HUM ELIMINATION TECH BULLETIN
When I say hum I'm talking about a 60 cycle hum or
buzz (or more likely a harmonic of 60 cycles), which is a pretty low frequency.
You could be hearing a higher frequency buzz or a tone,
which is often caused by RFI.
Or the noise can be induced interference from other pairs
in a cable, or a cable run parallel to a power cable (which would usually be 60
cycle hum). Data noise from something in the same or tie-wrapped to a phone
cable could be causing a problem?
Induced AC power onto a low voltage audio pair can be
measured with a meter. I'll talk about that in a little bit.
Hum is caused by one of three things:
1. An imbalance in a cable, where the tip side of the wire
is longer than the ring side. In telephony both of the wires coming from
telephone equipment or the phone company have to be exactly the same
2. An imbalance in AC powered telephone equipment, where
one of the two wires for the phone line is closer to ground than the other
(which creates an imbalance on the whole line).
One wire can be longer because it's shorted to another
wire in the cable with voltage on it (called foreign voltage), shorted to an
unused conductor, shorted to ground through an insulation fault or water damage,
and shorted to foreign voltage through water.
Since no equipment that plugs into AC is perfectly
balanced, the equipment's imbalance sometimes causes a ground loop when
the ground is on the wrong wire (tip or ring) on a piece of equipment, compared
to the piece of equipment at the other end.
3. EMI (from a nearby piece of equipment), RFI (buzzing or
hum is usually an artifact of analog TV if you're near a TV station antenna, but
there are very few analog TV stations left out there), or crosstalk from an
adjacent pair of wires in a cable.
Other things to look at:
AC voltage on the line (should be less than .5VAC from tip
to ground and ring to ground), which would only cause a hum on a line with an
imbalance when the phone or device is AC powered (has a reference to ground
through the power plug and/or a ground cable).
Is the AC Outlet Wired Correctly?
If the AC outlets are wired wrong it's likely that you'll
get some kind of ground loop, or an imbalance will be made worse because the
engineer who designed the device assumed the outlet would be wired properly.
Always carry an Outlet Checker and use it to check
the outlet! It could save you hours of chasing your tail!
Do You Have Ground Loop?
If the ground in the building is bad you could
really chase your tail. You might go outside to make sure there's a wire
connected to the ground rod(s) that is going inside the building (usually on the
opposite side of the wall from the electrical panel). Electricians have been
know to forget to connect it to the ground rod.
If the equipment has a separate ground, not just the 3rd
prong on the AC plug, you could try disconnecting that to see if it helps your
problem. While it may not be safe to leave it that way on some equipment, it's
OK to do it for troubleshooting.
Here's the proper troubleshooting for hum:
If you skip any step you
could chase your tail for a long time...
anything! (You can guess how
I know that)
1. Check the pair for each line you hear hum
on with a butt-set from the demarc (either the phone company, or the phone
system station equipment / ATA) for hum with the line disconnected from the
If you hear any hum at all on your butt-set,
when the equipment is connected it's likely to make the hum worse because the
equipment has a reference to ground.
Keep in mind that anything you hear on the butt-set in
monitor mode (not off-hook) is pretty much meaningless. A phone line or station
port must be terminated by a phone for any noise faults to be diagnosed. When
the butt-set is in monitor mode and you're not listening to a conversation or
whatever while another device is off-hook on the line, you are likely
to hear hum and noise that just doesn't mean anything.
If you hear hum, even a little, you must fix it or
get it fixed. It ain't going to get any better when you connect your equipment,
and it will likely get worse because of the equipment's reference to ground
through the power cord.
Remember that lightning protection diverts the tip and/or
ring to ground when there is more voltage than the protector is rated at. There
are some really junky lightning protectors, especially with MOVs (Metal Oxide
Varistors) that eventually go out - shorting one or both sides of the line to
ground. Jump around any lighting protection / remove the ground temporarily to
make sure that's not your problem!
The easiest way to eliminate crosstalk from adjacent pairs
or a cable tie-wrapped to a power cable is to run a CAT5 on the floor between
the two ends of the permanently installed cable. If the hum is gone on the new
cable laying on the floor, it's time to take a look at the permanently installed
cable, or install a new one (preferably in a different route).
If you can see that all the cables from the frame go right
over a welding machine or X-Ray / MRI machine of some kind, I'd suspect that
machine. Have them operate the machine to see if the problem occurs, and that
it's gone when the equipment is powered down (this is usually EMI or RFI).
