[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 16:22:06 EST 2009


Joseph,

Joseph M Gwinn skrev:
> 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.

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.

Great.

>> 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.

As always.

> 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
>> 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.

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.

Cheers,
Magnus




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