Longitudinal Imbalance (AC Voltage):
When you've tried EVERYTHING
Telephone companies have been dealing with AC on phone lines
since they first started serving the same customers as the electric company, locating
their lines on the same poles as the electric wires. Since phone lines are often built
along the same route as power lines, varying amounts of AC voltage are easily induced onto
the phone lines. This AC voltage usually can't be heard since it's traveling on balanced
phone lines (where both sides of the line are exactly the same distance from the central
office), called "common mode." The same amount of AC is on the tip as the ring, so you don't hear it in your
Butt-set (that has no reference to ground). AC voltage can go as high as 50V before the phone company (in most states) has
to do something to bring the voltage down - mainly for safety reasons (touching 50VAC with
a sweaty arm could hurt!).
The farther a pair for
a phone line travels parallel to a power line, whether it's on poles or
underground (or buried in the same trench to a gate), the more AC will
be induced onto the phone line. The more current that's being used on
the power cable, the more AC will be induced onto a phone line running
parallel to the power cable.
Induced AC voltage will be present on all of the lines in a
cable. Whether you hear it depends upon the balance of the line. An
unbalanced line will convert the induced AC to metallic voltage and current, which you
hear as hum, buzz, crackling, etc.
If a phone line has AC on it, and it's perfectly balanced,
you won't hear the AC on your Butt-set. When you connect a phone system or other
electronic device that has a reference to ground (and is slightly unbalanced) to that same
line, you'll start to hear hum.
When you do hear hum on the line, you are usually hearing a
harmonic (multiple) of the original 60 cycles, since 60 cycles is too low to be heard on
the telephone line.
The phone company has a number of solutions available to them
when they have too much AC on a line with hum. They usually install a big inductor or
choke (a really big and heavy transformer, sometimes called a "humbucker").
the past, the only thing hanging off the phone line at the subscriber end was a single
line phone or an electromechanical PBX that had simple relays that clunked open or closed.
Noise and voltage problems had little effect on their operation. AC voltages of 5 or 50
volts didn't bother them much.
Modern phone systems use electronic components on the trunk
cards. Some of these components cause a small imbalance on the line. This imbalance may
not even cause a hum, but if there is a lot of AC voltage on the line, some AC potential
is created. Now you've got this AC voltage traveling between the tip and ring, which is
something that the electronic PBX or Key System may not be designed to deal with. This 5
or 20 VAC can have adverse effects on the system like lines that won't ring,
false ringing, lock-ups,
cut-offs, Caller ID problems, and who knows what. Even if the line is perfectly balanced, the electronics on
the trunk cards can be stressed by the induced AC voltage on the line all the time.
the AC voltage on the line would be different at each premise having problems, the
symptoms may be different even though the premises have the same exact system.
A common symptom of
high induced AC on a phone line is when Caller ID is intermittent or
doesn't work at all, but a cheap battery operated Caller ID box works
fine. The battery operated Caller ID box has no reference to ground, so
the induced AC stays common mode - exactly the same on the tip as the
ring, and Caller ID works fine. When the phone system trunk port is
connected, it's not perfectly balanced so there's now more AC on the tip
or ring than the engineer designed the system to work at, and Caller ID
might not work.
The engineers in third world countries who design modern
phone systems don't even check to see if induced AC will cause a
problem. The problems occur at comparatively few places, so today's
phone system manufacturers don't want to put any money into making sure
it's not a problem with their system (in other words - they don't know
and they don't care).
One of the factors that makes it tough to troubleshoot AC
problems is that the AC that's induced onto the line may be different at different times
of the day. If there's a big factory on the power line before or after it gets to the subscriber,
and the phone line is running along the same route as the power line, you may see a large
increase in the induced AC voltage when the factory is at peak operation (using peak
power). Maybe the factory uses the most power in the morning, or the afternoon? That means
that you may see strange intermittent problems at only certain times of the day. This is
the worst kind of problem to troubleshoot, but if you know about induced
AC and longitudinal imbalance
you at least have some chance of fixing it. If you don't know about it, you're in trouble.
