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From: (Floyd Davidson)
Subject: Re: High Loop Current
Date: Sat, 02 Aug 1997 19:00:38 GMT

John Schauwecker <> wrote:
>I have been having problems with my modem and when I had the phone company
>come out and test the line they said my loop current was a little high but
>still in specifacations.  I think he said around 55ma.  Is there any device
>I can get to tone down the loop current so I can get better modem

Wow...  I've just used dejanews to read the various responses to
this article (only a couple had shown up on the news servers I
use), and I'm a absolutely aghast at the absurd variety of
suggestions for how to destroy a telephone line.

DO NOT PUT, in series or in parallel, into a telephone local loop:

    1)  Batteries of any kind
    2)  Polarized capacitors
    3)  Diodes of any kind
    4)  1/4 watt resistors (or any other 10 cent resistor!)
    5)  Lamps

With the exception of the lamps, all of the above are a safety
hazard in addition to being very likely to make the phone line

And not to mention none of them are needed anyway because 55 ma
is a perfectly normal loop current and a modem that cannot
handle that amount of current is defective.

Suggestions on how to reduce loop current all need to be weighed
against a few basic details and technical specifications for
telephone lines.  The simple things are that the line is a
balanced transmission line which can have up to 120 ma of DC
current from up to 56 VDC (actually in some cases up to 90 VDC)
and up to 120 VAC RMS (ring voltage) in the way of various
voltages and currents.  Those voltages and currents can be any
polarity and might be applied all at one time.

But to get the point across with better perspective...  if you
hang any of the above devices across the telephone line the
result will be that your device is going to pick a fight with
the battery plant at the telco office.  Telco batteries are just
like yours in some ways...  about 1.5 volts per cell more or
less (the telco ones are more like 2.1 volts).  But...
most telco batteries are two feet tall at least, and with 24 of
them in a string, in a fight with them your little device will
likely get red hot and explode in your face literally.

In particular polarized capacitors (or any cap rated at less
than 250 volts DC working volts), and batteries of any kind,
should be avoided because of the potental for a an explosion.
The other devices are merely a fire hazzard, with the exception
of the lamp, which probably will merely make the line quit

Besides, if you don't want the line to work right it isn't
necessary to put zener diodes, lamps, polarized capacitors,
diode rings, and ten cent resistors on it.  Just strip the wires
back a bit and give them a twist...  it works every time (as
opposed to the other ideas, all of which only have the same
effect about 90% of the time).


     Floyd L. Davidson                           Salcha, Alaska

From: (Floyd Davidson)
Subject: Re: High Loop Current
Date: Sat, 02 Aug 1997 17:49:16 GMT writes:
>jim cornelius <> scribbled:
>: John Schauwecker wrote:
>: > come out and test the line they said my loop current was a little high but
>: > still in specifacations.  I think he said around 55ma.  Is there any device
>: > I can get to tone down the loop current so I can get better modem
>: > connections?
>I would try this to see what it does.  It may cut the current by a few mA.
>Put a 100 ohm 1 watt resistor in each side of the line.  Two resistors
>totalling 200 ohms.  Radio Shaft sells a pack of two, cat# 221-252 for a
>half buck.  If there is any concern for loss of the audio signal, then I
>would bypass each resistor with a capacitor, 10 uF should do it, but make
>sure you connect it up with the right polarity.

If capacitors are used, 1 or 2 uF should be fine and they
must be NON-POLARIZED (plastic or tantanlum caps are fine).

However...  I suspect the only possible benefit from this has
nothing to do with loop current and any improvement will be
caused by a dB or two loss in signal level.  A telephone loop
can have 90ma as a _normal_ condition (and well over 100ma is
not unheard of).  I'd toss a modem that won't handle that and
get something engineered better, but...

On the other hand virtually all v.34 modems will suffer greatly
from hot signal levels.  I suspect that a local loop with higher
than normal loop current will also have higher than normal
signal levels.  Hence, leave the caps out and it might help.
But the right way would be to measure the level, and if it is
too high insist that the telco pad it correctly.

