How To Fix a Ground Loop

Written by Don Schultz, trueCABLE Technical Sales Representative & Fluke Networks Certified Technician

Quite often I get questions about how to prevent a ground loop, or how to perform a ground loop fix when using Ethernet cable. It is great that people are reading Shielded vs Unshielded Cable. That is the starting point. Knowing you could run into this issue is half the battle. Fixing or avoiding it is the other half.

What I did not detail in that blog, however, was to show how to avoid a ground loop altogether. I promised I would do so, and here it is.

What exactly is a ground loop?

Ground loops may occur when you are running shielded Ethernet cable in the following scenarios:

● The shielded run is between two buildings that are on their own AC mains (meters), or have two or more different subpanels that are separately earth grounded.
● The shielded run is inside the same building that has multiple AC subpanels (a great example is a factory), and these subpanels are using different grounds.
● The shielded run goes to a WiFi AP or camera outside, and there is a lightning protector involved that uses its own ground point. Then that same run is grounded yet again to the AC ground inside your house/building.

The home installer is likely to run into the third scenario. Professional installers are likely to run into all three.

Do you see the overriding pattern? There are multiple points of grounding that occur in each installation. This has the potential (pun not intended) to create a situation that has the following outcomes:

● Unexplained bit/transmission errors in your network. Worse yet, these errors are typically intermittent.
● Equipment damage (far less likely, but possible)
● Personal injury (extremely unlikely, but a remote possibility in extreme scenarios where very high AC or DC voltages are involved). Fortunately, the conductors inside Ethernet cable are quite thin in comparison to electrical wire. The conductors will likely turn cherry red and melt before you become a charcoal briquette. Though, you might get a nasty burn or jolt.

How and why does this happen?

Electricity is your friend, but it can also harm you. For reasons that only an electrical engineer can explain fully, having multiple points of ground can cause ground potential differences in your cable system. Those ground potential differences are then literally looped back as common mode voltage, and get injected across your Ethernet cable. It’s not voltage you want--we are not talking about PoE in this case.

What is the solution?

Never run shielded cable? No. That is not the solution. In my blog linked above, there are scenarios when you must use shielded cable. The primary one, and one that I consider inviolate, is when you are running Ethernet cable outside in open air scenarios. Air movement causes electrostatic discharge (ESD) build-up on your cable, especially in the dry seasons. This ESD needs a way to drain off, and this would be via the cable shield/drain wire and your ground. I learned the hard way on this one. The cost? A dead $200 Ubiquiti outdoor Wi-Fi AP.

You can get away with unshielded CMX outdoor cable in direct burial scenarios, assuming the cable is buried and making contact with dirt, and that cable is not in proximity to an underground AC line.

The solution is to be aware this situation can occur and mitigate it before you have trouble. Here are two infographics showing common scenarios and how to wire stuff up:

Scenario #1. You are running shielded Ethernet cable between two buildings with multiple subpanels or AC mains involved.

Take note in the above scenario, Building A. The patch panel should be a shielded patch panel if one is used. If the shielded patch panel has an auxiliary ground wire attachment (most do) then either do NOT hook it up or hook it up to your building’s AC ground wire. Do not separately ground that auxiliary wire to another ground (like a ground rod). You will only create another ground loop.

Scenario #2. You are running shielded Ethernet cable to a PoE device outside, like a Wi-Fi Access Point (AP). The AP is protected by a lightning/surge protector mounted outside and is grounded directly to earth.

In the above scenario, absolutely none of the outdoor shielded Ethernet cable is grounded inside your structure. It is isolated to the outside ground provided by your lightning/surge protector. This will isolate your inside network equipment. No, it will not protect you from a direct lightning strike, but some protection is better than nothing.

So, there you have it. This is how to avoid ground loops in two common scenarios. Of course there are more scenarios than the two I described above. When planning your network, judicious use of shielded Ethernet cable will serve you well. Be sure to use shielded where needed, and unshielded where needed. Both types of Ethernet cable have their place. Happy Networking!

 trueCABLE presents the information on our website, including the “Cable Academy” blog and live chat support, as a service to our customers and other visitors to our website subject to our website terms and conditions. While the information on this website is about data networking and electrical issues, it is not professional advice and any reliance on such material is at your own risk.