sundance-communications.com/forum Forums General General Telephone & Systems Information loop current

hello. i have a customer who had blown line port one on 2 ksus. the loop current is at 65 miliamps and the tel co says they cant do anything about it. what is the best way to handle this situation. thanks dave

Sandman has loop current regulators. I have used them and they work real well. Sandman loop current regulators

i may have to go that route but at 40 bucks a line i hope to find an alternate solution.

Not unless you can convince the LEC that is too high, I tried for a very long time. Now I buy the regulators. If you'll search on loop current, you'll find quite a bit of discussion on this.

Why not add a resistor or potentiometer?

Okay, I'm gonna bite. Please show us how to wire that in MP.

Actually you can mess around with resistors to lower the loop current just like adding loop length to lower it, but you also lower voltage and volume. The loop current regulators at Sandman are constant and keep the current right at 25 milliamps. Even if you could get resistors to get it right the LEC (at least here) is always messing around and the current changes. How Sandman's work, don't know, don't care, they work.

according to sandman the telco can be up to 125ma and be within FCC specs so I don't think your going to get them interested in dropping it

what kind of equipment is seeing damage at 65ma ?


I carry a half dozen or so of his and install them when needed Ive never had a customer balk at the price

(his early ones had dip switch's and you had to set it up to regulate the current down to 25ma , problem was if the telco dropped then the regulator would drop it a corresponding amount. the new ones just put out 25ma no matter what is going in , like Bill said I don't care how he does it , It works)

The way you would use a resistor or potentiometer (variable resistor) is to just add it in series to the line.

Lets actually calculate what size resistor would work. Presently, since R=E/I, E=48 V and I=0.065 A, it follows that R must equal 48/0.065 or 738 Ohms. That is the resistance of the entire telephone circuit. To get the desired current of 0.025 A, that resistance should be 48/0.025 or 1920 ohms. Therefore you must add 1920-738 = 1182 ohms of resistance to the circuit.

The wattage of the resistor needs to be at least 1182/1920 x 48 V x .025 A = 0.73 W, or as a practical matter, ¾ Watt. You might want to go to 1 Watt.

You can buy a few of these resistors for a dollar or two. The color code for a 1200 ohm resistor is brown-red-red.

If anyone would like to check my calculations, I’d appreciate it. I haven’t done this for a while.

I've used the resistor method to cut down on loop current and not run into any problems.

I was taught that you should add the same amount of resistance to both tip & ring so I always follow that advice.

If that's the way you want to do it Michael, go for it. Like I said previously, it can be done with resistors.

Using the same Ohm's law, wouldn't that lower the T/R voltage considerably?

65 ma is way too high. Watts X amps = Volts So If his ma loop current is high his voltage is also high. It's a proportional inverse relationship to a certain extent.

If im wrong please correct me. I have used the combination of correct resistors but Sandman's unit works like a charm every time. It even gets rid of the Caller Id chirp. I bet the customer is right near the Central Office.

I have 75ma around here pretty common. It's doesn't effect your idle voltage and off hook voltage is about normal also. I know this is another post, but I'll try to briefly do this again. The LEC's used to used reg's to boost loop current to distant locations, they found it is far cheaper to boost loop current for everyone and let us worry about any problems that occur. So 75ma close to the CO translates to them being able to push voice over copper about 10 miles, where as prior to raising the current you had to add regs after about 4 miles if there is no carrier involved. I installed tons of 2a and 5a regs, they are no longer used at all.

dexman is right if you use the resistor method then put a 600 ohm on each side of line this will lower current and volume the Sandman devices are worth saving the hassle

The biggest problem I have seen is these key systems don't have alot of tolerance built into them.

The addition of a resistor or resistors will decrease the voltage between the tip and the ring when the phone is off the hook. However it will not measurably affect the voltage when the phone is on hook. The on-hook voltage should always be right around 48 Volts DC, which is the battery voltage in the phone company's central office.