The ideal received signal level would be about -22 dBm (that is
a -13 dBm0 signal at a -9 Test Level Point).  It should never be
more than about 3 dB higher than that, so -19 dBm would be can
be considered the highest acceptable received signal level.
Somewhere between 4 and 6 dB hot there will definitely be peak
clipping, hence a level of -16 dBm is a guaranteed problem.

Most modems can generate some information which probably will
include a received signal level value, but since the modem was
not intended to be a precision measuring device it does have to
be taken as more of an approximation than as an accurate value.
If calls to several different distant end modems (remember that
the transmit modem code be the cause, so don't assume the cause
of high levels on a single specific connection is necessarily
the loop levels) indicate a pattern, then it would certainly be
worth taking some action.


     Floyd L. Davidson                           Salcha, Alaska

From: (Floyd Davidson)
Subject: Re: High Loop Current
Date: Sun, 10 Aug 1997 05:52:09 GMT

[Emailed and Posted] (Mike Sandman) wrote:
>You might want to check out the Loop Current Tech Bulletin at
>Very few modems will work properly at 55ma. As a matter of fact, not
>much modern electronic telecom equipment will work right at 55ma.
>Highest data rates and fewest disconnects on modems would occur under
>27ma.  55ma could actually cook some PCMCIA card type modems.
>Even ordinary 2 line feature phones will develop problems with their
>line select switches after a year or so. After that, they're junk.
>Mike Sandman

Is that sales gobble-d'-gook, or are you claiming it is true
fact in technical terms?  Since you are the only person I've
ever seen make these claims, and you are the only person that I
know who sells a device claimed to cure that particular snake
bite, I'm inclined to think you might be selling snake oil.

I have lines at work that run 70ma of loop current, though most
of them have 40ma.  We have _never_ had a problem with loop
current.  We have quite a few modems too, BTW.  We also have
more in house technical experience with the telephone system
than the average company.  That means when we do have a problem
it is usually corrected by someone who really does understand
what the problem was and how the corrective action fixed it and
we don't often have "fix actions" that are the product of a good
imagination and bear no relation to the problem as is common
when non-experts fiddle with telephone equipment.  Hence my
suggestion that we do not have problems from high loop current
as you state is a rather definitive statement.

We have never had problems with high loop current on PBX
line, answering machine, fax machines,  modems, multiline
phones, or any of the items you suggest all have problems.

I recently posted an article in comp.dcom.modems responding
to one of your cohorts who suggested reading your "Technical
Bulletin" on loop current.  I was not impressed with the
technical accuracy of your BULL-etin, and below is some of
the text from my previous article.

##### begin quote #####

And while reading those "Technical Bulletins" it is good to keep
in mind that their purpose is to sell something.  The amount of
good information in both of the bulletins specified is great, but
you have to be pretty well aware of it already to pick it out
from the significant amount that is pure bull.

An example, from the Loop Current Bulletin:

"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."

1.  If the current was 35ma at the CO it is either going to be
    35ma at the subscriber or (no not "wire loss and capacitance")
    there is a short to ground or between the pairs of _significant_
    proportions to allow only 18ma at the subscriber location.
    The line will either not work at all or will so noisy as to
    be virtually useless.

2.  The  "audio level may be well below -8.5db" will not make it
    hard to hear.  The _design_ level for a full level test tone,
    which is the absolute peak level allowed on a line, is -9 dBm.
    (Technically a "level" can't be "-8.5db", as that is a power
    ratio, not a level.  The term "dBm" means the power ratio
    compared to .001 Watt, and indicates a specific level.)

3.  Loading coils do not boost current or voltage.  They reduce the
    loss.  The difference probably is lost on anyone who is not an
    engineer designing equipment, but it actually is significant.

4.  If removing loading coils from a loop makes it unusable for
    a voice call, rest assured that it will also be useless for
    a data call using v.34 modems.