You can listen for the hum when they run each machine, one
at a time. Until you identify the cause of the hum, you have no chance of fixing
It's very unlikely you'll be able to get rid of EMI caused
by a machine of some kind. Re-running the cables may be the only fix?
If it's RFI, we have filters for phone and Ethernet that
covers interference from .5mhz to 150mhz, but some equipment could be causing
interference at a much higher frequency than that.
2. If the lines aren't the cause of the hum
it's time to take some voltage readings. If there is more than .5VAC from tip to
ground, ring to ground, or tip to ring, you need to eliminate it. See our
Longitudinal Imbalance Tech Bulletin for
information on measuring the AC, and eliminating it if you find more than half a
volt. Be sure to read the note in that Bulletin that most meters can't read AC
on a phone line correctly.
While you've got your meter out check the Loop Current and
on and off-hook DC voltage. It's probably not causing hum, but if you've got
your meter out you might as well check.
3. If there's less than half a volt of AC on the lines it's time to
start checking for a ground loop. The first thing to try is to reverse the
tip and ring on one end of the pair (phone line, CO trunk, paging
amplifier, etc.). A ground loop is pretty common between audio paging systems,
amplifiers, pre-amps, etc. and phone equipment (either a page port, station or
trunk level paging).
If reversing the tip and ring didn't fix it it's
time to eliminate a problem caused by a ground or the AC in the
building. Get out your outlet tester and make sure the electrician wired
the outlet correctly, and check the outlets on the battery
backup. Whatever stone you leave unturned could cost you a lot of time.
If there's a ground connected to any of the
equipment (separate from the 3rd prong AC ground) remove it temporarily. If it
gets rid of the hum you have a bad ground somewhere or a ground loop of some
kind. Time to call an electrician. All grounds on all equipment need to be the
If removing the ground didn't fix the hum it's time to run
the stuff on a battery with the AC disconnected (which also disconnects the 3rd
prong ground). If your stuff doesn't have a battery backup bring out a charged
up battery backup from your office to use for testing. You'd need to do this on
every piece of equipment connected to the system that uses AC.
If you forget to run the the paging system, the automated
attendant or anything connected to AC on battery (or disconnect it entirely to
see if the hum goes away) you're wasting your time. One little doodad connected
to the system could cause a problem, including an Ethernet switch if the system
supports VoIP, your POE devices, or a power cube attached to a phone that isn't
using POE. Forgetting to disconnect one thing can make you think the problem is
unfixable, when it's probably not.
It might be handy to carry a 3 Prong Adapter which will let you run
something with the 3rd prong ground disconnected for testing.
But it's probably a better idea to run everything on a
battery backup since then you know there's no interaction with the AC power /
On an alarm system, which could be contributing to
the problems, you could disconnect the RJ-31X so the panel is not connected to the lines. If
for some reason there is no RJ-31X pull the power cube from the AC outlet so it
runs on battery, and make
sure there's no separate ground run to the panel.
4. If you ran everything on battery, removed the grounds and swapped tip
and rings at one end of every pair, it's time to look for EMI or RFI produced by
If the problem is just in one office or one area it's time
to start turning off everything AC in the area. That would include
computers, monitors, fax machines, copy machines, radios, etc. The easiest way
is to just run your system on battery while you flip off all the breakers (got
to do that after hours!).
You can start by turning off one gizmo at a time, but it's
usually best just to have the customer shut down all his stuff in an orderly
manner and then kill all the breakers or the master to the building. You have to
shut off all the computers. Leaving one on, even backed by battery, could be
causing the EMI / RFI.
There could be stuff like fluorescent and neon light
transformers that are powered and you don't know it, and you won't know they're
still on unless you kill the breakers / main.
Transformers sometimes let out a ton of EMI. I've seen
some phone systems that were in the utility room of a building and there was a
HUGE transformer humming away in the room (3' square mounted to the
concrete floor). At some point you've got to shut that down and run your stuff
on battery to make sure that transformer or something in the building isn't
causing the hum.
Radio and TV stations are an unlikely source of hum. I
wouldn't consider that unless there's a big TV antenna on top of the building.
TV antennas need to be up very high, but they are short, so they like to put
them in metropolitan areas on the tallest buildings around.
Or maybe if you're right under some kind of antenna. You
may be hearing data noise from data being sent by RF?
There's pretty close to a 100% chance you can fix the hum, but it could be a
lot of work.
There's just no easy way to do it.