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TUCoPS :: Phreaking General Information :: loopcur.txt

Loop Current


               from Mike Sandman...Chicago's Telecom Expert

                       Copyright 1995, Mike Sandman

                  Contact info...Voice: 630-980-7710


Loop Current is the amount of electrical energy flowing through the
telephone and line, as opposed to the voltage which is the force behind the
energy. There is a definite correlation between the Loop Current and Line
Voltage (Ohm's Law), but the current is often the part that shows the
problem... not the voltage reading. The carbon transmitter in the phone has
been the controlling factor for years, since it needs over 20ma to sound


I was made aware of the loop current problem by Don Jordan of Newmont Gold
Co. in Las Vegas, NV (a long time phone man), a couple of years ago. I
didn't have a clue what was causing these problems before that, but as soon
as he explained it to me it was like a light bulb lit up, and I started
remembering all of the weird cases that I couldn't fix, that were probably
loop current problems.

Until about the mid 1980's, the big problem with loop current was that it
was too low. That was when the only way to get from point A to point B was
a pair of copper wires. Now, with the proliferation of electronic Central
Offices and electronic pair gain equipment: T1, Fiber Optics, Remote
Central Offices and SLICs in every suburban and even rural area (you can
see these Huts and Buried Vaults scattered around everywhere), over 90% of
the problems are high loop current. This is because the manufacturers of
the "far end" pair gain equipment have adhered to a very old specification
for loop current, but one that is still valid, that says between 23ma and
120ma are OK. When the phone company tells you that they are within specs,
while the smoke wafts off your trunk cards at 80ma of loop current, they
are right! The phone company, and the equipment manufacturers have no
incentive to bring the loop current down. All they have to do is make a
standard 2500 set work (which has no active electronic circuitry to burn
up), and the farther out it works... the better. Until the FCC sets a new
standard for high loop current, or CPE manufacturers take account of the
high loop current problem, the interconnect company will be left holding
the repair bag for these problems. Most CPE equipment was designed based on
low loop current problems... it works well right down to 23ma. CPE
manufacturers have been really slow to try to head off this problem. In the
rare case of low loop current, the phone company is required to bring it up
to 23ma.


Phone wires used to be much bigger than the 24 gauge standard today, to
carry the current the long distances that needed to be traveled from the CO
(Central Office). There is a definite loss of electrical energy per foot of
copper wire. The closer the CO got to the subscriber's location (because
there were more and more COs), the less there was need for the current
carrying capability of the bigger wires. Since the 1960's, phone wires have
gotten smaller and smaller, except in rural areas where the distance to the
subscriber still requires the bigger wires with larger current carrying
(and lower loss) capabilities (keep in mind that some very rural CO's
operate on their own totally different voltages and currents to get the
phone signal to their subscribers). At the end of a long loop, the current
that left the CO at 35ma might be 18ma because of wire loss and
capacitance, and the audio level may be well below -8.5db causing it to be
hard to hear. This current (and voltage) can be given a boost by the phone
company by putting a Loading Coil into the loop, usually on loops over 3
miles. The coil uses inductance to increase the audio voltage (sound level)
and current at voice frequencies, while rolling off higher frequencies. If
you need to use the higher frequency capabilities of the phone line for
high speed data, you may have to get the loading coil removed to allow
those higher frequencies to pass through (although the voice capabilities
of the line may then be unusable due to the distance). Data circuits have
always been adjusted 13 db below voice levels. It seems that high speed
data increases the loop current on its own.


The same phenomenon that lets high speed data work on a long loop can also
cause a problem if the loop current is too high. Generally speaking, loop
current above 27ma will cause problems ranging from intermittent garbage to
"no connect". It's going to be different in every case, so you have to keep
loop current in mind as a possibility when you have a problem. I have been
able to fix some of these problems by using a different brand modem, but
it's not convenient to carry around a box of modems for testing. A customer
called me about a location with an AT&T System 75 switch with single line
stations, where the users were having trouble connecting or getting garbled
data intermittently with laptop computers and modems connected to the
single line station ports. The loop current wasn't super high, I think it
was in the 35 to 40ma range, but the installation of Loop Current
Attenuators at the frame for those stations using laptops, solved the data
problems by reducing it below 27ma. Loop current problems are not limited
to CO lines, they also can occur on PBX or Key System single line station

If you're going to be installing your own T1 equipment, verify that the
loop current is down at a reasonable level before committing to it, or
you'll be burning up your phone system.

