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Author Topic: Fluorescent lights causing hum bars on VGA line  (Read 3866 times)

Jean-Pierre Coetzee

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Fluorescent lights causing hum bars on VGA line
« on: September 24, 2016, 04:45:38 am »

Hello all,

In one of our halls the fluorescent lights cause noticeable bars on our projectors that are run on VGA cable, the bars go away when the lights are turned off. I haven't climbed into the roof to see if the VGA lines are running anywhere near power but this has only began happening about 2 weeks ago and we had no issues before that and wanted to know if anyone had any other idea about what can be causing the issue.

Thanking you
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Cailen Waddell

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Re: Fluorescent lights causing hum bars on VGA line
« Reply #1 on: September 24, 2016, 09:25:56 am »

Has anything else changed?  New equipment in other places?


Sent from my iPhone using Tapatalk
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Jonathan Johnson

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Re: Fluorescent lights causing hum bars on VGA line
« Reply #2 on: September 25, 2016, 11:45:38 pm »

In one of our halls the fluorescent lights cause noticeable bars on our projectors that are run on VGA cable, the bars go away when the lights are turned off. I haven't climbed into the roof to see if the VGA lines are running anywhere near power but this has only began happening about 2 weeks ago and we had no issues before that and wanted to know if anyone had any other idea about what can be causing the issue.

I suspect a failing ballast in a light fixture. Replacement is pretty easy, but if you're replacing a magnetic ballast with an electronic one, the wiring may be different. The wiring diagram is on the label of every replacement ballast I've ever seen.

If your light fixtures use the 1+1/2" (T-12) tubes, you should replace them. T-12 are very inefficient these days, and they tend to have a more annoying flicker than T-8 (1") or T-5 (5/8") lamps. The sockets are the same for T-12 and T-8 tubes, so you can keep the fixture and replace the ballast and tubes.

If your fixtures have magnetic ballasts (magnetic ballasts weigh several pounds each whereas electronic ballasts are less than a pound each), replace them. They are also inefficient by today's standards.

Fluorescent lamps are on the verge of being a dying technology in favor of LED lighting. You've got a few options for retrofitting to LED:
  • Replace all of the fixtures with LED fixtures
  • Replace just the lamps with LED retrofit tubes (the kind designed to work with the existing ballast)
  • Remove the ballasts and lamps and insert LED retrofit tubes (the kind designed to work without the existing ballast)

I don't really like the second option, as it's slightly less efficient due to energy consumed by the ballast. If the ballast is failing, you wouldn't be able to use such a retrofit anyway.

There is a caveat: some LED fixtures/retrofits may introduce radio frequency or electromagnetic interference. I don't have any experience with that, so maybe some of the other folks here can shed some light on that issue. (Pardon the pun.)

Fluorescent lamps (the tubes) are considered hazardous waste because they contain mercury. Contact your waste collection company for disposal options in your area.
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Mike Sokol

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Re: Fluorescent lights causing hum bars on VGA line
« Reply #3 on: September 26, 2016, 10:15:43 am »

In one of our halls the fluorescent lights cause noticeable bars on our projectors that are run on VGA cable, the bars go away when the lights are turned off.

Sometimes it's not the fluorescent lights themselves causing the hum. It can just be the amperage load on the subpanel causing the ground to move around. I've seen a few installs where the AC power for the projector was tapped onto the lighting circuit subpanel, just out of convenience. If that subpanel also has a green Ground-Neutral bonding screw in place, then as the amperage loads change, the voltage on the EGC ground will also change. If your projector has a shielded connection back to the computer at your mixing console, then all sorts of ground loop hum bars can be created. I know this because I saw it myself once. Crazy to figure out at first, but pretty simple to find once you know what to look for. I now run a lot of Baluns for projectors over Cat5 and hum bars are a non issue.
« Last Edit: September 27, 2016, 07:29:54 am by Mike Sokol »
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Jean-Pierre Coetzee

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Re: Fluorescent lights causing hum bars on VGA line
« Reply #4 on: September 27, 2016, 04:47:50 am »

Sometimes it's not the fluorescent lights themselves causing the hum. I can just be the amperage load on the subpanel causing the ground to move around. I've seen a few installs where the AC power for the projector was tapped onto the lighting circuit subpanel, just out of convenience. If that subpanel also has a green Ground-Neutral bonding screw in place, then as the amperage loads change, the voltage on the EGC ground will also change. If your projector has a shielded connection back to the computer at your mixing console, then all sorts of ground loop hum bars can be created. I know this because I saw it myself one. Crazy to figure out at first, but pretty simple to find once you know what to look for. I now run a lot of Baluns for projectors over Cat5 and hum bars are a non issue.

Sorry for the late reply was a long weekend. In South Africa neutral and ground is separate all the way to the service inlet so there is no jumper between the two anywhere in the building, or at least shouldn't be...

