Multimedia wiring forces home safety issues
The National Electrical Code (nec) is revised every three years, and we on the committee of the National Fire Protection Association (nfpa--Quincy, MA) who are responsible for revising the nec are in the final phase of completing work on the 1999 code. In this revision cycle we have taken some significant steps to enable multimedia communications in a residential environment. Along the way there have been some challenges in developing a safety code that meets the needs of the many users and prob
The 1999 National Electrical Code establishes a single common grounding point.
The National Electrical Code (nec) is revised every three years, and we on the committee of the National Fire Protection Association (nfpa--Quincy, MA) who are responsible for revising the nec are in the final phase of completing work on the 1999 code. In this revision cycle we have taken some significant steps to enable multimedia communications in a residential environment. Along the way there have been some challenges in developing a safety code that meets the needs of the many users and probably will affect the way residential cabling systems are designed and installed.
What is multimedia? Let`s start by taking the word apart and defining its two halves, "multi" and "media." The first part is easy. "Multi," according to my dictionary, means "much" or "many." Other definitions suggest it means more than two, or many times more, or suggests the combining of elements. "Media" is the plural form of "medium," which itself means "means" or "agency." For this definition the dictionary gives an example: "Radio is a medium of communication."
So, multimedia refers to two or more types of transporting systems such as copper wire and optical fiber; or two or more means of communication, such as radio or television; or two or more human information receptors, such as the senses of sight, hearing, and touch; or two or more information environments, such as fixed and mobile. But the definition that I prefer is as follows: Multimedia is an artificial environment that may be supported by multiple transport technologies to enable a perception of realism that is made possible through a fluent combining of the human senses.
The nfpa and the nec
The nfpa has sponsored the nec since 1911. The original nec document, developed in 1897, resulted from the united efforts of various insurance, electrical, architectural, and allied interests. Changes to the code are made by proposing amendments that are published in the nfpa`s Report on Proposals (rop), in which are also published the actions taken on each proposal by the various Code Making Panels (cmps) and the Correlating Committee.
A period is set aside for comments on the proposed amendments and actions taken on them to date. At the end of that period, the cmps meet again to review the comments and once again take actions that are reported to the Correlating Committee. This activity is published, in turn, in the Report on Comments (roc).
Both the rop and roc are presented at the annual meeting of the nfpa. If adopted, they are incorporated for publication in the next nec. This sounds like a fairly straightforward process, but there have been some interesting turns in the development of the 1999 nec.
Multimedia in the home
Where is the problem with multimedia in the home? It can be illustrated with a true story about a young man and woman who were talking on the telephone during a thunderstorm. The young man put one ear against the speaker of a radio while the other ear remained pressed to the telephone. The young woman heard a sharp click on the phone and asked her friend about it but received no reply. The young man was found dead with one ear still against the radio and the other to the telephone.
The click was the sound made by an electrical transient as it traveled down the telephone line. The transient, also known as a "spike," resulted from lightning either hitting an aerial cable or striking near one. The high voltage traveled down the cable until it found a place where it could jump to ground. Even if the lightning strike was a near-miss, the path would have been almost the same. As the lightning bolt passed by the cable, the intense electrical field created by it induced a voltage on the wire, which passed down the cable until it found a path to ground.
In the investigation that followed this accident, it was determined that although the telephone was protected and grounded, the electrical and telecommunications systems in the young man`s residence were not bonded together. When the electrical transient entered the house on the powerline, it found a path to ground through the telephone system. Protection for the telephone system is usually provided by an entrance protector. But in this case, there was a station inside the house that was exposed to a transient from another source that was, in turn, grounded at the entrance protector. The transient`s eventual path to the entrance protector was from powerline to home electrical wiring to radio through the young man to the telephone receiver. This particular incident has led to the nec requirement that all transport technologies must be bonded together. This requirement is included in Section 800-40, "Cable and Primary Protector Grounding," and more specifically in Section 800-40 (d), "Bonding of Electrodes."
Multimedia and grounding
Now let`s consider our definition of multimedia in light of the transport technologies we find in our homes today. At a minimum, these technologies can include electric power, telephone service, cable TV, satellite and low-earth-orbit satellite dish systems, and off-air services such as radio and television. Not only has the residential multimedia environment grown to include all these services and technologies, but it also involves continuous contact between humans and the different transport systems. Both the number of systems involved and their interconnection distinguish this situation from what existed even in the recent past.
