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Ground loop question

post #1 of 63
4/1/08 at 11:40am
Thread Starter 
Just purchased my first home, and the previous owner was kind enough to leave behind his projector, screen, and surround setup. I've been working on hooking all my own gear up to the system, and am now encountering what I can only assume is a ground loop.

I have my Wii, HD-DVD player, and HD cable box all connected to a component switcher, which connects to the projector's component input. When I switch to the cable box, I have red/green horizontal bars scrolling slowly up my screen (bottom to top). The other 2 components have no issues. The problem remains when the cable box is connected directly to the projector.

Being a complete beginner to resolving ground loops, what are my first steps for resolution? I've been reading other threads, and doing some googling and wiki'ing, but there seem to be a million possible problems and a million corresponding solutions.

Is the problem most likely the coax ground, and I should have Comcast come fix it? Or try using a different coax cable? Should I check that the cable box is powered by the same outlet (or same circuit) as the rest of the gear? All the cables are run into the ceiling, so it's not as easy a check as I'd like. Should I just run and buy an isolation transformer, or wait to diagnose the problem further before doing that?

Any and all help is appreciated. A quick painless fix would be nice, but if I need to get dirty and learn some theory along the way, I'm all for it.
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post #2 of 63
4/1/08 at 12:31pm
First step is to verify the ground block for your cable where it enters the house is on the same common ground as the electrical system in the house. You should be able to track down the ground from the circuit breaker panel and the cable and see where they terminate.
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post #3 of 63
4/1/08 at 12:40pm
Thread Starter 
Ok, thanks. Am at work now, but will check first thing when I get home.

The basement where the equipment is was finished after the house was built, and has 2 new circuits. Is it possible that a new ground was put in for those circuits, so all my gear is on one ground but the coax is on the original ground?

(apologies if I ask any questions that make no sense at all...electrical theory and physics were never my strong suits, though not for lack of trying)
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post #4 of 63
4/1/08 at 2:24pm
Thread Starter 
Okay, plugging the cable box into the same surge protector as the rest of the equipment didn't help, but running the coax through the surge protector did fix the hum bars.

So that's at least a temporary fix while I diagnose the loop itself.
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post #5 of 63
4/1/08 at 5:00pm
Thread Starter 
So I've examined my grounding situation:

-Coax comes out of the house, runs through a grounding block which is mounted over the vinyl siding. Wire comes out of the block and runs into the ground.

-House ground is bolted to the water main in the basement, comes out of the house at the same place the coax comes in, then runs down into the ground, sharing a pvc pipe with the electrical meter.

So the two grounds run into the ground near each other, but do not share a hole. One is ground to the exterior of the house, the other to the water pipes. Both seem firmly mounted, no loose screws or bolts. Other than what I've said, I'm not exactly sure what to look for.

Any further instructions?
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post #6 of 63
4/1/08 at 5:51pm
Need to see if those two lines are connected to a ground rod together, they should be.
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post #7 of 63
4/1/08 at 6:05pm
Quote:
Originally Posted by newerakb View Post

Any further instructions?

A ground loop is caused when neutral current flows through the ground wire instead of the neutral wire. The small voltage drop causes a potential difference between grounds from the main panel to the outlets or from one power plug to another.

The neutral wire and ground wire can be connected only in the main power panel and nowhere else. If neutral (white) and ground (bare or green) are tied in an outlet box or subpanel, there will be a ground loop.

It's hard to open every box to make sure that it's wired right. If you have access to an Amprobe, clip it around the romex of each circuit. (All three wires) If the current is zero, that circuit is probably OK. If there is reading on the Amprobe, it means that the neutral current is getting back to the power panel through the ground loop instead of the neutral wire. You will usually see current in two circuits, the one with the fault and the second one that is looped through the entertainment center.
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post #8 of 63
4/1/08 at 7:09pm
Thread Starter 
Thanks for the help so far. Just went outside again and couldn't tell if both share a grounding rod. Are grounding rods exposed above the surface, or are they completely buried? I can see both grounds go underground, a couple feet apart, but if the rod is buried, it's possible that they meet up somewhere below.
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post #9 of 63
4/1/08 at 7:58pm
Quote:
Originally Posted by newerakb View Post

Thanks for the help so far. Just went outside again and couldn't tell if both share a grounding rod. Are grounding rods exposed above the surface, or are they completely buried? I can see both grounds go underground, a couple feet apart, but if the rod is buried, it's possible that they meet up somewhere below.

