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Author Topic: Fluorescent lights causing hum bars on VGA line  (Read 3671 times)

Jonathan Johnson

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Re: Fluorescent lights causing hum bars on VGA line
« Reply #10 on: September 30, 2016, 01:22:25 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.

The ballast output can be much higher than 120 VAC -- up to 600V to start the lamps. So the fact that you're putting 120 VAC on the wiring and sockets shouldn't be an issue.

This page shows a typical T-8 ballast, the label of which lists an open circuit voltage of 580Vrms maximum.

Just because something makes sense and is safe doesn't mean the AHJ will understand it. And what an AHJ doesn't understand won't be approved.  ::)

The OP is from South Africa, so American codes won't mean much to him. That's why it's important to understand how your local AHJ (Authority Having Jurisdiction -- also known as "the inspector") interprets your local codes. We can agree here that something is safe, but that doesn't mean that it complies with legal requirements.
« Last Edit: September 30, 2016, 01:28:45 pm by Jonathan Johnson »
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Mike Sokol

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Re: Fluorescent lights causing hum bars on VGA line
« Reply #11 on: September 30, 2016, 08:10:36 pm »

The OP is from South Africa, so American codes won't mean much to him. That's why it's important to understand how your local AHJ (Authority Having Jurisdiction -- also known as "the inspector") interprets your local codes. We can agree here that something is safe, but that doesn't mean that it complies with legal requirements.

I just did a corporate gig last weekend for a 3,000 employee picnic, and the only thing the company safety officer was worried about was making sure the generator fuel containers were at least 10 feet away from our small generators (a few 7KW, 3KW and 2KW to power to porta potties, duck race and such) and he supplied fire extinguishers that were placed at each genny. He never looked at nor cared about any of the actual AC power distro. 

Stephen Swaffer

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Re: Fluorescent lights causing hum bars on VGA line
« Reply #12 on: September 30, 2016, 11:08:57 pm »

The ballast output can be much higher than 120 VAC -- up to 600V to start the lamps. So the fact that you're putting 120 VAC on the wiring and sockets shouldn't be an issue.

This page shows a typical T-8 ballast, the label of which lists an open circuit voltage of 580Vrms maximum.


I know you know better, so I am going to be picky.  I have customers that answer the question "How much power do you need in this shop/outbuilding?" with "I need 220 volts."  That is meaningless as to the amount of energy available.  When you connect 120 VAC mains to the tombstones on a flourescent fixture with the existing wiring (22 or 24 AWG?)  voltage is not the issue-available fault current is.  You can dead short a ballast and you'll never see anywhere near 20 amps of current flow-dead short a 20 amp circuit to the tinfoil fixture with a tiny wire and you might have plenty of fireworks to start a fire before you ever get enough current to trip that breaker.  I could be wrong-but I strongly suspect that that is the concern.
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Frank DeWitt

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Re: Fluorescent lights causing hum bars on VGA line
« Reply #13 on: September 30, 2016, 11:19:26 pm »

I just did a corporate gig last weekend for a 3,000 employee picnic, and the only thing the company safety officer was worried about was making sure the generator fuel containers were at least 10 feet away from our small generators (a few 7KW, 3KW and 2KW to power to porta potties, duck race and such) and he supplied fire extinguishers that were placed at each genny. He never looked at nor cared about any of the actual AC power distro.

So using logic, he wanted a 10 ft long fuel line from the container to the generator and the generator could not have a built in fuel tank.

I know what he really meant was it was OK for the generator to have a built in fuel tank and be located next to a pickup truck full of fuel with a extra tank and pump mounted in the bed but extra fuel for the generator had to be 10 ft away.   That makes sense. 
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Jonathan Johnson

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Re: Fluorescent lights causing hum bars on VGA line
« Reply #14 on: October 01, 2016, 01:56:04 pm »

I know you know better, so I am going to be picky.  I have customers that answer the question "How much power do you need in this shop/outbuilding?" with "I need 220 volts."  That is meaningless as to the amount of energy available.  When you connect 120 VAC mains to the tombstones on a flourescent fixture with the existing wiring (22 or 24 AWG?)  voltage is not the issue-available fault current is.  You can dead short a ballast and you'll never see anywhere near 20 amps of current flow-dead short a 20 amp circuit to the tinfoil fixture with a tiny wire and you might have plenty of fireworks to start a fire before you ever get enough current to trip that breaker.  I could be wrong-but I strongly suspect that that is the concern.

