Electrical ground loops have a detrimental effect on most AV Equipment, ranging from signal degradation to severe equipment damage. A ground loop is created when AC or DC current unexpectedly flows through a ground wire. This can happen due to the error in electrical wiring, improper grounding, or poor connection between ground wires and earth.
Figure 1 below shows a typical connection between a computer and a Plasma display. If the ground potential Vg1at Power Entry 1 is not equal to the ground potential Vg2 at Power Entry 2, and the two are connected together, there is a possibility for a ground loop.
Figure 1 – Typical connection between a computer and plasma display
Once the AV cable is connected between the source (Laptop) and AV display (Plasma), the ground current will flow between both devices through the coaxial cable shield (see Figure 2 below). The circuit inside an AV Display may not be equipped to handle ground current coming through the cable and electrical components may be damaged as a result.
How to Measure Ground Loops
Before connecting any AV equipment to a remote display (greater than 25 ft. away), always measure the ground potential between the video cable shield (or signal) and ground at the display power source, using a digital multi-meter. See Figure 3 for suggested points to measure.
The measured voltage should be less then 1-2 volts. Measure both AC voltage and DC voltages. If any voltage is greater than 2 volts, do not connect AV equipment together. If you are not familiar with a multi-meter, please contact a licensed electrician.
What NOT To Do If There is a Ground Loop
It is not appropriate to lift or disable ground on any AV equipment, or otherwise modify or defeat the grounding feature to resolve ground loop problems. Lifting ground wire from power cords, cables, or any electrical equipment removes the safety feature designed to protect personnel using this equipment. Improperly grounded equipment presents a severe shock hazard.
Common ground lifter, as shown in Figure 4, should not be used at any time. Even though it is polarized, it still presents a safety hazard.
The original intent for those types of plugs was to connect a grounded 3 prong plug to an older outlet that does not have grounding. The center tab should be connected to a conduit ground using a receptacle screw.
How to Fix Ground Loop Problems
The best way to address any ground loop problem is to talk to a licensed electrical contractor. They are experts in electrical systems and will propose the best solution for improving grounding between AV equipment.
For audio signals, an isolation transformer can be used in the signal path. This type of tool will break the ground loop and provide clean sound (as shown in Figure 5). The isolation transformer can be installed on the front end or at the power amplifier end.
For video signals there are several choices that can provide favorable results. Ground Loop Isolation (GLI) devices can be used inline with video signals. Although effective, most of the time they are effective only until a ground loop exceeds the input specifications of the GLI device. On average, GLI devices can withstand about 3-4 volts. Anything above that level will damage the input circuitry.
Another way to solve this problem is to use a fiber optic transmitter and receiver. Since fiber optic cables provide clean galvanic separation between the two devices, there is no possibility of a ground loop when they are used. However, in many cases, fiber optic solutions can be cost prohibitive.
Ground Loops and Twisted Pair Video/Audio Transmission
All twisted pair video transmitters and receivers utilize differential drivers and receivers capable of withstanding minimal ground loop voltage. Typically, a ground loop voltage of greater than 4 volts will damage a twisted pair transmitter, receiver, or both.
Always measure the ground loop voltage before installing any twisted pair video/audio equipment. This measurement should be done from the twisted pair conductors to the ground of the power outlet at the display end. If the ground loop voltage exceeds 4 volts, do not connect AV equipment together.
Ground loop current and voltage can damage equipment in a split second and is typically not covered by manufacturer’s warranties. Make sure to measure for the existence of ground loops before connecting cables together every time when signal cable length exceeds 25 feet.