Skip navigation
Essential Tests for Structured Cable

Essential Tests for Structured Cable

Exploring the business opportunities that exist for electrical contractors/electricians through properly installed and tested cable

Would you like to expand your business and capture a piece of a market that, even in an economic downturn, is expected to grow to nearly $30 billion worldwide in just a few years? If so, Report Buyer, a London-based market research firm, suggests looking into the structured cabling business. According to its most recent forecast, the international structured cabling systems (SCS) market will grow from $15.3 billion in 2008, at a compound annual growth rate of 13.7%, to $29.1 billion by 2013.

For electrical contractors and electricians, low-voltage wiring requires many of the same skills you already possess. Adding structured cabling to your list of services allows you to bid more jobs. By doing this work yourself, you also rely less on subcontractors, which, in turn, adds to profitability. But to make this new business opportunity worthwhile, it must be done right the first time. As with all projects, a no-charge callback can kill profits quickly.

How can you make the most of this opportunity? What are the steps to ensure the highest level of customer satisfaction and profitability? Common sense says good workmanship and attention to detail will go a long way toward eliminating problems, including a thorough visual inspection to spot flaws such as kinks, cables cut by nails and fasteners, or low-voltage cable routed with high-voltage wiring. Then comes the next logical question: How should you test the installation if, in fact, it needs testing at all?

Cost of testing vs. payback

Historically, the typical argument about testing in most applications comes down to economics. As such, many label it an expense to be avoided — it takes too much time, there's too much equipment to buy, and you continually need training. Although some of the reasons behind this mind-set may have been true in the past, in today's market there is a stronger argument for testing each new installation. These days, tools are simpler and less expensive than they used to be, you don't need a lot of equipment (just the right equipment), and multi-functional devices allow you to do a lot of work with a small amount of equipment. In fact, an installer who tests his work and repairs four jobs can typically more than offset the money he would have lost performing four no-charge callbacks.

When you get down to the heart of the matter, there are really only four essential tests that need to be performed as part of the initial installation:

  • Which wire am I working with?

  • How long is it?

  • Is it connected correctly?

  • If not, what's wrong, and where is the problem?

Which wire is which?

Testing installed cabling begins with identifying the cable under test. In most structured cabling installations, large groups of cable are routed back to a patch panel or some other distribution point. Two quick methods are available for identifying cables: using a toner and probe set or a cable ID device.

Toner and probe sets have been available for many years. The tone-generating device is attached to one end of the cable. A probe is used on the other end to identify the cable carrying the tone. Once the cable is identified, it can be labeled or terminated in the proper connector.

Toner and probe sets are inexpensive and easy to use; however, there are some drawbacks. Traditional sets have relied on simple analog tones that are susceptible to electromagnetic interference (EMI). If a fluorescent light or a computer power supply is close to any of the cables over the length of their run, the probe may not be able to distinguish this interference on one cable from a legitimate tone on another.

Fortunately, there is a relatively new solution. Newer toner/probe sets are based on digital transmissions. The toner puts a unique digital signal on the cable. The probe will only respond to that digital signal. This makes the set immune to EMI and the resulting false readings. The even better news is many tools that perform the other three essential tests (more on those in a minute) can generate digital tone.

The second method of identifying a cable is to use a cable ID device. This device connects to the far end of a cable. A matching test tool is connected to the near end, completing a circuit on this cable. The test tool reads the cable ID device, displaying “Cable 1,” “Cable 2,” etc.

How long is the cable?

There are many reasons to know a cable's length, the most obvious of which is to make sure it does not exceed the maximum length for a given cable run.

For most common network applications, that maximum length is 100 m, though different applications have different requirements.

There are two primary methods of determining the length of a cable: time domain reflectometry (TDR) and capacitive testing. A TDR places a signal on a wire and measures the amount of time it takes for the signal to reach the end of the wire, reflect off the end, and bounce back to the TDR. Electrical signals on a copper wire travel at a significant fraction of the speed of light, meaning the times being measured are very short. TDR measurements are quick and typically accurate.

Cable length can also be measured by capacitive testing. This method is simpler but potentially less accurate. Both the diameter and chemical makeup of the wire can affect capacitance; therefore, a good capacitive length tester needs to be able to compensate for these factors.

Is it connected correctly?

The majority of the cable used in structure cabling projects is unshielded twisted pair (UTP). UTP consists of four pairs of conductors — eight wires in all — with each wire pair twisted at a slightly different rate (in twists per meter) than the other three pairs. The twists in each pair tend to cancel EMI that might be generated/transmitted by the pair. Because of the different twist rates, interference between the pairs is limited.

Because every cable has eight conductors, it's essential that all eight are terminated correctly at each end of the cable. This is where a test called wiremap comes in. More than just a simple continuity test showing each conductor is terminated at both ends, a wiremap test ensures that each conductor is terminated in the right position relative to its intended wire pair and the other six conductors. One area where a good wiremap tool can save a great deal of time is identifying and solving the problem of split pairs.

UTP was originally designed for and used in telephone systems. The telephony standards define that one pair is terminated on the center pins of an 8-pin modular connector — pins 4 and 5 of an RJ45 jack or plug. The next pair is to be terminated on pins 3 and 6, the next one on pins 2 and 7, and the last pair on pins 1 and 8. With the advent of data traffic, this pairing scheme was partially changed. To maintain compatibility with existing telecom equipment, while better serving the needs of data transmission, conductor pairs were defined as 1-2, 3-6, 4-5, and 7-8.

The important thing to remember is there are four wire pairs in a UTP cable, and they are terminated on the following pin number combinations: 1-2, 3-6, 4-5, and 7-8. Failure to understand this can result in split pairs. If an installer were to punch down conductors based solely on the simpler, but erroneous, pair-grouping of 1-2, 3-4, 5-6, and 7-8, the cable would appear as shown in Fig. 1. Because these pairs are split, this cable likely will have excessive crosstalk and poor data transmission. A good wiremap tester will detect split pairs, plus other connectivity issues.

Where is the problem?

A good test tool will show you more than just the problem. It can also tell you where to find it and, sometimes, what to do about it.

An example of this would be an error message on screen such as “impedance mismatch at 15 m.” With this information, you would know to look for a splice in the cable approximately 15 m down the length of the cable from the test unit. Similar messages can graphically show where on the cable to look for opens and shorts (Fig. 2).

Tools to get you there

There are dedicated tools for each of the four essential tests discussed in this article; however, there have been advances in these products so that all four tests can be conducted with a single verification tester.

One of three levels of testing associated with structured cabling, verification testing is the type of test most frequently employed by electrical installers. The best verification testers are compact, intuitive, and affordable so that each installer can be equipped with one. Many are able to generate tones for cable identification; some are able to generate digital tone, solving problems from EMI. All should perform wiremap and length tests. A few even provide diagnostic information.

It must be noted that there are other levels of testing applicable to different types of projects. Qualification testing should be the final step for residential installations after all construction is complete. More rigorous yet is certification testing, the final test for commercial installations. This is a task electrical installers can perform (and a topic for a separate article), or it can be subcontracted out to a certification specialist.

As you can see, there are new business opportunities in structured cable testing just waiting for electrical installers. A host of new, simple, and affordable tools make this level of testing possible. Use them to your advantage to grow your business.

Mukherjee is with Fluke Networks in Everett, Wash. He can be reached at [email protected].

TAGS: content
Hide comments


  • Allowed HTML tags: <em> <strong> <blockquote> <br> <p>

Plain text

  • No HTML tags allowed.
  • Web page addresses and e-mail addresses turn into links automatically.
  • Lines and paragraphs break automatically.