Installing telecom cabling near power conductors

Q: I am currently working on a project in which ducts for telecommunications cabling will be installed near ducts containing 6-kilovolt supply conductors. What should be the separation between the two ducts?

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Q: I am currently working on a project in which ducts for telecommunications cabling will be installed near ducts containing 6-kilovolt supply conductors. What should be the separation between the two ducts? Can you suggest a technical guide?
Hernando Amaris Duarte
Polysisus, Spain

A: In the United States, we use the National Electrical Safety Code. The NESC covers basic provisions for safeguarding of persons from hazards arising from the installation, operation, or maintenance of conductors and equipment in electrical supply stations, and overhead and underground electrical supply and communications lines. The NESC also includes work rules for the construction, maintenance, and operation of electrical supply and telecommunications lines and equipment.

NESC Section 32: Underground Systems provides the following guidance in Section B-"Separation From Other Underground Installations" Part 1, General:

"The separation between a conduit system and other underground structure paralleling it should be as large as necessary to permit maintenance of the system without damage to the paralleling structures. A conduit that crosses over another subsurface structure shall have a separation sufficient to limit the likelihood of damage to either structure. These separations should be determined by the parties involved.

Exception: When conduit crosses a manhole, vault, or subway tunnel roof, it may be supported directly on the roof with the concurrence of all parties."

Part 2 of Section B is titled "Separations Between Supply and Communication Conduit Systems." It reads:
"Conduit systems to be occupied by communication conductors shall be separated from conduit systems to be used for supply systems by

Exception: Lesser separations may be used where the parties concur."

The most recently published version of the NESC was 1997, and is available from the Institute of Electrical and Electronics Engineers (IEEE-New York City). See the Web site:

I attempted to locate an IEC equivalent to the NESC, but had no luck. If anyone knows of one, please drop me an e-mail.

Red or black?

I was recently asked to locate standards for Red/Black-TEMPEST. No, it wasn't a search for Shakespeare in Vegas, but rather, for a how-to guide for cabling sites that process classified National Security information.

TEMPEST is an official acronym for "Telecommunications Electronics Material Protected from Emanating Spurious Transmissions"-a fancy name for protecting against technical surveillance or eavesdropping.

I must say that with all the questions over the years, this one has presented me with the biggest challenge. After weeks of researching, I discovered why it was so challenging. Until December 2000, the National Security Telecommunications and Information Systems Security Advisory Memorandum (NSTISSAM) TEMPEST/2-95 RED/BLACK Installation Guidance document was classified material.

The general idea is that devices-CPUs, monitors and the like-give off electromagnetic radiation, which with the right antenna and receiver, can be intercepted from a remote location. This not being a good thing, there is a body of standards-most still classified-that explains how to design cabling systems in such a way as to remove any TEMPEST threats.

Black equipment processes only unclassified information, and black optical-fiber lines and wire lines carry on unclassified signals, while red equipment, optical-fiber lines, and wire lines process and carry unencrypted National Security information that requires protection. The level of protection required depends on the data's classification: sensitive, confidential, secret, or top secret. That level of granularity is also reflected within MIL-HDBK-232A.

TEMPEST/2-95 specified RED/BLACK equipment/system installation guidance, provides an explanation of the TEMPEST policy concept, and addresses RED/ BLACK considerations for facilities in which national security information is processed. In that document, Table 3-1, "RED/BLACK Installation Recommendations" provides separation specifics based on the level of equipment and the facility zone. See for the complete text.

While this question came from someone who is working on a United States Air Force project, the same separations would be applicable to any installation where heightened security is required.

To CMP-50, or not?

Regarding the new CMP-50 rating for cabling, I was asked since we couldn't leave it in place forever, then why would we use it at all? In a word: survivabilityellipse of the structure.

Tests conducted by the Fire Protection Research Foundation (FPRF) demonstrated that plenum cable insulated and jacketed with limited-combustion materials (CMP-50) produced less smoke and contributed less fire load than conventional plenum cable (CMP). The FPRF is an independent, non-profit foundation that investigates fire-safety technologies and strategies for the National Fire Protection Agency (NFPA).

In the FPRF tests, a CMP cable generated up to 21 times as much smoke as a CMP-50 cable, and its flame-spread and fire-load values were larger. Yet all cables tested met current requirements for use in plenums.

I feel that end users will vote with their conscience and their checkbooks. They can select CMP-50 cables for installations in which fire and smoke present special risks, including some public buildings, hospitals and other health-care facilities, university buildings, and buildings housing electronic and switching equipment. Our responsibility as designers is to present them with sufficient information so that they can make informed decisions.

Copies of the report, International Limited Combustible Plenum Cable Fire Test Project, are available from the FPRF. The organization's Web site is

Proposals have been made, and tentatively accepted, for inclusion in the National Electrical Code that all cable in plenums should be removed when no longer in use (abandoned). Design it in and design it out-even if this was not a requirement, it is simply good design to clear the abandoned cable from the pathways.

Tiny telecommunications room

There is a movement afoot to locate LAN electronics closer to the user. At first glance, the idea is great-fewer terminations in the telecommunications rooms, less cable in the plenums, shorter channels, and easier plug-and-play for users.

The general idea is to locate the LAN switch within a cluster of open-office furniture and feed it with fiber. This is sort of like adding active equipment to a consolidation point-which would then actually be a telecommunications room, minus the floor space, lighting, controlled environmental air, grounding, and, in some cases, physical security and fire safety.

From my perspective, closer is better, but only if the level of security, reliability, and safety provided in the TIA/EIA-569A-defined telecommunications room is maintained. And who better to do that than the committee that oversees that document? At its March meeting, TR-42.3 established a task group to begin work on standardization.

The idea is still great, and once the issues are resolved, these "tiny telecommunications rooms," or "active consolidation points," or whatever they will be called, will make an excellent addition to a designer's toolbox.

But if you just can't wait, at least address the following issues during the design phase:

  • How to access the electronics;
  • What to do when the furniture cluster's switch or fiber fails;
  • How to provide dedicated "clean" power for the electronics;
  • How to provide environmental-conditioned air 24/7;
  • How to provide grounding and minimize static discharge.

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Donna Ballast is a communications analyst at The University of Texas at Austin and a bicsi registered communications distribution designer (rcdd). Questions can be sent to her at Cabling Installation & Maintenance or at PO Drawer 7580, The University of Texas, Austin, TX 78713; tel: (512) 471-0112, fax: (512) 471-8883, e-mail:

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