That is one spec I'd like to see. I called Mike Sandman and spoke to him (very smart man), but I can't get anyone to show me that spec. There is an old graph I found in the ANSI standard which I interpret to read 23 low 45 high. But there is also a part of the graph that can be interpreted to read 125ma and I'm almost sure that is what the LEC is going by. I sent the graph and all supporting documents to Qwest and a few months later I asked them to send proof that 75ma was with-in limits. I got back the copies of the same documents I sent and I mean the same, my notes still in the margin! So like I said, I tried and tried to get someone, anyone to give me the supporting documents and have yet to receive them. OK just search "loop current" for the rest of the story.

well I hope you didn't use my name

have had some good conversations with mike over the years .

so he couldn't give you the spec ?

I just read it on his site and took it as fact , when I find high current Ive never called the telco about it just put on the regulators .

evan if we could get the telco to turn it down I would be concerned it might go up at a later date and start causing problems again

The voltage drop across any resistor is always proportional to the amount of its resistance. Here is a circuit diagram I borrowed from Wikipedia. Now in this figure, R1 is the total resistance from the central office to the phone set on the ring conductor, R2 is the resistance of the phone set, and R3 is the resistance from the phone set back to the central office on the tip conductor.

If you increase R1, then v1 will increase while v2 and v3 will decrease. If you increase R1 and R2 equally, then v1 and v3 will increase equally while v2 will decrease.



Now when the phone is on hook, R2 is extremely high and so v2 is practically equal to v4, the battery voltage. When the phone goes off hook, R2 is generally around 200 ohms, so v2 drops quite a bit, generally to around 5 or 6 volts.

If the sum of R1 + R3 is too low, then both v2 and current will be too high, meaning the telephone set is getting too much power.

Skip, It was sometime ago when I was still trying to convince the lec, so had nothing to do with you. He is a very interesting person to talk to. I don't remember where he said he got the actual information, but he did say he also couldn't get his hands on actual standards to support it. When I was checking it out I got the same info and I also couldn't tell you now from where.

Remember that the loop current you're measuring is OFF HOOK, and that's NOT 48V. It's going to be more around 6 to 12 volts. So actually terminate the line with a 600 ohm load, then measure voltage and current, then do your calculations to arrive at the resistance you want to stick in the circuit.

Then, divide the resistance by half, and put a resistor in EACH side of the circuit - one on the Tip, and one on the Ring. Otherwise, you're going to have an unbalanced circut.

When calculating the Wattage, keep in mind that you also have to pass Ringing current as well.

According to my source, the off-hook voltage should be between 3 V and 9 V.

Also according to my source, a typical telephone will have a resistance of about 200 ohms, although it may be between 100 ohms and 1000 ohms.

The off-hook current will vary depending on the resistance of the telephone or test set used. My Lil Buttie Pro test set displays off-hook current and has an "apparent" resistance of 275 ohms, according to the manual.

In order to test out the line, fist measure the on-hook voltage in parallel to your telephone or PBX. Make sure that it is around 48 V. If not, it must be corrected because there is probably a a break in the line. Then measure the off-hook current in series with your telephone or PBX. This should be between 20 mA and 75 mA. The current is very much a function of the resistance of your particular telephone or PBX.

(Remember not to measure current in parallel with your telephone, or you will blow out your meter.)

If you think 75 mA or 65 mA is too high, then you can lower it with resistors or the Sandman's device.

The idea of using balanced resistors instead of one resistor has a lot of aesthetic appeal, but I am not sure it is sound. Can anyone explain this?

MP:

Does your "source" work for Verizon? If so, I will pay you a lot of money for their phone number (joke).

To answer your question about balanced resistors, since tip/ring is a balanced pair that sometimes travels for miles, it's very important that the individaual conductors be approximately the same length door to door. If this isn't maintained within reasonable tolerances, the pair becomes unbalanced and subject to AC induction (hum).

Load coils are placed every several thousand feet of cable to equalize imbalances that occur over long distances of twisted pair; these bring the pair "back in balance" so to speak. You can't miss them on poles, they are about the size of a trash can, usually bolted parallel to the pole but sometimes attached to the strand (the little ones anyway).

