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Home > Archive > Electrical Engineering > August 2006 > Noisy AC
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| MIke Brown 2006-08-24, 5:25 pm |
| I hope this is the right place to ask this question...
I'm not an electrician, but am pretty knowledgable. I'm working on a
mobile video unit (creating one), and we are getting some pretty noisy
AC power from where we take our electrical feed. All of our cameras
have noise in the picture due to what looks like ground loop
interferance, however even if I disconnect our main ground completely
the problem still exists. It seems somehow we are getting a ground
connection to the building through the neutral wire. I can confirm this
by running a continuity test from the building to our system ground,
and continuity only breaks when I disconnect neutral and ground.
Is this improper operation for single phase AC? As I understand it, the
neutral should go straight back to the center pole of the transformer
provided by the electric company, and I don't think there should be any
direct connection between ground and neutral. I'm only aware of
something like this in 3 phase power, am I mistaken?
Is there a way on our end to clean up the power/noise/ground to provide
our equipment with cleaner power? We're running APC 2200VA UPS's on all
of our equipment, but it doesn't seem to affect the problem (I figured
they had some good filtering / ground isolation). I've read moving to
"balanced AC" could help, but I'm not familiar exactly with how that
resolves ground problems and noise.
Thanks!
-Mike
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| operator jay 2006-08-24, 8:25 pm |
|
"MIke Brown" <mikeb@360replays.com> wrote in message
news:1156452518.958779.229450@i42g2000cwa.googlegroups.com...
> I hope this is the right place to ask this question...
>
> I'm not an electrician, but am pretty knowledgable. I'm working on a
> mobile video unit (creating one), and we are getting some pretty noisy
> AC power from where we take our electrical feed. All of our cameras
> have noise in the picture due to what looks like ground loop
> interferance, however even if I disconnect our main ground completely
> the problem still exists. It seems somehow we are getting a ground
> connection to the building through the neutral wire. I can confirm this
> by running a continuity test from the building to our system ground,
> and continuity only breaks when I disconnect neutral and ground.
>
> Is this improper operation for single phase AC? As I understand it, the
> neutral should go straight back to the center pole of the transformer
> provided by the electric company, and I don't think there should be any
> direct connection between ground and neutral. I'm only aware of
> something like this in 3 phase power, am I mistaken?
>
> Is there a way on our end to clean up the power/noise/ground to provide
> our equipment with cleaner power? We're running APC 2200VA UPS's on all
> of our equipment, but it doesn't seem to affect the problem (I figured
> they had some good filtering / ground isolation). I've read moving to
> "balanced AC" could help, but I'm not familiar exactly with how that
> resolves ground problems and noise.
>
> Thanks!
>
> -Mike
>
I would expect neutral to be connected to ground at the service entrance to
the building. You should say where in the world you are, and what sort of
building you are in, it may make a difference.
| |
| TimPerry 2006-08-25, 3:25 am |
| MIke Brown wrote:
> I hope this is the right place to ask this question...
>
> I'm not an electrician, but am pretty knowledgable. I'm working on a
> mobile video unit (creating one), and we are getting some pretty noisy
> AC power from where we take our electrical feed. All of our cameras
> have noise in the picture due to what looks like ground loop
> interferance, however even if I disconnect our main ground completely
> the problem still exists. It seems somehow we are getting a ground
> connection to the building through the neutral wire. I can confirm
> this by running a continuity test from the building to our system
> ground, and continuity only breaks when I disconnect neutral and
> ground.
>
> Is this improper operation for single phase AC? As I understand it,
> the neutral should go straight back to the center pole of the
> transformer provided by the electric company, and I don't think there
> should be any direct connection between ground and neutral. I'm only
> aware of something like this in 3 phase power, am I mistaken?
>
> Is there a way on our end to clean up the power/noise/ground to
> provide our equipment with cleaner power? We're running APC 2200VA
> UPS's on all of our equipment, but it doesn't seem to affect the
> problem (I figured they had some good filtering / ground isolation).