Since fax transmissions are just high speed data, it's common for high loop
current to cause problems with fax machines.

Keep in mind that some station ports are 24VDC, instead of the normal
48-50VDC voltage of a CO line. As far as I've been able to tell, the same
loop current specifications apply to the lower voltage... between 23 and
35ma, but I do know that the dip switch settings on our Loop Current
Attenuator are sometimes too "coarse" at the lower voltage.


Most phone systems work OK with the loop current in the 23 to 35ma range,
although there are some that will cause problems above 27ma. One that comes
to mind is the Walker Poet. One of our customers reported that he had
intermittent crosstalk, even after replacing everything - KSU, cards and
power supply. Walker tech support suggested that he check the loop current,
which was found to be 32ma, normal in most cases. Walker said to get the
current down to 27ma or less, and the problem was cured. In one case on
another type of phone system, crosstalk was repaired by replacing the trunk
cards, but the problem kept reappearing. Constant high current would
eventually break down the trunk card and crosstalk would return.

There have been numerous reports of memory failures in equipment like
dialers, fax and answering machines.

I've had quite a few reports of Panasonic 2 line phones getting damaged by
loop current over 40ma. It usually effects just one line on one phone
first, but you will eventually replace all of the phones. Many CO line
(Trunk) cards are burned up on all types of systems every day... some under
maintenance and some billable. I've heard of other system cards being
damaged also. In some areas, the loop current is running in the 80 to 105ma
range (at that point there's no way you can make money with a maintenance

I've had a number of calls from Long Distance dialer providers whose
dialers had all kinds of strange problems with high loop current. Likewise,
the payphone industry seems to be hit hard due to the sophisticated
electronics in their phones, and the high speed modems they use to transmit
call detail data.

Symptoms of High Loop Current Include:

*  Burned Out Key, PBX or Data Equipment

*  Garbled Data and Modem Failures

*  Cut-offs & Squealing on Lines

*  Crosstalk, Echo & Hollow Sounding Lines

*  Numerous Intermittent Circuit Failures

* Off Premise Equipment Problems

Symptoms of Low Loop Current Problems:

*  Poor Voice Transmission Quality & Low Volume

*  Ghost Rings

*  Wrong Numbers

*  Lost Calls During Transfer

* Data Loss

Considering the problems that loop current can cause, if you have weird
unexplained problems you should probably go right out and take a reading on
the lines... it's easy! Dave Williams of Frontier Communications in
Belgrade, MT says he is checking the loop current on the CO lines on every
system he installs, which will head off customer disputes. It's not cheap
to bring down the loop current in many cases, the main dispute being who
pays for the repair (everybody points at the phone company first, but your
finger will fall off before you get them to pay for it!). If there is low
loop current, below 23ma, the phone company is responsible for getting it
up to 23. I've talked to guys in rural areas who said that the phone
company told them they just couldn't get it up to 23, and I'm mot sure what
you can do about it. You may have some recourse through your state's
Commerce Commission, but who has the time for that?


Measuring the loop current is easy. You need a loop current meter designed
to test telephone line loop current, like the Triplett Model 2 or 5 (which
also reads voltage and db loss - $200 to $600 new), or a good quality
DIGITAL volt-ohmmeter with a milliamp scale. An analog meter, except one
designed for telephone work, will usually not give you accurate readings. A
Fluke, or a few of the Radio Shack meters will do a fine job. If you have
purchased our HAC Tester (Hearing Aid Compatibility), that meter will work
fine. If you didn't get a set of slide-on alligator clips with your meter,
call me and I'll send you out a set for free. Our Ziad PHD Butt-Set with
DTMF Analyzer, also displays loop current. Radio shack makes a couple of
meters that will measure audio level in db, which would be nice if you
don't have a db meter already. If you don't use our Loop Current Attenuator
and try to use resistors to bring the current down, it's helpful to watch
the db loss as you increase the resistance, so you don't get it too low to
hear properly (try to stay above -7db, below that you will have a hard time
hearing on long distance calls). I'll talk about measuring db loss a little

To measure loop current, you need the meter in series with the telephone
line. To measure voltage, whether the DC line voltage or the AC audio
voltage, you need the meter in parallel (across) the telephone line. You
really don't want to make a mistake in your hook-up as the readings would
be meaningless - and could cause you to take an improper action. Make sure
you meter is set on DC ma, not AC!