Nothing has changed recently, I also noticed that it was causing a buzz on the monitors which went away when the florescent lights are turned off and those two circuits aren't even on the same phase so the only possible link between them is the grounding system, seems like I am going to have to get an electrician in to check every single fluorescent light as this might be related to a sporadic GFIC trip we've been getting at the distribution panel. Added to the confusion the last time I spoke to the electrician he said that the fluorescent lights are not connected to ground at all since they are double insulated and therefore do not need to be grounded according to the code.

So further faultfinding:

1. Monitor amp also buzzes and goes away when fluorescent lights are turned off and are not even on the same phase
2. Nothing has changed recently
3. Lights are not grounded so issue must be on the neutral?
4. There are sporadic GFIC trips that seems to have no cause and have no repeatable pattern

Other notes:

Live 1, Live 2, Live 3, Neutral, Ground are all separate conductors through to the inlet from the electrical company.
I am not sure if the metal frame of the fluorescent lamp is bonded to ground anywhere since this only came into code in South Africa after the lights were installed.
The electrician replaced the GFIC about a week ago to try and sort out the sporadic trips and we though it was solved but evidently it was not, this also did not change anything regarding the hum issues.

I'm thinking along the same line as Jonathan in that a ballast might be failing but there are 40 fluorescent lights in the venue so which one will probably be a matter of time to find out since none of flashing or struggling to start up.
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Jonathan Johnson

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Re: Fluorescent lights causing hum bars on VGA line
« Reply #5 on: September 27, 2016, 03:38:32 pm »

I'm thinking along the same line as Jonathan in that a ballast might be failing but there are 40 fluorescent lights in the venue so which one will probably be a matter of time to find out since none of flashing or struggling to start up.

It will mean a little work for your electrician, but rather than disconnecting each ballast until you find the faulty one, you may be able to get into a junction box and disconnect half the lights. If the hum bars stay, it's on the lights that weren't disconnected. If they go away, it's on the lights that were.

By isolating half (20 of 40), then half again (10 of 20), then half again (5 of 10), then almost half (3 of 5), you may be able to identify the culprit (out of 40 lights) in 5 or 6 tries -- depending on how the lights are wired. I use this method for all sorts of troubleshooting.
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Jean-Pierre Coetzee

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Re: Fluorescent lights causing hum bars on VGA line
« Reply #6 on: September 30, 2016, 04:29:46 am »

It will mean a little work for your electrician, but rather than disconnecting each ballast until you find the faulty one, you may be able to get into a junction box and disconnect half the lights. If the hum bars stay, it's on the lights that weren't disconnected. If they go away, it's on the lights that were.

By isolating half (20 of 40), then half again (10 of 20), then half again (5 of 10), then almost half (3 of 5), you may be able to identify the culprit (out of 40 lights) in 5 or 6 tries -- depending on how the lights are wired. I use this method for all sorts of troubleshooting.

Seems reasonable to me, another suggestion I got was to check the voltage between the live and neutral on each branch and one should be quite a bit above 0, your way seem to be a little less work.
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Frank DeWitt

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Re: Fluorescent lights causing hum bars on VGA line
« Reply #7 on: September 30, 2016, 11:09:20 am »

I realize changing the lights to LED is a bit drastic but just FYI I noticed our maintenance guy at church changing a whole suspended ceiling light fixture from florescent to LED so I asked why he didn't just disconnect the ballast and change the bulbs.  He explained that the defuser on the old ones were cracked, and hard to change and the whole fixture was low quality and the new fixture cost $46 with "bulbs"  The LEDs are mounted to the reflector and permanently wired in, no bulbs.
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Stephen Swaffer

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Re: Fluorescent lights causing hum bars on VGA line
« Reply #8 on: September 30, 2016, 12:12:32 pm »

Another consideration is modifying the fixture to put 120 VAC on the pins of the lamps.  Personally, I have no problem with it-but I have heard that some AHJ take issue-and you have the whole "listing" question mark.  Just make sure where you stand on that before you choose a course of action.

Just because they make it and sell it doesn't make it ok or a good idea.  Big box stores still market non-weather resistant  outlets for outdoors and non-code compliant outdoor covers.
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Frank DeWitt

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Re: Fluorescent lights causing hum bars on VGA line
« Reply #9 on: September 30, 2016, 01:01:59 pm »

Another consideration is modifying the fixture to put 120 VAC on the pins of the lamps.  Personally, I have no problem with it-but I have heard that some AHJ take issue-and you have the whole "listing" question mark.  Just make sure where you stand on that before you choose a course of action.

Just because they make it and sell it doesn't make it ok or a good idea.  Big box stores still market non-weather resistant  outlets for outdoors and non-code compliant outdoor covers.

Good point.  I did that at home, but In a public place like a church I guess the new fixture is a better way to go. It's listed, and BTW it has no pins, sockets, or lamps.
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