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I need somebody to explain properly how the ground loop works, especially if this happens when is only a DC circuit like a power supply for a motor and a coaxial cable which carries a PAL signal. Because at start I thought that my signal will be very affected but nothing really happend. To be more precise let's say that I have 200m of cable which contains 2 conductor with 1.8 \$ \Omega \$ resistance at 200m and a coaxial cable with inner and outer(shield) conductor. If I tie the shield of coaxial at both ends will my ground loop appears and influence my video signal.

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  • \$\begingroup\$ Is your last sentence (before the thanks) a question or a statement of fact? \$\endgroup\$ – Fizz Sep 28 '15 at 18:37
  • \$\begingroup\$ Yes,sorry about that.I want to know if this fact happens in circuits with DC power supply. \$\endgroup\$ – Stefan Merfu Sep 28 '15 at 19:18
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A ground loop occurs where there are two paths to ground over a section of cabling. This can occur, for example in your case, where you have a ground shield on a coax cable. If that ground shield were connected to a common ground between the two pieces of equipment (such as earth ground or some other shared ground at both ends of the cable), then there would be a potential ground loop due to multiple return paths from one end of the cable to the other and the resistance of the two paths is not the same. There is also a potential for a ground loop problem if there is induced AC on the shared ground (this is often the source of hum on audio lines).

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However, if the coax ground is not connected at the far end, or the equipment at the far end is not connected to a common ground point shared by both ends, then there is only one ground return path and no ground loop exists. (However it is then possible to have a situation where you have no ground between the two pieces of equipment -- both paths are broken -- and your signal doesn't work as intended.)

enter image description here

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  • \$\begingroup\$ Thank you for explanation.This coaxial cable is connected to a monitor.So if i break the loop on shield of coaxial cable the video signal will be affected(i tested) when the motors will start to run(brush motors). \$\endgroup\$ – Stefan Merfu Sep 28 '15 at 19:21
  • \$\begingroup\$ +1, "and the resistance of the two paths is not the same," is the most important aspect. It took me a while to understand ground loops a while ago, because most sources never brought up the different resistances, and hence different potentials. \$\endgroup\$ – Jarrod Christman Sep 28 '15 at 19:21
  • \$\begingroup\$ @StefanMerfu If you are saying the interference only happens when the shield is removed, then I am guessing the signal ground of the monitor is not tied to earth ground, and when the shield is removed you have only the signal wire connected with no ground at all. So there would be no ground loop in that case with the shield connected. That is the preferred situation. \$\endgroup\$ – tcrosley Sep 28 '15 at 19:34
  • \$\begingroup\$ Let's be more precise,my application is powered up by two batteries of 12V,the remote device which is equipped with a video camera is powered up with 24V.The coaxial cable is tied to ground in the remote device and the shield is again tied when reaches the monitor(to monitor ground).To use this monitor i have a convertor which rises my 12V to 220V.The interferences are not present because i think that the convertor is isolated somehow.Until find this soluntion my shield of coaxial cable was connected in both ends and i had a lot of noise on my motor starts(my image starts to get brighter). \$\endgroup\$ – Stefan Merfu Sep 28 '15 at 19:41
  • \$\begingroup\$ @StefanMerfu It doesn't sound like a ground loop issue with the cable, more like some sort of induced voltage due to the motor. \$\endgroup\$ – tcrosley Sep 28 '15 at 21:11
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Ground loop- more than one return path for current. For example, there may be a motr taking high current and due to bad wiring design the current returns on encoder's cable, which is not rated for high current and will just burn.

But that is a good case. One can find this problem quickly. Worst case - nothing burns, but electromagnetic interference appears where you less expect it, because certain harmony of current pulse found its way on unintended wire.

In most exotic situations a standing wave may appear on the loop, causing more EMI interfere.

In any case, just avoid it. For each current carefully design the return path. Use star co nection.

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