This new residential environment has greatly increased our exposure to electrical shock. For a human to be exposed in this way, there must be a pathway for the electricity to travel through the body. The combination of the personal computer, where we locate our hands while typing or manipulating the joystick, and the telephone, which we have next to our ear or connected to a headset, provides a human path between multiple and separately grounded systems. To make matters worse, this path incorporates our two most essential organs, the heart and brain.
In the relevant standard of the American National Standards Institute (ansi--New York), ansi t1.321-1995, "Electrical Protection for Network Operator-Type Equipment Positions," the abstract states: "Electrical disturbances may appear at network operator-type equipment positions arising either from electrostatic discharge [esd] or from other sources such as lightning or AC power disturbances. This standard provides esd-mitigative measures that are intended to control esd in the network operator-user environment, and electrical protection measures that are intended to minimize potential differences at a network operator-type equipment position."
This standard is most definitely applicable to the residential multimedia environment because in both situations you have a person closely linked to equipment that is supported by a network linked to multiple transport systems. The only difference between these two situations is the fact that the exposure for a network operator-type equipment position comes from multiple sources on the same network, while for the home worker the exposure is from different transport systems. In fact, the network operator position is safer because it is likely that all the network entrance facilities are linked to one common grounding point and that the only other transport technology present will be the AC power.
This ansi standard goes on to specify that a personal bonding terminal will be established at each operator-type workstation and that all technologies appearing at the workstation will be bonded to it together.
Amending the nec
Roger Witt, a representative of an insurance company, submitted several proposals to the cmp for the 1999 nec. These proposals were directed at Section 800-10C, "Point of Entry"; Sections 800-40 (b), 810-21 (f), and 820-40 (b) (f), "Electrodes"; and Sections 810-55 and 820-11 (c), "Point of Entry". In his proposals Witt tried to accomplish the same thing as the ansi t1.321-1995 standard: To get one common bonding point for all transport technologies within 5 feet of the AC power entrance. After many hours of intense debate and artful maneuvering, the Panel 16 attendees defeated these proposals by one vote. That`s when the real work began.
Shortly after the Panel 16 meeting, I contacted Witt to review the actions of the cmp and find out why he submitted his proposals. Witt investigates insurance claims made on his company and establishes their causes. He also is charged with developing methods to prevent recurring claims. His investigations showed that lack of effective single-point grounding in residences had led to extensive losses for his company.
I shared with Witt my opinion of the reasons behind the narrow defeat of his proposals, and to turn the situation around we decided to call on the expert abilities of Harry Katz. A principal on cmp-16, Katz works for the South Texas Electrical Joint Apprentice Training Committee and is the nec representative to the International Brotherhood of Electrical Workers. As such, he not only knows the nec intimately, but also understands the reasoning behind individual sections--both the theoretical issues and the actual, practical applications of the code.
Katz and Witt met to exchange ideas, and Katz used his understanding of the code to reword Witt`s proposed amendments. Then Witt circulated the modified proposals to other interested parties and gained their support. This support was evident at the meeting of cmp-16 following the comment period.
The chairman of cmp-16, Dean K. Wilson of Industrial Risk Insurers (Hartford, CT), recognized that an important safety issue was involved and got the different parties in the debate to develop a consensus. To accomplish this, all parties must first be brought to the same level of understanding of the problem. Then everyone needs to understand the viewpoints of the other participants. It may then be possible to put forward a compromise position that can be supported by the majority of committee members.
In this case it took some time to reach a compromise, but by the time we did, everyone understood the serious exposure to electrical shock when multiple transport technologies are linked to an individual, as is the case when someone works at a computer while wearing a headset. If you think about children playing computer games at home, it becomes clear that this exposure to risk is a real and immediate concern.
The cmp eventually approved the Katz proposal to establish a single common grounding point within 15 feet of the power entrance of a residence. While this solution is not perfect, it goes a long way toward mitigating the exposure to risks that residential multimedia users face. I urge every reader to get a copy of the ansi t1.321-1995 standard and understand its message. This document, along with the new and revised sections of the nec, due out in 1999, will give you proper guidance in planning, designing, and installing residential multimedia environments.
Jim Romlein is the president of mis Labs (Watertown, WI), an active participant in standards-making for the Telecommunications Industry Association (TIA--Arlington, VA), and a member of Panel 16 of the nfpa.