Connections +
Feature

Maintenance & Testing – Taming the Wired Beast

Cable technology is evolving at a pace that would dizzy Darwin, but following a simple guideline when testing residential wiring will ensure both functionality and compliance.


July 1, 2001  


Print this page

Modern residential cabling installations use a technique called a “home run”. This means that all communications services coming into the home arrive at a central location, which, by default, also becomes the demarcation point for the various utility companies

The heart of this scheme is a multimedia junction box that is often called a “structured cabling centre”, but has a variety of different names, depending on the vendor. All of your communications wiring should emanate from this box in a “star” topology that extends throughout the house. Any adds, moves or changes that become necessary can be made easily from this single location.

The TIA/EIA 570A grade II standard calls for a new residence to have two of each of the following cables:

4-pair UTP (unshielded twisted pair) cables rated for Category 5 (100 MHz) or better for telco, LAN, Internet and security wiring. Cables rated for a bandwidth of 350 MHz are available, and will meet the Category 6 specifications (when ratified). These can also be used for video applications using a video balun, which uses the four pairs for red, green, blue and sync signals.

75-Ohm coaxial cables for use with satellite dishes, CATV, CCTV, Internet and home theatre systems. In the past, an installation would typically have RG-59 cable, but a modern installation should use RG-6 (both use type “F” connectors).

Multimode fiber-optic cables for use in LAN, audio and home theater applications. While few homes are currently using fiber technology, this allows for higher bandwidths down the road.

Some wire manufacturers and distributors will actually wrap all of these cable types into a single bundle that can be pulled through the house all at once.

CERTIFICATION

AND VERIFICATION

In a more complex commercial job, the installer concludes by performing a masterpiece of modern physics called a “certification”. This process involves the testing of all cabling to certain standards that specify parameters like frequency, impedance, crosstalk, return loss and attenuation. These tests are performed with specialized instruments that cost thousand of dollars, which yield results that are traceable to National Institute of Standards and Technology (NIST) standards.

It should be noted that while most installers already own a cable certifier, certification testing is overkill for the residential market. Compared to a commercial installation, the cable runs are shorter, the bandwidth requirements are lower, and the homeowner simply does not want to pay for it because it is very time consuming. The equivalent (and perhaps more appropriate) level of testing for the home or the small office is called “verification”.

Verification testing is typically performed after the homebuilder is finished. All of your cabling must be cut, stripped and terminated. If the cables were not labelled when they were pulled, you may also want to trace them out with a toner and ID them prior to testing. A simple 2-piece continuity checker can alert you to opens or shorts, but to verify UTP cables and terminations you must perform a test called a “wiremap”. Many low-cost testers will perform this test, but here is a quick summary of what is needed:

For twisted pair cables, the wiremap test checks for opens, shorts, crossed wires and split pairs. (This requires a two-piece tester, as there must be a special terminator at the other end). Detection of split pairs is accomplished with a near-end crosstalk (NEXT) measurement, so a simple DC continuity test will not work. This type of fault was not a problem in 10BASE-T systems, but will cause trouble with modern Fast Ethernet LANs.

For coaxial cables, a simple continuity test for opens and shorts is fine. Some testers will automatically send tone on an open coax, and even detect the RF television signal to let you know that it is a live cable.

For multimode fiber cables, a simple test set (with both an 850 nm led source and a receiver that reads out on an LED bar graph) is sufficient.

UNIQUE CHALLENGES

While the testing of residential cabling systems may be simpler (and therefore less costly) than a commercial job, it does present unique challenges to the installer. If all goes well, all of your wiring will verify and the job will be completed. If, on the other hand, it is one of those days, you may encounter a few of the following complications:

Nails and staples are probably the worst culprits in a residential installation. In a new home, the cables are pulled after the structure is framed, but before the walls and insulation are added. Upon returning to terminate and test, the installer may find that some of the wires have been cut or shorted by nails or wire staples. This particular type of failure is easy to detect, but tough to locate. In this example, a tester that incorporates TDR capability is handy because you will want to know how far it is to that open or short.

Code requirements can be a nuisance because they vary by state, by municipality and even by individual inspector. Chapters 7 and 8 of the National Electrical Code cover low voltage (24V or less) wiring from fiber to plenum, and while it is an excellent framework, you are still subject to local requirements. For example, there are still some “old school” inspectors out there who require that all low-voltage wiring be stapled just like the power lines. Not only does this increase the likelihood for short circuits, but if you smash a communications cable, its specified characteristics go out the window. At any point where the normal geometry of a cable is altered, an impedance discontinuity is created, which will generate return loss and degrade performance.

Wall Fishing may be necessary when trying to work in a completed house. This is especially difficult in a residential job because walls are filled with plumbing, chimneys and insulation, and there is really no allowance for separate cabling space. By following the 570A guidelines, you should not have to go back and pull additional connections, but if a contractor has caused damage, be prepared to go fishing.

Cost containment is always an issue, but when testing the cabling infrastructure in someone’s home, you will feel like the proverbial “bull in the china shop”. As mentioned, the homeowner will not want to pay for exotic performance testing, so a verification of all the connections is adequate. You will also find yourself in a more delicate environment than a commercial building. One installer tells of a job where he was wiring someone’s kitchen and fractured a cooking surface valued at more than one thousand dollars.

While the technical requirements for residential installations are only a subset of what is required at the commercial level, there are facets of the job that tend to make it trickier. Be sure to arm yourself with the right tools and do the proper planning — and then go forth and verify.CS

Brandon Crowe earned a degree in physics from Purdue University. In 1991, he formed Darkstar Technologies, West Lafayette, IN, which designs and manufactures hand-held test instruments for the communications industry.


Print this page

Related