Video Baluns Getting Very Hot

You need to quantify 'Very hot'.
If you can't keep your hand on them, then I call that 'very hot' but if you can keep your hand on them I call that just 'hot'.
I believe measurement scales have been constructed for the concept of 'hotness'.
Del

They are very hot to touch that you cannot place hand on them.

12v at 20mA shouldn't get 'em that hot. Have you checked the supply voltage and the signal level in?
Del

Thanks Del.

I checked the supply voltage which is 12V. How can I check the signal?

Don't s'pose you have a scope?
Look at the spec' of the device the signal is coming from.
Does it get hot with 12v on it but the signal dissconnected? That will provide a clue as to where the power is coming from.
Del

You might have a ground loop...

http://www.muxlab.com/assets/files/application_guides/VE_CCTV_Balun.pdf

It most probably is a ground loop. The text immediately under this illustration in the linked article furnishes more information and proposes solutions:

"One remedy is to correct the ground loop problem or replace the cable by coax, although the ground loop problem will continue to exist. Another solution is to install a groundloop blocker on the line. FM Systems Inc offers one. Information about FM's GB-60 ground-loop blocker can be found at http://www.fmsystems-inc.com/. Other vendors include; http://www.elect-spec.com/video.htm (Video Guard Hub Buck) and http://www.jensen-transformers.com/iso_vid.html (Jensen). An article about video ground loop problems and how to correct them may be found at the following link: http://www.epanorama.net/documents/groundlop/video_isolation.html. MuxLab's LongReach Active CCTV Balun (500015), CCTV Pass-Thru/GLI Balun (500132) and Passive CCTV/GLI Hub (500133) also provides ground loop blocking,"

Another possibility could occur if you are trying to supply DC power to some device via the coax. If that is the case, there must be a capacitor (or possibly something more complex) to allow injection of the video signal and separation of the DC and video components. Something like this must be at both ends of the coax.

Improper installation of these would probably prevent the system from working. If the system is working but still getting hot, then the improper grounding is a more probable (and more dangerous) cause.

We just went through this same sort of problem with several recently installed security surveillance video units at our location.

I found the system PS voltage selection switch located inside the DVR enclosure set to the wrong position (lower setting) causing application of voltage higher than design to the PS.

The switch was set to 24 VAC and the contractor had connected the units to 120VAC which is acceptable if the unit is physically configured for it.

The cover had to be removed from the DVR unit in order to access the switch.

Unfortunatley we lost 1/3 of the units before anyone reported the issue.

Dear members

Thanks for your suggestions. I am sorry for delaying update about the problem as I was out of the site from some days. To start with troubleshooting please advice me if I will connect all the cameras with a common ground wire (ground wire running from one camera to another and so on) and then connect this common groung wire to the ground wire of the supply system, will it solve the problem of Ground loop? What if I will isolate all the cameras from ground, so that there will be no connection to ground?

The outside shield of the coax will do + a single point ground. Pay attention to power supplies, and #10.

Dear members

We checked the system and tried some work arounds including your suggestions. The observations are as under:

We have 10 cameras in total, 8 connected via STP CAT5 and 2 via Coax. Group A (camera No. 1,2,3 & 4) is connected on one STP CAT5 backbone up to DVR and Group B (Camera No. 5,6,7,8) through another STP CAT5 backbone. Other 2 (Camera No. 9, 10)are connected directly to DVR through Coax RG59.

Before correction:

Group A and Group B + 9 & 10 were connected to different AC power outlets and groung wires connected. Voltage between Group A baluns' body on DVR and ground was different for different cameras like 10, 7, 13, 15V and about the same for Group B and about 15-20V for other 2. The voltage across baluns' body of each group and across baluns' body of different group was not the same.

After correction:

Ground wires of all the cameras were removed, Cameras were isolated from the steel poles and power supplied from common outlet to all the cameras and DVR/monitor. Now there is same voltage between baluns' body of Group A + group B and ground about 5V, camera 9 & 10 removed. Voltage between baluns' body of each group is about 0V but about 12 V across the group. After connecting the shield of cable of group A to group B the voltage dropped to 6 V, camera 9 and 10 is still removed. The shield of the cables is not grounded anywhere and makes no change if connected to ground.

I am not sure if the ground loop still exist and everything is ok to connect all the cameras safely without damaging the baluns again. Please advice me if I am going in the right direction.