5.  Data levels are not adjusted to "13 db below voice levels".
    They are 13 dB below test tone levels.   The reason for that
    is because the peak power levels in data signals can be shown
    to approach 12.5 dB above the average power levels (as the
    data signal becomes more random and approaches a Guassian
    Distribution).  The actual level of the signal when speech is
    involved can range from about 5 to 20 dB below test tone
    levels.  Speech at test tone level is very uncomfortable...
    (Get two people on a pair of local extentions on one line,
    and have one talk _loud_.  The other person will hear
    something almost at test tone level.)

6.  The last sentence in that quote is hilarious.  Data does not
    change the loop current, no way shape or form.  And never will.

If someone reads that and gets the urge to run out and buy one
of those loop current reduction devices...  they should check
with me before the all day lollypop is gone because I have a
bridge and some "waterfront" land that will make an ideal pair
of test locations for those devices.

##### end quote #####

Mike I would be very interested in your defense of the statements
made in your BULLetin.


     Floyd L. Davidson                           Salcha, Alaska

From: (Floyd Davidson)
Subject: Re: Loop current and attenuation question
Date: 16 Apr 1998 08:47:31 GMT

GreivAngel <> wrote:
>Kinda related and really stupid: wouldn't stepping up the loop current TOO much
>cause crosstalk?
>Please copy all responses to mail

In a word: no.

For telephone local loops, crosstalk is related to how well
balanced the circuit is.  Loop current does not affect that
balance, even if excessively high.

The detrimental effects of excessive loop current would be
distortion caused by saturation of transformers ("repeat coils"
in the vernacular).  Within the range of acceptable loop current
(up to 120ma), no transformer used in a telephone equipment
should become saturated.  If an inferior transformer is used, or
if loop current were significantly higher than 120ma, then
distortion could be expected.  Neither situation is common, and
in fact I have never seen either happen.

Crosstalk is rare on digital facilities, but when analog
transmission facilities were common it was not rare at all to
hear crosstalk caused on those facilities by hot levels.  The
most troublesome systems were small, relatively narrow band
microwave radio systems designed for less than 600 channels of
analog L carrier.  The power in a single channel is a
significant portion of the total, so a tone that hot on just one
channel can cause significant problems.  That is as opposed to a
hot level on an 1800 channel system, where a single channel
would go into peak limiting long before there is enough power to
overload the entire system.

The FM modulator in analog radio systems has a characteristic
that when over driven there is a point (rather sharply defined
too, and is referred to as a "knee") where signal distortion
suddenly begins to rise much more rapidly with a further
increase in input.  One effect of an overload is crosstalk.
That commonly results in very distinctly audible traces of other
conversations being heard, but more commonly it results in lots
of strange whistles and squeals known as "birdies" being heard.

In the late 1970's and early 1980's I maintained a 300 channel
microwave system into Eielson AFB, Alaska.  On that system were
private line circuits for the FAA, and also circuits which
originated (via a very old USAF owned microwave/carrier system)
on a remote bombing range known as Blair Lakes.  It was not
uncommon for the FAA to complain of birdies on their shoutdown
lines between the area control towers (which means the
controller has to work with this thing whistling in his ear all
the time, or else turn down that circuit and resort to alternate
comm channels).  That complaint always meant the Blair lakes
lines had been mal-adjusted (it's a remote site, and the
personnel there aren't telephone people, but they are bored),
usually by having the gain cranked up, and when the perpetrator
then put the phone on hook the circuit would break into
oscillations which sent an excessively high level tone to the
300 channel radio system, and caused it to be overloaded.  As
luck would have it, out of 300 channels, only about 3 or 4 were
affected, and 2 of those where FAA circuits!

Hence, if the control tower heard birdies singing, I'd plug off
the telephone lines to the bombing range...  And I never did
decide who would be more annoyed, the FAA controller or the
remote tweeker.


Floyd L. Davidson                      
Ukpeagvik (Barrow, Alaska)

From: (Floyd Davidson)
Subject: Re: Loop current and attenuation question
Date: 18 Apr 1998 02:51:47 GMT wrote:
> (GreivAngel) wrote:
>> Kinda related and really stupid: wouldn't stepping up the loop current TOO much
>> cause crosstalk?
>High loop current has been the culprit in *some* cases, the reason being that

It is difficult to imagine such a case.  Consider that any
repeat coil used has to be able to handle ringing current as
well as loop current.  Any component underspecified enough to
saturate within the normal range of loop currents (ie, up to
120ma) would be severely deficient in other ways.  I am not
inclined to expect anyone to encounter such a part in equipment
actually sold for use with real telephone lines, if for no
other reason than the interface has to be FCC certified.

>the more the current the greater the magnetic field and the possablity of
>transformer action to ajacent circuits, this can of course be midigated by
>insuring that all pair in close proxcemity have a good tight twist (cat 3 or
>better) Quad or residental wire is not sutable.

Non-twisted pair is not suitable, but CAT3 is actually overkill
for telephone voice circuits.  In any case the magnetic field
from loop current is 1) not varying and hence there is no such
thing as "transformer action", and 2) is greatly less than
normal levels from 60Hz powerlines and would be the least of
the two if another cable is unbalanced enough to have a crosstalk


Floyd L. Davidson                      
Ukpeagvik (Barrow, Alaska)

From: (Floyd Davidson)
Subject: Re: How to tell if a line is in use ?
Date: 10 Jun 1998 13:56:12 GMT

John Rice <> wrote:
> wrote:
>>When the line is on-hook it has less than 20ma of loop current,
>>and when off-hook it has more than 23ma of loop current.  Those
>>are precise figures (for the USA), and constitute an exact
>>indication of supervision.
>Precise ? I don't think so. Highly dependent on Switch Type. Maybe close, in
>the newer Digital Switches, but contrary to popular belief, there are a LOT of
>old switches still out there in service and some of those switches, once
>off-hook, will remain off-hook at 10ma of loop current.

However, that is _precisely_ the definition of loop supervision.
And it would be a very unlikely circumstance where a functional
line draws more than 23 ma of current initially and then drops to
less than 20 ma, much less down to 10 ma.  The fact that old
switches would stay off hook at that current isn't really very
important. However, a practical design for an idle line detection
circuit might well use 10-15 ma as a lower limit and 20 ma as
an upper limit, which would result in a extremely reliable

>>Depending on the resistance across which one measures, the
>>voltages stated above will roughly be found.  However, since a
>>given telephone set can be designed with more or less resistance
>>than some other set, and since the resistance of the cable
>>involved can be significantly greater than the resistance of the
>>telephone set itself, measuring voltage from tip to ring on a
>>telephone line is a very unreliable way to determine line use
>If the loop voltage is under 30V, it's pretty unlikely that the line is idle.

If the loop voltage is 24 volts instead of 48 it will _never_
get as high as 30V.  Odd loop voltage isn't nearly as common
today as it once was, but 48 VDC is not a requirement for loop
voltage.  Another odd occasion would be a short loop that uses a
low resistance battery tap, which will result in both high loop
current (which could be over 100ma and still be reasonable) and
a very high voltage across a relatively high resistance tel set
(which could be as high as 350 ohm).  That would result in a 35
volt drop across an off-hook line that is functioning within

But the point was and remains, that loop start supervision is
determined by current, not voltage.


Floyd L. Davidson                      
Ukpeagvik (Barrow, Alaska)

From: (Floyd Davidson)
Subject: Re: Safe load resistance on line?
Date: 22 Sep 1999 11:46:12 GMT