Adding a resistor to only one side of the line could introduce enough of an imbalance to cause hum.

thats a lot of feedback. thank everyone so much. i need to print it out and get to work. thanks again. dave

If you click on the word "source" you can see my source.

I still don't get this balanced line thing. What does "balanced pair" mean? The current is the same everywhere in a loop.

I mean, what is the difference between

------------600 ohm-200--ohm--600 ohm------------

and

--------------1200 ohm--200 ohm------------------

Few things to keep in mind ya’ll… There’s a lot of different signals (both DC and AC) coming down them there phone wires. Most of ya know stuff but as I read down through here seems like some ya’ll get mixed up a bit.

DC current will ONLY be seen when off-hook and should range between 20mA minimum and 65mA. The first is almost a thing of the past now days with cordless phones and key systems out there, is to power the teleset when in use and two is to let the switch know that the teleset has gone off-hook and it now needs to do something about that.

The battery source is located at the C.O. (or remote) is 48 V DC. The only variance to change the DC loop current from the Telco side is the inherent resistance of the conductors that make up the pair (total round trip.) Then there’s also the DC resistance* of the equipment attached to factor in the complete circuit. You always here folks say NEGATIVE 48 volts when referring to the voltage. That’s the way the C.O. batteries are set up as a reference. 48 volts is 48 volts the negative is just seen when you measure it cross tip and ring.
* Don’t confuse the impedance rating expressed in Ohms as the DC resistance… two different things.

Placing resistors in series (with conductor) on both tip and ring will reduce the amount of loop current. To all signals (the AC signals too) these resistors will electrically appear to be a longer loop. Adding a 300 Ohm resistor to each side (600 total) would be ‘bout the DC equivalent of adding 5, 000 feet of cable to the loop. Just fine if you’re ONLY concerned with DC current.

There also a couple different types of AC current superimposed (DO NOT FORGET THE SUPERIMPOSED PART) over the 48 VDC.

Everybody’s favorite AC voltage RINGING! Yaaaaa! 104 V AC supply with 30Hz on for 2 seconds then off for 4 seconds. If enough gets there to initiate the teleset to ring, you’re good. When talking about adding resistors to the circuit as long as the telephone rings and the resistor is capable of handling the current, who cares.

Then the other and most important AC component you’ll see on the telephone line is the one were all here for right. The signal! The voice traffic! 300 to 3000 Hz AC signal of varying voltage levels… We (the telephone company) will not ever measure this in terms of voltage or current. The will me measured and referred to as a dB (decibel) level. Decibels is a ratio and is in reference to a 0 dB.

For frame if reference over 0 dB voice level is a hot signal and at risk of overdriving equipment. 13-14 dB (negative or below the zero refernce) is usable but low. 6-8 dB negative (say that as “6 to 8 dB down”) is right round perfect.

Adding a 300 Ohm resistor to each side (600 total) would be knock you’re voice signal down roughly 4 dB. Is it was low to begin with you’re going to be WAY in the dirt… If you were a bit on the hot side, well… it will talk just fine then too.

at a basic level A balanced pair means the tip is the same length as the ring...when a pair is unbalanced you basically are building a long antenna which will cause noise and transmission issue

88 micro farad load coils are placed in a standard loading scheme 3000 ft from the C.O. and then every 6000 ft after that. They have little to no effect on the DC component being passed down the pairs. They are there to off-set the capacitance that is inherent to two conductors being twisted together for thousands of feet.

They are used to ensure the voice level rolls off (goes down) flat over greater distances. Meaning that a 250 Hz signal losses roughly the same amount as the 2500 Hz as the signal travels through the copper. If those frequencies didn’t stay close to the same the voice may make it to the other end but the characteristics of the voice of the person speaking would be lost.

BTW, Gee whiz stuff, the smaller silver upside down looking trash cans on the poles are usually repeater housings for Digital Data services.

