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

[Magnus Danielson]

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. 
> 
> 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. 

For most purposes an isolational 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.

> 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.

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.

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.

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 stdandard MIL-STD-1310 for ships:
http://www.earth2.net/parts/basics/milstd1310g.pdf

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 sucsess stories to be told.

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.

Cheers,
Magnus