Re: Separation from electrical

Re: Separation from electrical


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Posted by Roman Kitaev, RCDD on March 13, 19100 at 07:58:50:

In Reply to: Re: Separation from electrical posted by Dmitri Abaimov, RCDD on March 10, 19100 at 12:56:16:

Phil,

the old version of the EIA/TIA 569 standard contains
the minimum separation distances between telecom pathways and
power wiring of 480 volts and less.

For "electrical equipment in proximity to open or
nonmetal telecom pathways" (i think it's the
situation valid for TCs), it dictates the following
separation distances:

< 2 kVA 127 mm (5 in.)
2-5 kVA 305 mm (12 in.)
> 5 kVA 610 mm (24 in.)

Best regards,
Roman Kitaev, RCDD

: Dear Phil,
: The issue you raised is really on of the hottest ones in the industry. And I hevent seen any formula yet. And I doubt it can exist at all. There are so many parameters involved, like amperage, load type, size, ferromagnet material used, encasing and so so on.
: There are some reference points, however.
: 1) There is an ISO/IEC 801-3 standard that sets the EMI field strength limit for the systems (not only cabling) for frequency range 27MHz - 1GHz. Yes, it's the way different from 50(60)Hz, but still... This standard requires commercial equipment to meet Severity Level 2, which is 3 V/m.
: 2) There is a set of tables in the SYSTIMAX Design Guidelines. Very old document originated by AT&T. So these tables say for the toughest situation possible in an office building - 380V, 1 phase, load is heavy machinery (not quite an office, but still), 12xAWG0 cables with 105 Amps in each, with the length of exposure (running in parallel)50 meters (152.5 feet) the minimum separation deistance is only 1 meter. It's not quite easy to match transformer to power line, but I would suggest it's like maybe 10 meters of exposure or maybe less. We end up with less than 1 meter of separation. Are patch panels more susceptive to EMI than cables? Well, sounds reasonable because we have to untwist some conductor length and because there is a place inside the jack where conductors run in parallel. But still, in the closets the exposure is very limited, only few meters if it's planned well, so I believe 1~2 meters of separation is enough between the panels.
: 3) If you keep that physics and math knowledge, gathered in a university, you may try to calculate that induced voltage to see whether it's within the limit of 50mV thoughout the cabling system. Old AT&T requirement for voice circuits. Why to use that old stuff? Just because these days there was only DIW (pre-CAT3) cable available, and modern ones are much better balanced. Besides, the only application that really susceptive to 50(60)Hz (plus some harmonics) noise is an old analog voice. In this digital world, all other devices send signals of such a high frequencies that this kind of noise is just filtered out.
: I'd be glad to see more comments from the cabling industry community on that.
: Best regards,
: Dmitri Abaimov, RCDD

: : I have been in several discussions with co-workers and architects and I have yet to receive an acceptable answer.

: : When designing and Structured Cabling System, I invariably try to get DEDICATED Telecommunications Closets (I know it's a room, but the "standards don't say that). When I am successful, it's always adjacent to an electrical closet.

: : My question is this:
: : What are the MINIMUM distances away from transformers and large electrical distribution panels that communications cabling should be? Is there any formula that can be used to measure anticipated EMI of X-size transformer at X-feet? If there is, what is the MAXIMUM EMI tolerance for cabling?




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