Can we help?

I contacted a technical department who informed me I had 'hum bars' and to fit isolation amplifiers. This solved the problem. My question is how are these hum bars caused and how can they be prevented in future installations?

A: This is one of the most common problems with installations connected to two or more electricity supplies, installations in noisy environments or cameras grounded to different buildings.

The hum bar you had would probably have been caused by a ground loop. When video cables bridge two power sources, earth ground loops occur i.e. one building supplies the mains voltage to the monitor and another building supplies the mains voltage to the camera.

In the building containing the camera, its chassis reading is 0 volts when connected to the electrical supply ground. The monitor chassis in another building is also at 0 volts.

Ground loop problem
However the level of electrical ground in one building could be higher or lower than that of the ground in the other, hence a voltage potential can exist between the camera and monitor chassis.

When you connect your coaxial cable between the camera and the monitor, the cable shield connects the two grounds and an alternating current flows in the shield between the units. This is a ground loop and the cause of your unwanted hum bars.

It is also possible to have a ground loop within the camera housing.

If the camera power is taken from the same mains electricity supply as the monitor but the camera housing and bracket are fixed to an earthed structure with its own mains supply, an earth loop could be present.

Fit isolation amplifiers
Electrical interference is another cause of hum bars. Cables run near such things as fluorescent lights, fridges, power lines and radio transmitters etc should be avoided if possible.

The ground loop problem is not related to the equipment installed, it is an effect of the particular environment. The ground loop problem can be eliminated inexpensively by using the isolation amplifier.

Or if it is a new installation, it can be eliminated by anticipating the problem by using fibre, or sending the video over twisted pair via launch amplifiers.

Lee Middleton, Technical Sales Engineer at Dedicated Micros, answers your queries on CCTV