Practical view of shielded vs unshielded cabling dispute

Practical view of shielded vs unshielded cabling dispute

There are many discussions and confusions concerning the use of shielded
versus good-balanced UTP cabling. We tried to get a practical
picture of real things based on different lab testing.
We found several reports that document the results of practical testing for UTP,
ScTP and STP immunity and emissions.

Here are web site links where you can find these reports:

1. DELTA European Cabling
"Proposed cabling setup for electromagnetic characterisation of cabling and
EMC measurements on LAN systems".
http://www.delta.dk

2. 3P Third Party Testing
"EMC performance of cabling"
http://www.3Ptest.dk/3Ptest/index.html

3. Wiremold
"How EFT disturbances affect Ethernet and Fast EThernet Performance in
Multichannel Plastic Raceway"
http://www.wiremold.com

4. SIEMON Company
"UTP Cabling and the Effects of EMI"
http://www.siemon.com

Basically, if your external field strength does not exceed 3 V/m, your
good-balanced UTP cabling will function without any problems.

Some tests from Lucent show that it extremely difficult to find an office
environment with field strength exceeding this value. Testing in an airport
(where EMI is harsh) has shown that field strength is below 3 V/m.

It needs to be remembered that simply putting the shield around cabling does
not help to improve immunity and reduce radiation. There are several critical
conditions:

- shield must be continuous along the whole link (without any pigtail
connections!)
- shield must fully enclose cabling (360degrees) along the whole link - shield
must be grounded at both ends of the link
- the building must have a good-quality grounding system (according to
IS11801, max. voltage difference between any grounding points must not
exceed 1 V)

If one of these conditions is not satisfied, then shield performance will be
badly degraded. For example, tests from 3PTest shown that if the shield
continuity is broken, the emissions from shielded cabling system increases
as much as 20 dB on the average.

Conclusion
==========
Shielded cabling may behave better with regard to immunity and radiation ,
but it takes too much efforts to provide the required conditions for this
behavior in terms of money, maintenance and regular checking.

Good-balanced UTP system provides good EMI performance in the most
office and industrial environments (where field strengths is below 3 V/m).
Where field strength is more than 3 V/m, it is safer to use fiber that is
immune to EMI. Also, be aware that most LAN equipment is also rated for operation
with field strength up to 3 V/m.

Back to Designer's Tips

Practical view of shielded vs unshielded cabling dispute

Home Cabling Guide

Learn more ...