shorted. So, we throw a tone one at one end, go to the other end and listen. When we find the wire, we 'short the pair' and make sure the light comes on at the other end. It is primarily just a DC continuity test of the circuit.




It is only my opinion, based on my experience and education...I am always willing to learn, educate me!
Daron J. Wilson, RCDD
daron.wilson@lhmorris.com

 

[RSKing]

Wires,

Thanks for correcting my impedance/DC Resistance error. I am now free of the gravitational pull of the 'Stupid Star'... at least until I start typing without engaging my brain again

Richard S. King, RCDD/LAN/OSP
TKG Consulting Engineers, Inc.

 

[AvayaNovice]

Yeah, I know why he said it -- but it obviously wasn't something that TTT did, which is why he was pointing it out in the first place.

 

[TouchToneTommy]

But as long as everyone is looking for a "standards violation", the electrician DID put it in EMT from the 2nd floor suite down to the MPOE room. He was just an idiot for using black filled cable in the first place ("it was the only 25-pair I could find&quot instead of just running 6-Cat-5's instead (since he already had the boxes of cable), and not tying off the pairs before he removed the icky-pic just make my day. Oh well, we're charging by the hour.....

 

[Bobg1]

One more thing on the "OHMS"
Using ones basic ohm meter you can easily measure resistance.
Measuring impedance on the other hand is another story since you are now working with several factors being involved (capacative and inductive reactance).
The moral of the story is don't expect your basic vom to read impedance properly on cable.