Actually, I think it's 18 AWG. 22-24 AWG is like phone wire, and that's certainly not what's in a fluorescent fixture.

I hadn't really thought of fault current issues like that. But that table lamp next to your easy chair is wired with an 18 AWG cord plugged into a 20A circuit. Oh, yes, the incoming leads on the ballast are also 18 AWG.

I don't think they're really too concerned about fault current inside of the metal housing of a fluorescent light fixture. The likelihood of a short fault is very low (the wires are protected from damage), and if there is arcing, it's contained in the metal housing.

The biggest problem I see is that someone pulls out an LED retrofit tube from a fixture that's had the ballast bypassed and tries to put a fluorescent tube in. The worst that'll happen is it won't work.
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Frank DeWitt

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Re: Fluorescent lights causing hum bars on VGA line
« Reply #15 on: October 01, 2016, 08:04:53 pm »


The biggest problem I see is that someone pulls out an LED retrofit tube from a fixture that's had the ballast bypassed and tries to put a fluorescent tube in. The worst that'll happen is it won't work.

The ones Installed at home have 120 AC to one end and neutral to the other, so your right, there would be no voltage on the heaters and not enough voltage to light a standard bulb.

They are also available with 120 volts to one end and nothing on the other.  That would quickly burn out the heater but otherwise not work.


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Craig Hauber

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Re: Fluorescent lights causing hum bars on VGA line
« Reply #16 on: October 14, 2016, 05:32:49 pm »

Actually, I think it's 18 AWG. 22-24 AWG is like phone wire, and that's certainly not what's in a fluorescent fixture.

I hadn't really thought of fault current issues like that. But that table lamp next to your easy chair is wired with an 18 AWG cord plugged into a 20A circuit. Oh, yes, the incoming leads on the ballast are also 18 AWG.

I don't think they're really too concerned about fault current inside of the metal housing of a fluorescent light fixture. The likelihood of a short fault is very low (the wires are protected from damage), and if there is arcing, it's contained in the metal housing.

The biggest problem I see is that someone pulls out an LED retrofit tube from a fixture that's had the ballast bypassed and tries to put a fluorescent tube in. The worst that'll happen is it won't work.

It is 18ga,
Never seen smaller after working on thousands of them over the years. 
Really old ones actually may have thicker wires. 
And that 18ga is perfectly adequate to trip a 20A breaker -even in 277V ones
-witnessed this often when called to bail-out facility maintenance guys who can't seem to figure out a magnetic t-12 to electronic t-8 conversion (the wiring is always different -but always shown on the new ballast label)
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Mike Sokol

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Re: Fluorescent lights causing hum bars on VGA line
« Reply #17 on: October 16, 2016, 11:58:00 am »

-witnessed this often when called to bail-out facility maintenance guys who can't seem to figure out a magnetic t-12 to electronic t-8 conversion (the wiring is always different -but always shown on the new ballast label)

Amazingly, many people are simply terrible at following a schematic. This is especially troublesome if they happen to be in the maintenance department. I'm teaching a basic audio electronics class to College juniors this semester, and even getting them to understand how to wire the most simple circuit imaginable is tricky. Yes, a battery, momentary contact switch and a light bulb. And they can get that wrong on a desktop electronic playground. 

Jonathan Johnson

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Re: Fluorescent lights causing hum bars on VGA line
« Reply #18 on: October 17, 2016, 03:49:16 pm »

Amazingly, many people are simply terrible at following a schematic. This is especially troublesome if they happen to be in the maintenance department. I'm teaching a basic audio electronics class to College juniors this semester, and even getting them to understand how to wire the most simple circuit imaginable is tricky. Yes, a battery, momentary contact switch and a light bulb. And they can get that wrong on a desktop electronic playground.

No wonder people have trouble figuring out 3-way switches. That's probably the most confusing electrical thing most people will ever come across.

Reference the drawing below: "It must be right. I flip the switch and the light comes on. I flip it again and it turns off. But why does the switch get hot when it's off? And why does the battery die?"

« Last Edit: October 17, 2016, 03:51:47 pm by Jonathan Johnson »
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Tim McCulloch

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Re: Fluorescent lights causing hum bars on VGA line
« Reply #19 on: October 17, 2016, 05:19:56 pm »

No wonder people have trouble figuring out 3-way switches. That's probably the most confusing electrical thing most people will ever come across.

Reference the drawing below: "It must be right. I flip the switch and the light comes on. I flip it again and it turns off. But why does the switch get hot when it's off? And why does the battery die?"
You need a longer circuit   8) :o ::)
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