From time to time someone will pop up on a forum or user group with tales of fried SDI boards, dead monitors or dead audio devices. Often the reason for the death of these units seems obscure. One day it all works fine, the next time the monitor is plugged in it stops working.
A common cause of these types of issue is the use of individual power supplies for each device. Most modern power supplies use a technology called “switch mode”. Most “wall wart” power supplies are switch mode. Computers use switch mode power supplies, they are probably the most common type of power supply in use today.
The problem with these power supplies is that the voltage they produce is not tied to a common earth or ground connection. A 12 volt power supply may have an output voltage that measures 12 volts across it’s positive and negative terminals, which is great. But the negative terminal might be many volts above “ground”. Used singly this is not normally a problem but if you use a couple of different power supplies with negative terminals floating at different voltages, if you connect them together current will flow from one to the other as the establish a common base voltage.
As an example if you have a monitor powered by one power supply and a camera powered by another, when you connect the monitor to the camera current may flow down the SDI or HDMI cable from one power supply to the other causing damage to the chips that process the SDI/HDMI signals.
Even if there is no damage this current can lead to audio hum or other electrical noise.
How can you prevent this?
First use only high quality power supplies. Wherever possible try to run everything off a single power supply. Powering the camera from a high capacity power supply and then feeding any connected accessories via D-Tap or Hirose outputs on the camera is good practice. Also powering everything by batteries helps. If you must use separate power supplies then connect everything together before connecting anything to the mains and before turning anything on. This should ensure that any current runs through the shield and ground paths in the cables rather than possibly travelling down the delicate signal part of a connection as you connect things together.