The ground rod(s) could be buried. The thing to do here is to tie the 2 grounds together outside. That gives you what you need - a single point ground. If someone did put in a second separate ground rod without connecting them together, then it could cause problems. NEC (National Electrical Code) calls for all ground rods to be connected together with at least #6 wire or larger if I recall correctly.
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post #10 of 63
4/2/08 at 5:56am
Thread Starter 
Quote:
Originally Posted by Arnold R View Post

The ground rod(s) could be buried. The thing to do here is to tie the 2 grounds together outside. That gives you what you need - a single point ground. If someone did put in a second separate ground rod without connecting them together, then it could cause problems. NEC (National Electrical Code) calls for all ground rods to be connected together with at least #6 wire or larger if I recall correctly.

As I mentioned earlier, the homes line is grounded to the plumbing in the basement, before it goes outside. Why would it be grounded to that, as well as to a grounding rod outside?

Edit: Or is the plumbing ground through the grounding rod, and the home ground connects to the plumbing to share its ground?
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post #11 of 63
4/2/08 at 8:00am
Save yourself this headache. You could chase your ass all week looking for the exact source and routing of this loop and find nothing. It's a common problem with cable TV connections because of the way cable is distributed. The simplest fix is to break the loop with a cable TV isolator available from several sources and designed for this very issue. The VRD1FF from Jensen is one example.

http://www.jensen-transformers.com/iso_vid.html
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post #12 of 63
4/2/08 at 8:39am
Thread Starter 
I've come across some of those in my searches, and wasn't sure about them. Would I put it just before it reaches the cable box? Or would I put it where the line comes into the house?

Won't adding an isolator just fix the symptom without addressing the cause?

I just don't want to spend $60 on a quick fix if I could fix the root of the problem and not have to buy one of these every time I hook a new TV up.
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post #13 of 63
4/2/08 at 9:31am
You could give it some time and try to find the exact reason. Basicly you have a difference in ground potential, maybe as small as a few tens of millivolts between the cable ground and your HT ground. There could be lots of reasons for this, none of which really matter or are harmful to anything else. Once you eliminate the easy stuff like a miswired outlet or corroded cable ground block the process of tracking down and eliminating the ground loop can be a tedious process.

Ground isolating two remotely grounded pieces of electronics like your HT system and the cable distribution drop is a technically valid method of defeating a ground loop. The only downside being the initial expence of the isolator. Typical isolation devices include transformers, opto-isolators and common-mode chokes. Installl any isolator you might buy just before it enters your cable box inside the house and leave the grounding block outside intact. If you have multiple TV cable boxes you should be able to fix the problem with a single isolator placed just after the cable enters the house at the distribution point.
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post #14 of 63
4/2/08 at 9:37am
Thread Starter 
Quote:
Originally Posted by NightHawk View Post

You could give it some time and try to find the exact reason. Basicly you have a difference in ground potential, maybe as small as a few tens of millivolts between the cable ground and your HT ground. There could be lots of reasons for this, none of which really matter or are harmful to anything else. Once you eliminate the easy stuff like a miswired outlet or corroded cable ground block the process of tracking down and eliminating the ground loop can be a tedious process.

Ground isolating two remotely grounded pieces of electronics like your HT system and the cable distribution drop is a technically valid method of defeating a ground loop. The only downside being the initial expence of the isolator. Typical isolation devices include transformers, opto-isolators and common-mode chokes. Installl any isolator you might buy just before it enters your cable box inside the house and leave the grounding block outside intact. If you have multiple TV cable boxes you should be able to fix the problem with a single isolator placed just after the cable enters the house at the distribution point.

Okay, that's exactly what I needed to know. So placing an isolator at the distribution point will fix hum bars throughout the entire house, and will have no negative side effects?
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post #15 of 63
4/2/08 at 9:47am
Quote:
Originally Posted by newerakb View Post

Okay, that's exactly what I needed to know. So placing an isolator at the distribution point will fix hum bars throughout the entire house, and will have no negative side effects?

It will eliminate one cause of ground loop hums, so unless there is another independent cause, it will eliminate all hum symptoms. There will be no negative effects. This is not a "ground lift". Each item that was grounded before it was installed will be grounded after. It just breaks an unneeded and troublesome lateral connection between grounded components.

