[time-nuts] Standards sought for immunity of shielded cable links to power-frequency ground loops

Brian Kirby kirbybq at bellsouth.net
Wed Jan 7 03:54:41 UTC 2009


During my experiences involving audio/phone, video and data 
transmission, we were taught to ground the shield at one end only so we 
would not cause a ground loop.

I ran into problems everywhere I went with this and as much as folks 
disdain transformers, they are your friend in this type of problem.

Don White Consultants/Interference Control Technology published a whole 
series on EMI, Grounding, and EMC for the military.  They are located in 
Gainesville, VA.

Brian



Joseph M Gwinn wrote:
> First the background: 
> 
> In some timing distribution applications, the primary source of 
> interference comes from different ground voltages in different parts of 
> the facility, such as a ship or a megawatt radar. 
> 
> The effect of differing ground potentials on a shielded cable is to pull a 
> large current through the shield, so there is a significant voltage 
> between the ends of the cable.  No matter how good the shield is at RF, 
> one consequence is that the same power-frequency offset voltage appears on 
> the conductors within that shield, because the skin depth at 60 Hz vastly 
> exceeds the thickness of any reasonable shield.  Unshielded twisted pair 
> will suffer the same common-mode offset voltage, perhaps more.   This 
> offset often contains significant harmonics of the power frequency, 
> nominally up to the seventh harmonic, not just the fundamental.
> 
> If the cable is shielded twisted pair, such as twinax, the offset appears 
> as a common-mode voltage on the two conductors, and (if not too large) is 
> eliminated by the CMRR of the receiver. 
> 
> If the cable is coax, the offset voltage appears added to the timing 
> signal voltage, and if the offset isn't too large the signal receiver will 
> be sufficiently immune to this conducted EMI. 
> 
> 
> And now the question: 
> 
> What standards exist governing required immunity of signal ports to these 
> ground-loop induced power-frequency (hum) voltages?
> 
> All the conducted suseptability standards I've found cover only 
> frequencies exceeding 10 KHz, not power frequencies and their harmonics.
> 
> 
> Thanks,
> 
> Joe
> 
> 
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