Here is the hookup to measure loop current. If you have bridging clips on a
66 block, just open one of them, and connect one lead of the meter towards
the CO, and the other towards the equipment. It doesn't matter which color
lead goes where since the meter will display a + or - that you don't care
about (except on an analog meter which will make the needle go backwards
and get damaged).

With the line on-hook, you should see almost no current flowing through the
meter (it will read maybe .01ma). When you go off-hook with either the
phone equipment, modem or your butt set, you should get a reading of
between 23 and 35ma if the line is OK. One customer said that they only got
an accurate reading using the actual system trunk to pick up the line,
using the but-set showed lower readings. Repeat and record the readings for
each line, since not all of the lines have loop current problems in many


If you have a high loop current reading, you have three choices:

*  Do nothing

*  Put resistors on the line, keeping in mind that too much
             resistance will lower the volume of the line too much

*  Put a commercially available Loop Current Attenuator on
             the line

The only two choices are obvious. Most opt for the commercially available
Loop Current Attenuator. I sell both a modular version which can be plugged
inline with the RJ11 either for testing - or for good, and a non-modular
version which can be punched or screwed down. You can also punch down the
modular version, which will cut off the mod ends.

The Loop Current Attenuator has a set of four dip switches, which you flip
one at a time while watching the loop current come down to below 27ma on
your meter. At that point you just leave it, and it's fixed. You do need to
buy one Attenuator per line that has high loop current, which can get
expensive, but this unit is the only one that will not make the audio level
(db) of the line unusable. I've actually flipped the switches on the Loop
Current Attenuator while watching a thousand cycle tone from the CO, and
the audio level actually went up a hair all the way down to 20ma... that's
pretty amazing! Don't mistake the Loop Current Attenuator for an audio
attenuator (pad). Those devices are sold to reduce the level of a line,
usually by a fixed amount printed on the device, when the audio level of
the line is too high (like for data).

Most phone guys don't like playing with resistors, but if you'd like to
try, here's what you've got to do - and it will work in some cases! As an
example, the TIE CX/HX family of phone systems includes all kinds of keys
and hybrids like the 818, 1232, 1648, 2056, 2464 and 32128. If you have
high loop current, you will often hear a squeal when you conference two
lines together. Back in 1985, I had a few systems with that problem, and
TIE's fix was to replace the bridging clips with 100 ohm (or whatever
worked) resistors. It worked like a charm on every system, and I just used
Radio Shack 100 ohm 1/2 watt resistors in all cases. I figured that putting
resistors in series with the line couldn't help the volume of the line, so
I tested it with a phone at the KSU by listening to the thousand cycle
tone, and using alligator clips to jump out the resistors. I couldn't
notice a big drop, so I figured it was OK. In some cases of high loop
current, the audio level is already high, like -4db, so you have some room
to fool with resistors before the line isn't loud enough. If you are
already at a -6 or -7db, you really don't have any room to play with the
audio, and you should probably go right for the Loop Current Attenuator
which has some kind of network to keep the level up. At the time, I wasn't
smart enough to think about what I was doing, or why I was doing it, so I
didn't draw the correlation between that problem and others that I was
having. I could kick myself now!

What I've seen when playing with resistors myself, is about a 1db drop in
audio level for every 100 ohms put in series with the line. Please note
that this means to use 2, 100 ohm resistors - one for the tip, and one for
the ring side of the line. This will keep the line in balance. A phone line
develops noise and hum on it when it is unbalanced. In telephone work, each
side of the line - the tip and ring - must be exactly the same length. A
difference of even a few inches could cause noise, and 100 ohms is probably
like a couple of miles of cable - you would hear a loud hum on the line! In
the simplest case, if the tip side of a line was shorted to the tip side of
another line, the tip side would be exactly twice as long as the ring
side... causing an imbalance and hum. On a wet cable, water shorts one side
of the line to ground or a foreign voltage.


One of the more common complaints that a telephone man hears is "Can't
Hear". It may have been on only one call to Zanzabar, but here I am on a
service call and I've got to talk nicely to the customer. One of the
problems with this service call is that I'm usually pretty powerless to do
anything about it anyway, but I can't tell that to the customer.

Here's a little information that will let you actually do some testing, if
you think there really is a problem. The ideal level for a telephone line
loop loss seems to be about -5db. Local calls aren't too loud, and you can
still hear OK on intercity and long distance calls which have a lot of loss
compared to local calls.