Here is a $12.25 edition of the Jensen product, without the glamorous brand name. http://www.cencom94.com/gpage.html8.html

BTW, I've never found a neutral and ground wire tied together in any of the hundreds of systems in which I have observed and remedied the symptoms of a ground loop. I have found a hot and ground wire swapped, and if not for the fortuitous tripping of a 20 amp circuit breaker, I wouldn't be around to write this post.
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post #16 of 63
4/2/08 at 9:56am
Quote:
Originally Posted by AntAltMike View Post

It will elminate one cause of ground loop hums, so unledd there is another independent cause, it will eliminate all hum symptoms. There will be no negative efects. This is not a "ground lift". Each item that was grounded before it was installed will be grounded after. It just breaks an unneeded and troublesome lateral connection between grounded components.

What he said.
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post #17 of 63
4/2/08 at 10:38am
Thread Starter 
Thanks for all the help everyone. Maybe I'll give the Dayton VIT-1 Isolation Transformer on Amazon a try.
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post #18 of 63
4/2/08 at 7:39pm
Quote:
Originally Posted by newerakb View Post

Thanks for all the help everyone. Maybe I'll give the Dayton VIT-1 Isolation Transformer on Amazon a try.

Or you could just tie the 2 grounds together. Since water pipes exit the home underground to go to the meter, they serve double duty as both water pipe and ground rod. Connecting the two grounds together will give you the desired single point ground. If you want a no cost proof it will work, use a long cable such as a couple of auto battery jumper cables to connect the water pipes and the cable shield together and see what happens.
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post #19 of 63
4/3/08 at 1:35am
Quote:
Originally Posted by Arnold R View Post

Or you could just tie the 2 grounds together. Since water pipes exit the home underground to go to the meter, they serve double duty as both water pipe and ground rod. Connecting the two grounds together will give you the desired single point ground. If you want a no cost proof it will work, use a long cable such as a couple of auto battery jumper cables to connect the water pipes and the cable shield together and see what happens.

Adding additional grounds like you describe is just hacking at the problem and rarely improves anything. It simply creates additional nested loops. Any new path to ground is a new loop and as likely a new opportunity for noise intrusion as noise suppression. The safety ground in your home is NOT a noise sink.

The real problem arises from a fundamental flaw in coaxial cable. That is that the shield is also a signal conductor. The fact that this shield always enables unintended ground currents to flow between equipment results in common impedance noise coupling and is rarely completly eliminated without properly isolating the grounds between equipment sensitive to 60 Hz noise, i.e. audio/video equipment.
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post #20 of 63
4/3/08 at 2:19am
Quote:
Originally Posted by NightHawk View Post

Adding additional grounds like you describe is just hacking at the problem and rarely improves anything. It simply creates additional nested loops. Any new path to ground is a new loop and as likely a new opportunity for noise intrusion as noise suppression.

That should be put in bold letters and tatooed backwards on the foreheads of anyone who uses the remedy of arbitrarily adding grounds other than in desperation, so they can read it every time they look in the mirror.
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post #21 of 63
4/3/08 at 8:33am
Thread Starter 
The presence of a ground loop just means theres some sort of difference between the cable ground and the house ground, right? So can I be certain that in my situation, the cable ground is the problem, and not the house ground?

The reason I'm asking is because I've noticed that I have no hum bars on the tv in my living room, which is also hooked up to an identical cable box through identical component cables. If the cable were improperly grounded, wouldn't I experience hum bars on all my sets? Or are projectors more prone than lcd's?

The projector is in a basement that was finished after the house was built, so it was wired at a different time, by a different electrician, using a different make of circuit breaker. So is it possible that my problem could be caused by some error by the basement electrician? Like, not grounding the new circuits?

I'm borrowing a multimeter tonight and will check resistance and voltage on all the lines, as well as test whether any outlets have their hot/neutral lines flipped. But until then I'm just curious how confident I can be that the cable line is the problem.
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post #22 of 63
4/3/08 at 9:08am
Can you get to the distribution point and connect only one cable box at a time to the cable comming in?

