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Line Loop resistance.

Started by jjacob, November 14, 2013, 02:51:55 PM

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jjacob

Need some education from our more knowledgable forum members.

In the schematic for my AE LPB-82-55 "NOTE 3" states "On line loops of less than 500 ohms, disconnect relay contacts..."

Can someone explain to me exactly what Line Loop resistance is, and also what would be the proper way to measure it?

I did do a search and there is some information on the forum. Seems to refer to the DC line current? Current would be amps, but this note refers to resistance (ohms).

Not sure if this helps but I am 3.3 miles from the CO and I am literally the last house that can get DSL.

I have a VOM meter so can measure voltage, amperage and resistance.
Attached schematic is from the TCI Library.

John

unbeldi

#1
Every piece of wire has a certain resistance to the flow of electric current. The longer the wire, the higher the resistance. Therefore the farther your location is from the central office the higher the resistance of the wire loop (local loop) is.

Loop length effects transmission loss and distortion.

Voltage, current, and resistance are related by Ohm's Law:  Voltage = Resistance x Current.  So on long loop you have less current available for signaling and talking.

If you knew the voltage at the CO (probably close to 48V-52V) you could just produce a short on your line with an ammeter and measure the current, or measure the voltage across a standard resistor of, say, 600 ohms, connected to your line. Line current will probably be 20-100 mA when shorted.  20mA is the lower limit when a typical transmitter stops to function properly. This limits the maximum loop resistance of a functional telephone line to around 2500 ohms.

This is for direct current.  At voice frequencies the loop also produces a capacitive reactance which varies with the frequency of the signal.

G-Man

     Loop resistance is the combined resistance of the two-wires coming from the central office to your telephone.

    Assuming that the telco is delivering your dial-tone over a 22-gauge pair to your premises, your loop resistance would be roughly 576-ohms

     At only 17,424-feet from the central office, loop-current or transmission is not a concern.

  It serves no useful purpose for your application so I would disconnect it since your paystation is not being used with a paystation line adapter at the c.o.

     Unless you observe it to be problematic in some way, either leave it alone or disconnect it if you wish.

jjacob

Thanks G-man and Unbeldi.

For what it is worth I measured the voltage at the network interface and it is 52 volts.

John

jjacob

Please correct how I have measured voltage and amperage below if I am wrong. I am just curious.

I put my leads between Tip and Ring at the network interface and I read 52 volts.
I put my leads between Tip and Ring at the network interface and I read 33.5mA (0.0335 amps).

V= Resistance X Amps
So the resistance = 1552 ohms
Is 1552 ohms my line loop resistance?
Is that correct?

John

unbeldi

#5
Quote from: jjacob on November 15, 2013, 12:40:24 PM
Please correct how I have measured voltage and amperage below if I am wrong. I am just curious.

I put my leads between Tip and Ring at the network interface and I read 52 volts.
I put my leads between Tip and Ring at the network interface and I read 33.5mA (0.0335 amps).

V= Resistance X Amps
So the resistance = 1552 ohms
Is 1552 ohms my line loop resistance?
Is that correct?

John
Yup.

Technically you should read the voltage at the CO where the battery is, because there is a voltage drop across the line in the measurement which must be considered when the resistance of the voltmeter is appreciably low. However with today's high-resistance digital voltmeters, the current drawn from the line by the measurement is so small that it can be neglected. If you're using an old analog meter this needs to be reconsidered and you need to measure the current through the voltmeter at the same time and use Ohm's law to figure out the voltage at the source.

The ideal voltmeter would have infinitely high resistance and the ideal ammeter would have infinitesimally small resistance.

dsk

I'm not sure how you should measure that exact, My line is compensated in some way, I measure 29mA at shorted line. (only A-meter) and I measure 39mA when putting in a resistor measured to 148 ohms.
http://tinyurl.com/nytmoan

dsk

G-Man

Your measurements indicate that your telco is using a very fine gauge to deliver dial-tone to your home. Here are the (nominal) resistances for various gauges based upon the 3.3-mile figure that you provided.

22-gauge = 576-ohms
24-gauge = 894.548-Ohms
26-gauge = 1,422.146-Ohms

Quote from: jjacob on November 15, 2013, 12:40:24 PM
Please correct how I have measured voltage and amperage below if I am wrong. I am just curious.

I put my leads between Tip and Ring at the network interface and I read 52 volts.
I put my leads between Tip and Ring at the network interface and I read 33.5mA (0.0335 amps).

V= Resistance X Amps
So the resistance = 1552 ohms
Is 1552 ohms my line loop resistance?
Is that correct?

John

Weco355aman

In most of the Bell System the first 3000' was 26Ga
The 3000' was also the first load coil for loop's over 10+k'
Then they would use 24Ga for the rest of the plant.
If you were over 20k' then you would be on 22ga.
I know on one place that had 19ga cable.
From C.O. 26,24,22 and 19ga for very long lines.
Most toll cable was 22 or 19ga.
In to days network if your connected to a 5E2K switch at the C.O Mdf
your line voltage will be -46-47vdc and ringing at best 80vac.
Dms 10/100 will be -52vdc.
If you are served off of a Pair Gain (slc96 type system) you will have -52vdc.
And the ringing will be approx 105vac. Then you add the cable pair to these figures.
The C.O. Power is set at -52.7vdc with the rectifiers on.

Phil

Russ62

Hi, The purpose of the extra contacts on the AE. 2 nickle upper relay is to provide extra current to the main coin relay on long phone lines by shorting L1 and L2 together during relay operation this feature isn't needed for home use, even with a coin relay controller, and seems like a bad idea, so I either disconnect one of relay switch wires at the screw terminal board at the bottom edge of the lead coin track if present. or unsolder one wire from the nickle relay switch contacts if the housing has the newer continuous cable with out the screw term. strip. and tape the end of the disconnected wire. I don't just cut up original wire harnesses. Some times the spade lug of one wire is found already disconnected and placed in little plastic cover, depending where the phone was used.             Russell

jjacob

#10
Russell,

Thanks for that explanation. That explains it for me simply. So should I just disconnect the red wire from terminal 1A as the wiring diagram indicates (note 3) and insulate the spade end? Does that accomplish what you have explained?
By the way, I have also made the wiring change for one nickel operation (note 2) per the wiring diagrams instructions.

Thanks,
John

Russ62

#11
Hi,    Yes, that's exactly what I do in LPB 82's like yours.  In my post I was for some reason thinking of the LPC series which is the next model, even though the diagram clearly shows the LPB 82-55                       Russell