> I've read moving to "balanced AC" could help, but I'm not familiar
> exactly with how that resolves ground problems and noise.
>
> Thanks!
>
> -Mike
as far as i can tell "balanced AC" is strictly an audiofool term. the
biggest problem i see with any means of ungrounded feeds to a live truck it
that it would require that you travel with an electrician to properly ground
it at each venue. GFI is not a viable option in this case (picture yourself
hanging you head and saying "i'm sorry i missed the shoot on this one time
event someone touched something and a breaker popped")
if you mention balanced AC to an electrician he is going to assume you are
talking about load balancing: a totally different thing.
it would be more accurately be referred to as 'isolated AC'.
before proceeding with costly "fixes" i would suggest you hire an
experienced TV engineer to look over your system. broadcasters face this
dilemma on a reoccurring basis.
| |
| Beachcomber 2006-08-25, 3:25 am |
| On Fri, 25 Aug 2006 00:34:04 -0400, "TimPerry"
<timperry@noaspamadelphia.net> wrote:
>MIke Brown wrote:
>
>as far as i can tell "balanced AC" is strictly an audiofool term. the
>biggest problem i see with any means of ungrounded feeds to a live truck it
>that it would require that you travel with an electrician to properly ground
>it at each venue. GFI is not a viable option in this case (picture yourself
>hanging you head and saying "i'm sorry i missed the shoot on this one time
>event someone touched something and a breaker popped")
>
>if you mention balanced AC to an electrician he is going to assume you are
>talking about load balancing: a totally different thing.
>
>it would be more accurately be referred to as 'isolated AC'.
>
>before proceeding with costly "fixes" i would suggest you hire an
>experienced TV engineer to look over your system. broadcasters face this
>dilemma on a reoccurring basis.
>
I'm a former TV maintenance engineer that has had some experience with
this sort of thing. Ground loops on a remote truck can be common for
a large and complex system. The problem might be that the safety AC
ground on much of your equipment is electrically connected to the
shield ground on your video coaxial cables and possibly your audio and
control cables as well. The circulating current flowing in a ground
loop produces a 60 cycle (in the US) hum that can show up in video
monitors, distribution amps, waveform monitors, and audio lines.
With newer equipment, this is generally less of a problem as the power
supplies are designed with more isolation. Safety has first
consideration. You'll want to start with your truck's electrical
system and make sure that AC connection is grounded properly and
according to the national electrical codes. If you operate on
generator power, you still need a proper ground system. Watch out for
things like amps, prompters and monitors connected to a building's AC
system if the truck is on an isolated power feed.
Some devices may need to be connected to an isolation transformer to
minimize or eliminate ground loops.
For coaxial video feeds, there is a device called a hum bucker which
is kind of a brute force approach to minimizing a ground loop. Here
is one example: (I'm not necessarily recommending or not recommending
this particular branded device)
http://www.american-technical-resou...male-p-188.html
Make sure your jack fields are wired correctly by someone
knowledgeable who knows when lifted grounds are appropriate.
For extremely long video (and audio) feeds... the use of fiber optics
lines with the appropriate converters at either end are preferable to
coaxial cables. The price of this technology has come down over the
past few years.
Beachcomber
| |
| MIke Brown 2006-08-26, 3:25 am |
| Yea I wish there were broadcast engineers in our area, but it's rare to
even see an event truck hit our city, and when it does they're usually
too rushed to talk.
Our system was designed as a special effect, there are over 40 cameras
in our system, thus making filtering individual cameras a big task, and
alot of extra equipment. All of the cameras we use take in a 12vDC
supply, of which the negative terminal is tied to video ground, and
they are all supplied by the same DC power supply. The DC supply is
isolated, but it doesn't really matter as the video ground hits the
other side which attaches to computers and patch panels, and is
grounded to case. So it's very difficult to isolate this at our rack.
