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

Magnus Danielson magnus at rubidium.dyndns.org
Wed Jan 7 04:08:47 EST 2009


Joe Gwinn skrev:
> At 4:59 AM +0000 1/7/09, time-nuts-request at febo.com wrote:
>> Message: 6
>> Date: Tue, 06 Jan 2009 21:54:41 -0600
>> From: Brian Kirby <kirbybq at bellsouth.net>
>> Subject: Re: [time-nuts] Standards sought for immunity of shielded
>> 	cable links to power-frequency ground loops
>> To: Discussion of precise time and frequency measurement
>> 	<time-nuts at febo.com>
>> Message-ID: <49642781.2020305 at bellsouth.net>
>> Content-Type: text/plain; charset=ISO-8859-1; format=flowed
>>
>> 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.
> 
> Yes, it's impossible to do this in a system of any size.  In my 
> experience, the RF cables connect the arms of the star-grounding 
> system, causing loops.  So, the receivers had to be immune.  The 
> problem is to quantify and specify the required degree of immunity.

If you only where connecting the boxes through the power grid and have 
no other electrical connection at all, the problem would not be as big. 
Fiber connections does generally not cause any concern.

Real systems interconnect in some interesting mesh of often non-static 
structure. The way to handle that is to build some overall strategy. 
This takes the form of bondning strategy, EMC requirements and wiring 
practices. Additional tools of various sorts helps to releive specific 
problems.

>> 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.
> 
> DC blocks (usually a series capacitor) also work at RF.  But we would 
> have a lot of them.  And we would still need some kind of spec to 
> require, to know when we are done.

Trouble is that most DC blocks is only providing a diffrential block, so 
that would either render it fairly useless for DC purposes (if the 
terminating impedance is low at DC-lowfreq in which case the full inner 
conductor would keep approximatly the same common mode voltage level as 
the shield since the shield is a low impedance path, the termination is 
a low impedance path and the conductor is a low impedance path) or 
dangerous as it would translate a common mode to a diffrential mode 
voltage (for a high impedance termination at the DC-lowfreq as now the 
center conductor does not follow the shield and thus the diffrential 
voltage forms from a common mode source).

An effective cure needs to block common mode disturbances while not 
(significantly) convert them into diffrential mode where the user signal is.

Isolational transformers (basically 1:1 transformers, preferably 
double-shielded, where each shield is hooked to the respective cable 
shield side) is usually preferred in this context. It can be effective 
to connect the grounds on both sides to see if it causes the problem to 
reoccur, if not it is only a precaution to have the transformer.

This type of common-mode to diffrential-mode convertion aspect also 
comes into play when designing lightning arresters. Inappropriate use of 
com-gaps can result in one of them to lit but not the other and a 
diffrential mode spike is formed. The diffrential mode tolerance to 
over-voltage or over-current is usually much less than common mode 
tolerance.

That said, diffrential mode DC blocks have their use, but I fail to see 
how they can aid in this application. Do feel free to bring me up to 
speed on how they help.

>> Don White Consultants/Interference Control Technology published a whole
>> series on EMI, Grounding, and EMC for the military.  They are located in
>> Gainesville, VA.
> 
> But do they publish formal and official requirements documents? 
> That's what I need, versus training.

I hope my previous post was a rought pointer in the right direction. 
There is more and I hope I can help you further on that subject.

EMC is hairpulling experience at times and take some learning, but it is 
fairly basic physics applied in the end.

Cheers,
Magnus




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