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On 1/9/2022 3:26 PM, Richard (Rick) Karlquist wrote:
 The grounds for these independent systems are subject to considerable 
voltage differences and will draw considerable current if you try to 
connect them together.  Note that for this purpose, "ground" and 
"neutral" are the same circuit.
 
Engineers from BBC and ABC-TV were active members of the AES Standards 
Committee, on which I have served for nearly a quarter century. ABC-TV 
has (or had at that time) major facilities on opposite sides of a major 
street in Manhattan, and the shields of video tie lines between them saw 
exactly this problem in a big way. With audio, it could be solved with 
transformers, but the signal bandwidth was far to great for video. 
73, Jim K9YC
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