ground loop problem....fix

[VST_Man 1]

recently had a install where a dome mounted on a wall was showing heavy ground loop hum/rolls. applied the basics and nothing cleared it up "until" it was decided to replace the camera.......and when a specific screw was removed all the ground loop problems disappeared. put the screw back in and it came back. turned out that a high power conduit was behind that screw and when it touched the conduit it created the voltage difference.

 

just general information that may help

[rory 0]

Good to know. Was the conduit metal or plastic?

[VST_Man 1]

metal

[treehousepig 0]

try a Humbug from Tecton, that usually cures it

[jisaac 0]

that is usually what it is. Most ground loops are caused at the camera location. If you can make your camera in a floating environment (floating from any mtetal) then you should be ok. Its almost always because of the some metal touching apart of your camera or cable creating a double ground. Which induces voltage on the shielding of your cable.

[Thomas 0]

My understanding of the physics of the ground loop is that the camera has a differant earthing potential then the power supply. So a floating camera should in theory never have a differant earthing potential then the power supply because the power supply would be the ground for the camera.

[jisaac 0]

usually the cameras are grounded to the monitor/dvr

[Cooperman 0]

Thomas and jisaacmagee,

 

Both your answers are credible bits of the puzzle, but just to clarify for others who may be getting confused...

 

If you were using a 12v DC "floating" camera that was isolated from any contact with metal, then the likelyhood of any earth loop being generated is virtually / practically nil.

 

Now where a 24v, 110v or 230/240v AC camera is used, and the power supply to the camera is grounded (as would be required by wiring regs. in the U.K.), then if the monitor / DVR is grounded at the monitoring end some significant distance away, then the likelyhood is that the power supply's would be earthed at different points, therefore offering a very high probability of earth loop hum being induced.

 

This would be the case whether the AC camera were "floating" or not.

 

Even in a situation where a relatively short length of co-ax is being used, and both camera and monitor / DVR power supply's are being sourced from the same power distribution board; whilst there should be little chance of an earth loop, if the camera is then bolted to part of a buildings metalwork, that can then set up a secondary earthing path, which can itself induce hum.

 

Although isolation transformers will solve most earth loops, it's usually better to identify risks and design out any potential (pun not intended) problems.

[Thomas 0]

Otherwise known as why I sit behind a desk and not in the field where I can do harm. I had forgotten about the power supply ground.

[rory 0]

Otherwise known as why I sit behind a desk and not in the field where I can do harm. I had forgotten about the power supply ground.

 

Yall use it in the US?

[Thomas 0]

I belive some of them do.

[313-313 0]

would grounding be more prevalent in 12vDC or 24vAC. I have never had a problem and hence donot know.

[jisaac 0]

would grounding be more prevalent in 12vDC or 24vAC. I have never had a problem and hence donot know.

 

do you mean would ground loops be more prevelent in 12v or 24vac?

 

i think that is what you meant. If so this is my personal opinion.

It may not be 100% correct but from my experience it does not matter as ground loops are not caused by the type of power but how your equipment is grounded.

For example lets look at how vst's scenario happeaned. His camera was mounted and his cables were hooked up. there was a ground for his equipment that was at the dvr/monitor end. But when the screw touched the emt conduit it created a double ground which induced voltage on the shield of the cabling that was hooked up to it. Which created a ground loop in his 60hz cycle. When he unscrewed that screw touching the emt it solved the problem. Because he went back to only one ground. Which allowed any overflow in current to flow in the direction of the circuits ground.

Thomas:

you wrote" Otherwise known as why I sit behind a desk and not in the field where I can do harm. I had forgotten about the power supply ground"

If I waited until I knew everything before I started installation then I would grow old and die way before I was able to think about starting.

I just recently am starting to get a general understanding of how ground loops get into our video.

[313-313 0]

jisaacmagee. very well explained good job....

[Cooperman 0]

Agreed ....

 

Just to clarify 313-313, where AC powered cameras are grounded there will always be a possibility of earth loop hum being induced.

 

Generally speaking because of the design for a DC cameras power supply circuitry (and the fact that many low voltage adaptors are plastic cased, double insulated, and don't have an earth conductor), as such they are not 'grounded' to earth, unless they are metal cased and bolted to some conducting metal work.

 

Bottom line is although earth loop is relatively rare (perhaps less than 2% of installations), the vast majority of cases involve AC cameras, whereas DC cameras are virtually (but not totally) immune.

 

Hey Thomas,

 

There's a lot to be said for being a desk jockey; mind you I can't say I'd rush to go back to doing installs ... the novelty wears off after the first 25 years