We ground our trailer using the ground supplied by our 100 amp AC
input, which is basically tied directly to the building's ground. I've
noticed today that the noise (which your right, it's 60hz hum) comes
and goes at different times during the day, so I'm convinced it's
caused by some piece of equipment within the building, and not within
our trailer. Of course I have no control over that, we need a way to
isolate this noise before it hits our equipment. I'm thinking of
looking into some power conditioning device I can install at the panel,
I came across a few manufacturers today.
I wish I had a waveform monitor handy, I've only got a portable
oscilliscope (and it' s only a 20Mhz) so I can't really see the noise.
I've considered hum buckers, but I'm convinced this noise is also
affecting our data lines to the cameras and am looking for a solution
that filters noise at the source (our ac input feed). I'm going to look
into a 100amp isolation transformer later tonight (if such a thing
exists).....
Thanks!
-Michael Brown
Beachcomber wrote:
> On Fri, 25 Aug 2006 00:34:04 -0400, "TimPerry"
> <timperry@noaspamadelphia.net> wrote:
>
> I'm a former TV maintenance engineer that has had some experience with
> this sort of thing. Ground loops on a remote truck can be common for
> a large and complex system. The problem might be that the safety AC
> ground on much of your equipment is electrically connected to the
> shield ground on your video coaxial cables and possibly your audio and
> control cables as well. The circulating current flowing in a ground
> loop produces a 60 cycle (in the US) hum that can show up in video
> monitors, distribution amps, waveform monitors, and audio lines.
>
> With newer equipment, this is generally less of a problem as the power
> supplies are designed with more isolation. Safety has first
> consideration. You'll want to start with your truck's electrical
> system and make sure that AC connection is grounded properly and
> according to the national electrical codes. If you operate on
> generator power, you still need a proper ground system. Watch out for
> things like amps, prompters and monitors connected to a building's AC
> system if the truck is on an isolated power feed.
>
> Some devices may need to be connected to an isolation transformer to
> minimize or eliminate ground loops.
>
> For coaxial video feeds, there is a device called a hum bucker which
> is kind of a brute force approach to minimizing a ground loop. Here
> is one example: (I'm not necessarily recommending or not recommending
> this particular branded device)
>
> http://www.american-technical-resou...male-p-188.html
>
> Make sure your jack fields are wired correctly by someone
> knowledgeable who knows when lifted grounds are appropriate.
>
> For extremely long video (and audio) feeds... the use of fiber optics
> lines with the appropriate converters at either end are preferable to
> coaxial cables. The price of this technology has come down over the
> past few years.
>
> Beachcomber
| |
| TimPerry 2006-08-26, 9:25 am |
| MIke Brown wrote:
> Yea I wish there were broadcast engineers in our area, but it's rare
> to even see an event truck hit our city, and when it does they're
> usually too rushed to talk.
>
where are you?
> Our system was designed as a special effect, there are over 40 cameras
> in our system, thus making filtering individual cameras a big task,
> and alot of extra equipment. All of the cameras we use take in a 12vDC
> supply, of which the negative terminal is tied to video ground, and
> they are all supplied by the same DC power supply. The DC supply is
> isolated, but it doesn't really matter as the video ground hits the
> other side which attaches to computers and patch panels, and is
> grounded to case. So it's very difficult to isolate this at our rack.
> We ground our trailer using the ground supplied by our 100 amp AC
> input, which is basically tied directly to the building's ground. I've
> noticed today that the noise (which your right, it's 60hz hum) comes
> and goes at different times during the day, so I'm convinced it's
> caused by some piece of equipment within the building, and not within
> our trailer. Of course I have no control over that, we need a way to
> isolate this noise before it hits our equipment. I'm thinking of
> looking into some power conditioning device I can install at the
> panel, I came across a few manufacturers today.
