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Title Author Insulation testing requirements for telecommunications twisted pair cables Bryan Phillips Date April 2010 Keywords CATV, telecommunications, twisted pair, copper cable, video, voice, data, VOIP Abstract Accurately determining the insulation condition in twisted copper cable pairs is an essential requirement in the CATV and telecommunications sectors. Selecting test equipment that is tough, easy to use and capable is now possible. Email Bryan.Phillips @megger.com 2
A convenient method for accurately determining the insulation condition in twisted copper cable pairs is an essential requirement in the CATV and telecommunications sectors, as defective insulation invariably leads to performance impairments in services such as video, voice, data and VOIP. Among the most common causes of problems are insulation that has become weak or brittle, causing cracks or otherwise exposing copper within the cable, and the ingress of water into the cable. Insulation checks will identify these problems quickly and positively.a little care is, however, necessary when choosing an insulation test set, since types that have been specifically optimised for use in CATV and telecommunications applications will provide valuable time savings as well as offering enhanced operating convenience. A particularly useful feature is a dual display that shows two test parameters simultaneously. The primary display shows the main parameter being measured usually the insulation resistance while the secondary display shows a related parameter such as leakage current. Users save valuable time by measuring these two parameters with a single test. In the best instruments, the dual display system is rendered even more useful by the provision of both analogue and digital readouts for the primary parameter. In many applications, the response of the analogue readout provides the instrument user with a very rapid indication of the insulation condition. Infinity and beyond Another important feature to consider when choosing an insulation test set is the maximum value of insulation resistance for which it can provide a meaningful reading. Many types simply show values above, say, 1 GΩas infinity. If a developing fault on a cable has reduced the insulation resistance from, for example, 15 GΩ to 5 GΩ, a tester of this type will provide no indication of the problem, as both readings will be shown as infinity. The latest testers can, however, provide meaningful readings up to at least 50 GΩ. This means that incipient faults can be detected much sooner, allowing more time for action to be taken to avert a complete failure. Insulation test sets for CATV and telecommunications applications should have provision for adjusting the test voltage over a wide range so that the voltage can always be accurately matched to the application. Facilities for testing continuity are also important, as this is a frequent requirement when testing installations of all types. To ensure operator safety when testing at higher voltage levels, a CAT IV 600 V rating in line with IEC 61010 is desirable. 3
Finally, to allow convenient use in the field, the instrument should be rugged yet light-weight and compact, and it should have facilities for attaching it to a tool belt. Accessories should be expected to include a carrying bag with shoulder strap and a bed-of-nails test lead set. Selecting the right CATV instrument Several insulation test sets satisfy all of the requirements that have been discussed here. These are the MIT410TC, which offers insulation test voltages from Evaluate circuits in a main telephone distribution frame using a specialised 250 V to 500 V and measurements up to insulation tester. Make sure you specify a 500 V tester to detect moisture induced 50 G, and the MIT430TC, which offers insulation breakdowns in dry conditions. test voltages from 50 V to 1,000 V and measurements up to 200 GΩ. In addition, the MIT430TC includes memory, on screen recall of stored results and Bluetooth communications for downloading. Both products are supplied with bed of nails test leads for telecom applications and a convenient soft carry case with shoulder strap. 4
Insulation testing requirements for telecommunications twisted pair cables. MIT400TC-1_WP_en_V01.pdf