Networks Europe March/April 2019 - Page 32

32 CABLING "Non-standard cables like CCA cannot legally be installed to meet National Electric Code fire safety ratings, resulting in code violations and increased risk of fire" Length failures beyond the obvious While one might think that exceeding length limits obviously means that the cable is too long, that’s not the only potential cause. Another cause could be that the NVP is set incorrectly. NVP, which stands for nominal velocity of propagation, characterises how fast a signal is travelling down the cable relative to the speed of light in a vacuum, and it’s what allows the tester to calculate the length of the cable. Expressed as a percentage, NVP is set using the NVP specification provided by the cable manufacturer. And if it’s not set correctly for the cable under test, you might exceed length limits. When length reported is shorter than the known length, it could indicate a break in the cable. And if one or more pairs are significantly shorter than the others, there could be cable damage or a bad connection. Causes of the losses For return loss failures, there are several causes to consider, most of which can be attributed to poor installation practices. Patch cords could have been mishandled causing changes in impedance, cables could be kinked, or pairs untwisted. It’s possible, however, that patch cord or cable impedance is wrong (it should be 100 ohms) or not uniform. Bad connectors or poorly matched plugs and jacks can also cause return loss failures. And don’t rule out testing issues – you need to make sure the correct autotest is selected and that your link adapters are working. And if you have an unexpected return loss PASS, remember that knots and kinks don’t always cause a failure, especially when using good quality cable. Insertion loss, the loss of signal that happens along the length of any cable, can be a little easier to pinpoint. Since www.networkseuropemagazine.com