NO SPAM  <> wrote:
>Floyd Davidson wrote:
>> In article <>,
>> Jared Brosch  <> wrote:
>> >I want to do some testing on a live phone line
>> >and one of my testing units will put a 600ohm
>> >impedance across the lines.  Is there any
>> >danger or implications in doing this that I
>> >am not thinking of??
>> Roughly, you can commonly expect to see 40 ma of loop current,
>> which would be close to 1 watt of power in a 600 Ohm resistor.  It
>> is very unlikely, but you _could_ see as much as 120 ma of loop
>> current... which would toast even a 5 watt resistor!
>Hopefully there will be several thousand feet of loop between the CO
>and his MPOE, which means there should be a few hundred ohms of wire
>DC resistance.  SO if the line can deliver short circuit up to a
>hundred milliamps, then most of the voltage will be dropped in the
>wire resistance.
>If you were doing this on our digital PBX, then you would hit the mark
>very closely.  We can draw nasty arcs on the 66 blocks in the phone
>room, and most lines will give most of the 54 volts DC at several
>hundred milliamps, which tapers off as some kind of protection circuit
>kicks in.  Using anything less than a 5 watt resistor here is asking
>for smoke!

Using anything less that a 5 watt resistor on a regular analog
POTS line is asking for smoke.  Your evaluation comes down to
"the voltage will be dropped", but voltage is not the defining
factor.  Current is.

With a maximum loop current of 120 ma, the maximum power that
will be dissipated in the resistor is .12 * .12 * 600, which
works out to more than 8 watts of power.  (It is not a good idea
to assume that you know the voltage used to supply loop current,
because it is not necessarily as low as 54 volts, even if that
is the most common voltage that will be seen 99.99 percent of
the time.  There are equipments that use 100 volts!)

Clearly if there is any chance that such a line might be
experienced, and if the resistor is will be handling loop
current for any period of time, then even a 5 watt resistor will
not be sufficient, and more reasonably a 15 watt unit would be
the absolute minimum.  Obviously a commercial product would need
to view it in those terms.

For less stringent needs, the common range of loop current is
likely to be less than 60 ma.  That works out to a power of 2.16
watts, which the reason I suggested a 5 watt resistor at an
absolute minimum, with 10 watts being a safer option.

A digital PBX line is an entirely different beast, and putting a
600 ohm impedance across that line would be very unlikely.


Floyd L. Davidson                
Ukpeagvik (Barrow, Alaska)

From: (Floyd Davidson)
Subject: Re: Safe load resistance on line?
Date: 22 Sep 1999 12:40:00 GMT

Joe Golan, RCDD  <> wrote:
> wrote:
>> I want to do some testing on a live phone line
>> and one of my testing units will put a 600ohm
>> impedance across the lines.  Is there any
>> danger or implications in doing this that I
>> am not thinking of??
>> Thanks
>> Jared
>First I am going to assume that you are working with a pots type line
>which nominally has 48VDC across the idle circuit.

Typically it is not a 48 VDC voltage.  It is a 48 VDC battery plant,
which supplies 52 volts to the load under normal circumstances, and
might supply as much as 56 volts.  (And loop current could be supplied
from a 120 vdc power supply, so the 48 volt figure is not necessarily
significant anyway.)

>By applying ohms law to your problem E/R=I, 48/600=8 ma. This current

Bad arithmetic.  80 ma, not 8 ma.

>is well below the current drawn by a typical pots telephone which
>usually runs about 20 ma.

Obviously it is well above the typical loop current.  However,
your concept that 20 ma is a typical current is grossly in
error.  20ma is the point at which the switch marks the
difference between onhook (less current) and offhook (greater
than 20 ma.).  In fact, loop current for offhook is specified at
23 ma or greater, with 60 ma as a recommended high and 120 ma as
an absolute high.  Typical values are 28ma, and anything from 35
to 45 ma. is very common.

> The 600 ohm impedence will look like a
>telephone going off-hook to the central office.

It will only if the loop current is greater than 20 ma.  But it
happens that 600 ohms DC resistance (as opposed to impedance) is
not guaranteed to cause an offhook condition (240 ohms maximum
is guaranteed to do so).  The impedance of a device is its "AC
resistance", and that may not allow any DC current to flow,
which would never be seen as an off hook condition by itself.


Floyd L. Davidson                
Ukpeagvik (Barrow, Alaska)

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