I think you can still be confident an isolator will work. Besides it's not a huge investment if it doesn't and IMO it's good practice to use one at the cable entry point anyway.
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post #23 of 63
4/3/08 at 9:12am
You're thinking too much. And soon, you're going to be testing too much. Your house has a ground system that was designed to minimize fire and shock hazards. Audio and video equipment function best when it incorporates a ground system designed for technical performance. The two are not inherently incompatible, but quite often, a grounding system that is optimal for one purpose is not optimal for another.

Most likely, neither the cable line or the house ground is the problem. The problem is that your lateral connection between two perfectly valid safety grounds has completed the formation of a loop antenna that is inducing a hum.

You don't have the equipment needed to measure ground resistance, and even if you did, I don't believe that such measurements would lead you to a conclusion and remedy for the symptoms you are experiencing.

There may be over ten thousand posts archived in these forums regarding ground loops. Many were made by electricians and engineers who understood them. Unfortunately, they all got to the point where they just kept posting the same things over and over and they no longer contribute to this forum on this topic.
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post #24 of 63
4/3/08 at 9:20am
Thread Starter 
Quote:
Originally Posted by AntAltMike View Post

You're thinking too much

I have a habit of that. I don't like not understanding things, so when situations like this arise, I tend to spend much more time than necessary learning theory and all sorts of unnecessary information in order to fix the problem.

And if a lateral connection is causing the problem, putting an isolator at the house entry point wouldn't fix the bars, would it?
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post #25 of 63
4/3/08 at 9:32am
From the AVSForums Archives, September, 2004:

http://archive.avsforum.com/avs-vb/s...gs#post4409024

Ground loops can occur even when a dish installation is grounded in accordance with the Model National Electrical Code. A ground loop is not the same thing as a ground fault, though ground loops that traverse ground faults or non-unified grounds certainly exhibit more severe hums.

About a month ago, I posted a response on this subject alluding to a book titled: Audio Systems Design and Installation by G. H. Philip Giddings, that explained the difference between safety and technical grounding. It is out of print and not in anyone's inventory in the United States, but you can buy it from Great Britian for 39.99 GBP. The fact is, there are many A/V systems that are in conformity with the electrical code grounding requirements but still exhibit hums.

Excerpt:

"The audio system designer must understand the reasons for and requirements of all three types of grounding - technical, system, and equipment - if system designs that are not only quiet and stable but also safe and reliable are to be realized. Unfortunately, the detrimental effects on audio of improper grounding practices can often be reduced by creating another improper connection. Uninformed individuals often solve audio grounding problems with techniques that are at the expense of the safety and reliability of the system, placing the user of the system in potentially life-threatening situations. This is simply not necessary: power and grounding can coexist in harmony. The extra expense is mainly one of time, that required to understand the subject and implement designs carefully"

- G. H. Philip Giddings
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post #26 of 63
4/3/08 at 12:04pm
Quote:
Originally Posted by newerakb View Post

And if a lateral connection is causing the problem, putting an isolator at the house entry point wouldn't fix the bars, would it?

It would. Put it in the house after the grounding block. It breaks that connection.
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post #27 of 63
4/3/08 at 1:19pm
Quote:
Originally Posted by newerakb View Post

And if a lateral connection is causing the problem, putting an isolator at the house entry point wouldn't fix the bars, would it?

It's also possible that the ground loop in on the power pole outside the house. Don't try to fix that yourself.
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post #28 of 63
4/3/08 at 1:35pm
Quote:
Originally Posted by NightHawk View Post

Adding additional grounds like you describe is just hacking at the problem and rarely improves anything. It simply creates additional nested loops. Any new path to ground is a new loop and as likely a new opportunity for noise intrusion as noise suppression. The safety ground in your home is NOT a noise sink.

It's not adding additional grounds. It's connecting existing grounds together. Check out NEC codes 250.50 through 250.53
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post #29 of 63
4/3/08 at 1:55pm
Quote:
Originally Posted by Arnold R View Post

It's not adding additional grounds. It's connecting existing grounds together. Check out NEC codes 250.50 through 250.53

Well it adds additional ground paths. The NEC coundn't care less about A/V equipment performance so any guidence from there is irrelevant. Their focus is safety and once the minimum requirements are met with regards to safety do not look to electrical codes for how best to connect your home theater system.
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post #30 of 63
4/3/08 at 3:39pm
Thread Starter 
the NEC doesn't have an optional code set for those who find home theater performance more important than their safety?
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