[time-nuts] Standards sought for immunity of shielded cable links to power-frequency ground loops
magnus at rubidium.dyndns.org
Wed Jan 7 16:22:06 EST 2009
Joseph M Gwinn skrev:
> 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
>>> the facility, such as a ship or a megawatt radar.
> I left a useful detail out: The reference signal is a 10 MHz sinewave.
10 MHz into a transmitter. Should not be too hard to master. For some
reason I feel confident in that environment. :)
>> 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.
I see. It is fairly easy to induce common mode currents and DC voltages.
An isolational transformer from a source and then on the other side
simply DC offset or apply signal through a transformer if not directly
from an amplifier.
>> 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.
The reason I mention safety is that some people suggest solutions which
does not fullfill the safety criteria in spirit or standard. It gets you
into the right category of solutions.
>> One document to start with is the "Qwest Technical Publication
>> Grounding - Central Office and Remote Equipment Environment" at
>> 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:
>> 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:
>> Old standard MIL-STD-188-124B:
>> Newer standard MIL-STD-1310 for ships:
> I will be doing some homework. Some of these are tomes.
You could also look up ETSI EN 300 132-* and EN 300 386 which is
relevant for telecom boxes. Further on is EN 300 199-* probably good to
have around, but maybe not so applicable to this particular problem.
What you want to transfer is similar to an E1 or E2 on an intra-office link.
>> 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.
Sounds good. Will think about levels.
>> 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.
Actually, the most outer cabling links needs to be shielded or else they
would intduce into cables.
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