>
> I wish I had a waveform monitor handy, I've only got a portable
> oscilliscope (and it' s only a 20Mhz) so I can't really see the noise.
> I've considered hum buckers, but I'm convinced this noise is also
> affecting our data lines to the cameras and am looking for a solution
> that filters noise at the source (our ac input feed). I'm going to
> look into a 100amp isolation transformer later tonight (if such a
> thing exists).....
>
http://www.sola-hevi-duty.com/
light dimmers are often the culprit, particularly older systems.
| |
| Tomi Holger Engdahl 2006-08-29, 5:25 pm |
| invalid@notreal.none (Beachcomber) writes:
Been there, seen that. I have worked several times on
TV broadcasting system beign the tech person hunting out
those problems from the system.
I have written some documentation to the web what I have
learned over the years
http://www.epanorama.net/documents/...loop/index.html
(that document is mostly written several years ago
and I am slowly updating it..)
[color=darkred]
> I'm a former TV maintenance engineer that has had some experience with
> this sort of thing. Ground loops on a remote truck can be common for
> a large and complex system.
Ground loop problems are common for all large and compex systems.
> The problem might be that the safety AC
> ground on much of your equipment is electrically connected to the
> shield ground on your video coaxial cables and possibly your audio and
> control cables as well.
That's usually the case with all normal video, audio and computer
equipment. The defauls is normally that the video, audio and data
grounds are connected to equipment case that is connected to mains
ground (excpetion are doulbe insulated equipment with no ground).
> The circulating current flowing in a ground
> loop produces a 60 cycle (in the US) hum that can show up in video
> monitors, distribution amps, waveform monitors, and audio lines.
That's true.
The current flowing on ground can cause noise to signal lines
in two ways:
1. direct voltage difference between different ends of the cable shield
(U = I * R) , gets directly added to unbalanced signal on line
(audio or video)
2. circulating ground currents can create considerable mains frequency
magentic fields that can couple to all signal lines nearby
Both of those can cause considerable noise problems. The second
magentic field related problems are then ones harder to debug...
> With newer equipment, this is generally less of a problem as the power
> supplies are designed with more isolation.
Many newer equipment have power supply that is somwhat better
isolated than older equiment, that's true. But this better
isolation generally helps considerably only with non grounded
equipment (doubly insilated). With grounded equipment still
have the signal line ground connected to mains power ground
(there are good reasons why this is the norm).
> Safety has first consideration.
That's true.
The system should be always built in such way that it is safe.
From the possible electrically safe system setups use the
configuration that is least noisy.
> You'll want to start with your truck's electrical
> system and make sure that AC connection is grounded properly and
> according to the national electrical codes.
ThatÃÃ's true. Make sure that everythign is wired crectly
(no ground/neutral interconnections or wired siwtech in any
part of system) and in sensible ways (star grounding usually best
idea).
> If you operate on
> generator power, you still need a proper ground system. Watch out for
> things like amps, prompters and monitors connected to a building's AC
> system if the truck is on an isolated power feed.
>
> Some devices may need to be connected to an isolation transformer to
> minimize or eliminate ground loops
That's true.
> For coaxial video feeds, there is a device called a hum bucker which
> is kind of a brute force approach to minimizing a ground loop. Here
> is one example: (I'm not necessarily recommending or not recommending
> this particular branded device)
>
> http://www.american-technical-resou...male-p-188.html
Hum bucker transformer and video signal isolation transformers
are both very effective tools for fighting agains mains frequency
noise on video lines.
> Make sure your jack fields are wired correctly by someone
> knowledgeable who knows when lifted grounds are appropriate.
Use audio signal isolation transformers where necessary.
> For extremely long video (and audio) feeds... the use of fiber optics
> lines with the appropriate converters at either end are preferable to
> coaxial cables. The price of this technology has come down over the
> past few years.
--
Tomi Engdahl (http://www.iki.fi/then/)
Take a look at my electronics web links and documents at
http://www.epanorama.net/
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