TECH TIP - Dealing With Ground Loops In CCTV systems A ground loop is a system-grounding problem that occurs when one device in a system is connected to an electrical ground different from the ground of the other devices. At low voltage, the ground difference is detectable at voltages as low as 200mV. In camera systems can be identified as a “Hum Bar” or dark horizontal bar in the picture. Ground loops can also occur between cameras connected to a multi-output power supply if one or more of the cameras has an additional path to ground. Most multi-output power supplies do not isolate outputs from one another. Most common ground loops are caused by the camera being grounded to a voltage that is different from that of the head-end. If the [low-voltage] camera is grounded try disconnecting the camera from its ground. If the problem remains, disconnect the video cable (coax or UTP wires) at one end and check their voltages relative to the local ground. We expect no more than about 200 mV AC. If it is higher, then you have a ground loop. Are there multiple cameras powered off the same multi-output power supply? As an experiment, try powering a camera off a separate floating supply, or use one gel-cell battery (if the camera uses 12VDC for power) to power the camera locally. If removing the camera from the multi-output supply solves the problem, then we you may have a fault voltage coming from another camera or its wire. It's not uncommon for wire to get scraped as it passes through ends of conduit or T-bar ceilings. Check each camera one-by-one. When isolating, it may be necessary to disconnect both power conductors from the power supply. The easiest and most cost effective way to remedy a ground loop problem is to use a ground loop isolator transformer. This device connects anywhere between the head end and camera on the video cable and isolates the video ground. The ground loop isolator transformer is relatively inexpensive and simply connects via BNC connectors between the camera and head end device.