The first thing you need is a db meter. Either use one designed for
telephone lines, in which case an analog one is OK, or get a digital unit.
The impedance of the line, be it 600 or 900 ohms seems to make a difference
to some extent, and meters like the Triplett Model 2 or 5 are made for this
use, but I've found that the Radio Shack digital meters that have a db
scale are OK, especially if you compare it to a "real" meter to get an idea
of what kind of a difference in readings you will see. One you use it on a
few jobs, you'll get a feel for what you're looking at (I'm cheap). Our
ZIAD Linemaster DTMF Analyzer also has a digital db circuit loss meter that
reads in .5db increments.

When you are measuring the db loss of a line, you are reading only the AC
component - the audio - of the phone line. At the same time, there is the
DC talk battery component of the phone line of about 48VDC. Of all
measurements, the DC voltage seems to be the one that you really shouldn't
worry about (the voltage will follow the current... according to Ohm's
Law). As I stated earlier, you connect the meter in parallel (across) with
the telephone line to read db. When you are trying to attenuate loop
current by using resistors only, it's imperative that you have two meters,
a meter to read the loop current and a meter to read the resistors' effect
on db loss so you know whether it's gone too low to be usable. You could
use the same meter by changing the lead configuration and range, but that
gets time consuming and frustrating. Since loop current measurements are
not usually exactly the same line to line, you should repeat the process
for each line, assuming anything is going to come back and get you!

The ideal db loss to look for is -5.5db, when calling the 1000 cycle test
tone from the local CO. If you use the tone from another Central Office,
you will not be able to tell the real loss on the line, since there is an
additional loss from CO to CO. I believe that the phone company is required
to give you the correct phone number to use. In Chicago, I got a number
from COG, for a guy that sits in an office and gives out these numbers all
day long. He also has the numbers for silent termination, but CO specific
silent termination numbers aren't important - you just want to have a nice
quiet line to see if there is static or noise etc. on the local loop. They
used to have ring back numbers here in Chicago, but they've done away with
them years ago (they sure were handy for checking ringing!). That same
ringback number had a couple of other neat features... it would beep once
if you dialed the touchtone numbers correctly from 1 to 0,*,#, and had a
rotary dial speed test built in. These features aren't very important today
since electronic dials seldom go off frequency.

To see the effects of using the wrong test tone, dial up the test tone from
a CO across the city. Then dial a test tone farther away, and then long
distance to another city. I've been able to check the loss of various long
distance companies by calling the same test tone (like in New York),
through AT&T (10288), MCI (10222), Sprint (10333) etc. You usually get a
couple of db loss across town, and 5 or more db loss long distance... which
is fine as long as the line started at a -5.5db. If you start with a line
at -8.5db, which is the minimum that the phone company has to provide on a
"line", that extra 3db loss makes it very hard to hear long distance.

There are two classes of phone lines that you can order from the telco. A
"Line" must be maintained by the phone company at -8.5db or greater. A
"Trunk" must be maintained at -5.5db or greater, and can be either loop or
ground start. I've had the phone company switch over quite a few customers
with lines that were below -7 or -8db, to trunks. The phone company then
re-engineers the lines (adding loading coils or whatever) to bring it up to
-5.5. That 2 db can really make a difference if you talk primarily long
distance. It's a real strain, to strain to hear all day. Amplified handsets
and headsets help, but the increased gain from the amplifier also amplifies
line noise, so it's often still hard to hear. A "Trunk" is usually a few
bucks more per month than a "Line", plus the ordering and installation
charge if you are having it changed from a "Line".

A number of guys have asked if there's anything they can put on a line to
bring the volume up, especially on lines that are conferenced so the
outside parties can hear each other. I don't really know of anything that
works great. (I usually order 3 way calling from the CO for my customers,
they LOVE it!). The only two wire amplifier I'm aware of (that works OK) is
the Reliance (or R-Tech or something) VFR5050. I've used it a few times on
OPXs, although it's expensive and time consuming to install. I believe that
it only amplifies in one direction. I tried another unit that was cheap and
easy to install, but it didn't work at all! The VFR5050 is on a small card,
so you need to buy a card cage with a power supply. I think it works in
those card cages that the Long Distance companies install for their packs
(with 25 pair cables and all), so you might be able to use an abandoned
unit that's lying around (I'm not sure it works). A nice thing about using
the correct card cage is that the line gets passed thru automatically if
the card is pulled (an option?).

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