You'll sometimes hear hum on the trunk on the
phone system when you have a
longitudinal imbalance, but not all the time. It could be just strange
stuff happening that could be the symptom.
The first step is to take a reading of the
AC on the line by measuring from tip to ground, then ring to ground with an AC voltmeter.
Because the phone equipment may have a center tapped transformer which could screw up your
readings, you should open the line before taking your readings. When I measured our lines,
they had about .5VAC (half a volt) from tip or ring to ground. I got the same voltage on
both sides of the line. I measured it first on an idle line, and it read the same with the
pair closed down and also when I went off hook.
Note that some meters just won't read AC on a
telephone line correctly, and there's no way to know whether a particular
meter make/model will or won't until you try it. Most Fluke meters will
work, but even expensive True RMS meters might not. Since you're measuring AC riding on-top of the DC
talk battery, some cheap meters get confused. As soon as you
put a meter like this on a phone line, on the AC scale, the readings will
just hunt all over the place, never stopping, and never making any sense.
We tried lots of meters, and we finally chose one to sell to phone men
that works. You can see information on
this meter, the differences in meters, and how to measure AC ring voltage in our in our Ring Voltage Tech Bulletin.
HERE to see the meter information.
If you have a telephone company type line tester, you can
take a reading of the Circuit Noise and Power Influence. Personally, I've never used these
being an Interconnect phone man. If I hear hum on the line at the demarc with
the equipment disconnected, I just call the phone company and let them fix the problem out in
the field. You should never hear hum or noise on a phone
line at the demarc with the equipment disconnected, no matter what the
phone company says!
Phone company guys use the Noise and Power
Influence tests to tell them that there's a problem, and
when they've fixed it. You may be able to use it just to give you a general idea of
whether there is a problem on the line. There should be "green" areas on these
types of meters, indicating the good range. Our lines had 47dbrnC on the Power Influence
scale, and 1 to 2 dbrnC on the Circuit Noise scale - both in the "green" area.
You may be able to see
if AC voltage is effecting the operation of a phone system or piece of
equipment by running it on a Battery Backup unplugged from
the wall, with any grounds removed from the system. When you do
that the equipment has no reference to ground, and any AC on the phone
line may no longer cause symptoms?
The only real way to tell if a longitudinal imbalance is
causing problems on your phone system if you read over .5VAC is to put a device which
isolates the AC voltage in front of the equipment, to see if the problem clears. There
are two types of transformers that can help. The first is a choke, which uses the windings
on a transformer to prevent AC voltage from getting to the phone system. The other is a
drain, which drains the AC to ground. You may even need a combination of the two.
According to the experts at SNC, about the only manufacturer of this kind
of noise reduction equipment in the country, the only way to diagnose this is to put one
of these devices on the line...to see if the trouble clears.
SNC has a gizmo they call the SNIX, which combines
both the choke and drain in one case. They also have a tester called the Little Zapper
which has a SNIX inside, that allows you to hook up the choke, drain or both using
butt-set cords and clips. Once you determine that the hum is gone or the intermittent
problems have stopped, you can put a choke, drain or SNIX (choke and drain combination) on
the line. SNC's number is 800-558-3325, and they have engineers available to
discuss your problem.
Longitudinal imbalance can occur after
the NI too. I talked to a guy who ran some OPXs from a PBX over to a Merlin system, over
500 feet away. The Merlin system had some CO lines coming into it directly from the phone
company, from a different direction than the PBX. Those lines worked perfectly, and were
hum free. The OPXs all had hum. As a matter of fact, when he swung an actual CO line into
the Merlin using the same house pairs he also had hum. If he used a 500 set or a butt-set,
no hum. There was only hum when the line on the house pair was in the Merlin. Obviously,
the Merlin had a slight imbalance which allowed the AC voltage that was picked up on the
house pairs running through the plant to be heard as hum. This is the perfect scenario for
using the Little Zapper to troubleshoot the problem.