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

[Joseph M Gwinn]

Magnus,

time-nuts-bounces at febo.com wrote on 01/07/2009 01:27:52 AM:

> Joseph M Gwinn skrev:
> > 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. 

I left a useful detail out:  The reference signal is a 10 MHz sinewave.


> > 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 shieldis 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. 
> 
> For most purposes an isolation transformer would solve this issue. The 
> unfortunate signal characteristics of a PPS pulse makes this a little 
> more cumbersome, but not unachievable, but it is no longer a simple 
> passive device. For higher frequencies will RF chokes be an aid of 
> course, but the RF choke needs "bolting down" in order to be effective, 
> so that there is a common mode current for the RF choke to object to. 
> However, the RF choke is not as effective with lower frequencies and 
> essentially useless for DC.

The receivers have built-in RF transformers.  There is no 1PPS signal per 
se, although the transformer would probably pass such a signal well 
enough.  What is being carried is 10 MHz. 

The problem is to devise a test and spec that ensures that the actual 
implemented circuit in the receivers suffice.  There are many ways to 
botch this circuit.


> > 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 
theirharmonics.
> 
> You should look into the telecom set of standards. If you think of it, 
> they have been addressing this particular problem for ages. The words 
> which probably get you right on the target is "bonding network" since 
> you bond to the ground.

This is just the sort of lead I was hoping to find.

 
> In short, there are two grounding strategies: all gear is floating 
> relative the safety ground or all gear is internally tied to the safety 
> ground. There is benefits and problems with both strategies. Regardless, 

> a hierarchial star ground strategy emerges.

In our systems, everything is tied to ground for both safety and RF 
reasons unrelated to timing signals.  And we do have a star of sorts, but 
the story always ends up more complex than that, so it always ends up 
being a somewhat random grounding grid.

My problem is not safety, it is tolerance of conducted EMI.

 
> One document to start with is the "Qwest Technical Publication
> Grounding - Central Office and Remote Equipment Environment" at
> http://www.qwest.com/techpub/77355/77355.pdf
> 
> Not to say that it is the standard of any sort, but I think it is a good 

> document to start from as it is a public source of telecom bonding 
> practices to be used in many facilities, implementing existing 
> international standards and involving transmitting towers (which is 
> within your field).
> 
> IEC 60950 should be a standard reference regardless.
> 
> You should also consult Bellcore GR-1089. There are additional Bellcore 
> specs, but starting with GR-63 and GR-1089 is not totally off the mark 
> at least. Bellcore specs costs money, but if you need to comply there is 

> no alternative.
> 
> ITU-T has a set of documents, such as the K-series of standards. You can 

> download these for free at:
> http://www.itu.int/rec/T-REC-K/e
> 
> The European telecom world uses ETSI EN 300 253 as basis. They require a 

> login which you can get for free and then pull down all the documents 
> you like. There is also alot of specific EMC documents for various 
> contexts etc and they are all there. ETSI EMC is the TB handling them.
> 
> On the military side, MIL-HDBK-419 may be a guide:
> http://tscm.com/MIL-HDBK-419A.PDF
> 
> Old standard MIL-STD-188-124B:
> http://www.tscm.com/MIL-STD-188-124B.PDF
> 
> Newer standard MIL-STD-1310 for ships:
> http://www.earth2.net/parts/basics/milstd1310g.pdf

I will be doing some homework.  Some of these are tomes.

 
> In the end, all these documents forms a reference of standards and 
> practice in a varity of environments. I suspect that your environment 
> does has some bonding standard and practice and you need to figure out 
> what it is so that you know what you can expect, what you need to 
> fullfill (which is limiting freedom on what methods you may apply!) and 
> then it becomes easier to say what may help you. Also, you need to 
> figure out what is the type of problems you run into, how disturbances 
> actually induce into your lines. It could very well be that PSUs acts as 

> EMF due to bad conditioning for instance.
> 
> There are many anecdotes and horror stories to be told on the subject.
> There are also sucesses stories to be told.

We do have a bonding story, one that sort-of follows MIL-STD-1310, even 
though the system is land based.

 
> What makes the field a bit complex is that you need to think about 
> failures, EMC, bonding, interference, lightning strikes (on wire, in 
> tower, on building) which can cause a disparity of various indirect 
> effects. It's a bit like being a time-nut. We could probably have a 
> separate email list setup for that kind of discussions alone.

Fortunately for me, I do not have to worry about lightning.  That's 
handled elsewhere, as all these cables are within a steel-frame building 
with a lightning protection system built in.

Joe