Good info Bryan!

A balanced line is transposed at the end so that any noise on one side is the opposite phase of the other side therefore the noise is canceled out

Dang, this is gonna be fun. Another way to drop loop current. Silicon diodes! I use 1N4004, but anything rated 400 volts or higher and 1 Amp or better will work fine. BUT, (you knew this was coming didn't you), you have to use them the right way. The idea here is that a silicon diode has a forward voltage drop of 0.7 volts DC. The neat thing is, if it is forward biased, (current flowing the right direction), there is NO AC voltage drop. OK, micromicrovolts, but no drop for all practical purposes. The catch is, you have to connect them inversely in pairs so AC (Voice AND Ringing) can do its AC thing. And you have to put a set on Tip and then a set on Ring to maintain longitudinal balance. So, we put 1 set on Tip, and 1 set on Ring, for a total voltage drop of 1.4 volts and AC is not affected and balance is ok. What have we accomplished is lowering the voltage drop across the co network in the phone system. Lower voltage = lower current. What it looks like electrically, is a DC only resistor. The messy part is, where do you put 10 or more diodes for EACH line. 66m blocks work well, but build in the shop so that it looks neat and pretty. It really does work, but I would use Sandmans stuff if I had to deal with high loop current today. He wasn't around 20-25 years ago. But, if $ is a problem and time is cheap.....

John C.

I guess what you're saying is

/----|<-----\
\---->|-----/ or

/----|<-----\ /----|<-----\
\---->|-----/ \---->|-----/ or

/----|<----|<-----|<----\
\---->|---->|----->|----/ on each side.

Is that right?

i did get some resistors a little lower than 600 ohms. put one on each side before the ksu. it brought the current down to 30ma and seems to have good volume and rings in ok. going to let the customer use it a day and if everything works well i will do all of them. thank you for all the feedback. dave

Thanks for reporting in. It's nice to know that some of the "science" we learned in school actually works in the real world.

I am curious how you connected these resistors in. Did you use a beanie or a scotchlock? A picture would be great!

i used ur connectors on the c o end. they worked fairly well. if not careful they can shift out of the channel. i punched the other side at the ksu 66 block. the resistor hides neatly behind the plastic guide but the resistor wire doesnt cut real well with a 66 tool. i did end up with a row of 16 ur connectors next to the block.

MP, you got it exactly. I don't have a scanner, so I was at a loss as to how to show a picture. It's astounding what you can do with all the little characters that I ignore!
John C.

From a post I made on the Avaya Forum
Can someone explain the weird voltages?


We did some testing today to try to pin it down and found some weird voltages.

Some lines show 48 to 51 volts Tip to ring where others are around 37 to 38 volts, I thought the standard was 48 volts. The 48 to 51 volt lines show about 0.25 volts Tip to ground where the 37 to 38 volt lines show 7 to 8 volts Tip to ground, I thought Tip should always be close to ground.

I also get about 8 volts AC Tip or Ring to ground and 15 Volts on others.

I should mention that some of the lines are direct copper to the CO about 4 mies away and others come from a terminal unit located about 3000 feet away. When direct copper goes bad Verizon usuly switches us to the terminal. A few times when they have done this they failed to option the cards correctly and left us with either a ground start line or a line that would not cut over on ring, it would wait till the ring voltage droped before the call cut over.

Could the weird voltages be card option problems or is this new stuff 37 to 38 volts and not Tip grounded.

The thing that gets me is the 7 to 8 volts tip to ground on some lines.

On the Loop current subject, I have several Adtran Atlas 550's with FXO cards I have found that at 65ma they will open the loop for about 20 seconds every 2 or 3 minutes or so. I droped the loop to 25ma with resistors and the problem went away. You could pull the card right after it droped and the components where very hot, at 25ma they are just warm. I could have tolerated the loss on the AC talk side as I could have adjusted the Adtran to add some gain to the audio but I could see (hear) no change so I left it alone. Wound up adding 1000 ohms tip and 1000 ring.