Frequently Asked Questions

By using good CCTV power supplies you will minimise ground loops. If they still exist the Humbloc terminates ground loop currents in a cable and blocks the hum and interference carried in the ground loop.

A common problem many installers face with CCTV installations is earth or ground loop problems. Many CCTV installers have seen the classic symptoms of the rolling humbar or the tearing picture but don't understand the cause of the problem and the solutions to cure it. They may have even seen a small spark or felt a slight tingle when they disconnect interlinking cables from DVRs and other equipment; these are all signs of ground loop problems. (Please note - a tearing picture can also be caused by a weak signal and a humbar by over-voltage or an unregulated power supply used with a 12V camera such as a 13.8V alarm PSU).

So, what is a ground loop?

A ground loop occurs when there is more than one "ground" or "earth" between two pieces of equipment. Because the two pieces of equipment have multiple ground paths, loops (circuits) are formed in which current can flow.

The current flowing through this unwanted circuit in the CCTV system can have a devastating effect on the system's performance. The current flows through the shield of co-ax types of cable and instead of "dragging" any interference (usually 50Hz mains hum) down to a "true" ground at the receiving end of the cable, it actually picks this interference up (in its ground loop antenna!) and carries it to the sensitive inputs of CCTV equipment such as DVRs, distributors and Switchers. The DVR, distributor or switcher simply interprets this "interference hum" as part of the intended video signal entering the equipment and displays it on the monitor as a rolling "hum" bar.

The small voltage difference in the two groups sets up a ground loop current. This current picks up interference as the circuit created by the ground loop acts as an antenna. The 50Hz Mains interference picked up is displayed on the monitor as a humbar.

So how can you prevent ground loops?

In practice this is hard to do as each CCTV system is pretty much bespoke so you can't apply general rules. In systems that have different types of CCTV cameras installed with different types of power supplies, there is a greatly increased chance that some of the cameras and other pieces of equipment may have different ground potentials. Two pieces of equipment only need a "ground voltage" difference of 0.1V (which is tiny) and to be interconnected with an earth cable of 0.1 ohms resistance to form a ground loop current of 1A!! Well designed 12V CCTV power supplies are often "double-insulated" and the 0V output is in effect floating. This has the benefit that it can “float up or float down” to the ground potential of other equipment minimising ground loop problems. Some other power supplies such as "alarm-power supplies" may clamp the 0V output to an electrical earth causing ground loop problems with CCTV equipment. YOU SHOULD NEVER remove the electrical safety earth of mains equipment in an attempt to remove a ground loop as it can endanger lives.

By carefully using good CCTV power supplies you will minimise ground loops but if they still exist then a device that blocks the current flowing down the earth connection is an option. The Humbloc simply terminates ground loop currents in a cable and blocks the hum and interference carried in the ground loop.

The Humbloc prevents the ground-loop in the cable – blocking the "humbar" interference on the monitor: