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| Expert Advice - Archived Articles The article as it appeared in American Polywater Corporation's Technical Talk newsletter (PDF: 894/KB/4 pages)
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The Problem The savings and benefits of automatically applying lubricant during cable pulling depend on installation specifics. A contractor pulling a few hundred feet of building wire into small conduit will have quite different needs from a utility crew pulling several thousand feet of underground distribution cable daily. Yet in both cases, some level of automation in the lubrication operation may be cost justified. Automated lubrication can:
How Much We first need to know how much lubricant to use when pulling a cable. The equation below will tell us. Q = K x L x D Where This equation determines the amount of lubricant needed to completely coat the interior wall of a conduit with a lubricant film of 0.009 inches (0.2mm) thickness. The equation calculates lubricant quantities consistent with those used by experienced cable pulling crews. How Much/How Fast If we’re going to pump the lubricant to the conduit entrance, we’ll need the previous equation converted to a lubrication rate calculation: R = K x D x S Where At typical pulling speeds of 10 to 60 ft/min (3 to 18 m/min) and duct sizes of 2 to 6 inches (50 to 150 mm), the rate calculation provides expected lubricant demand from a low of 0.03 gal/min to a high of 0.5 gal/min (0.1 to 1.9 l/min). These are low volume flow rates that can be produced by a number of low-power pumps. But first, what about smaller and/or less frequent pulls, where automatic pumping is not practical. Is there any better way to apply pulling lubricant than dipping into the lube bucket and applying it by hand? Pump Selection When choosing pumping systems for applying pulling lubricant, a pump should be selected that can handle both liquid and gel lubes at the needed flow rates. It is a mistake to compromise lubricant quality based on limitations in a pump’s capability. The most important characteristics of pulling lubricant are that it is fully compatible with cable jackets and that it produces low friction in a broad range of field conditions, regardless of how it is applied. A pump should not shear the lube or build pressure in the outlet hose when flow is reduced with a restriction valve. Pressure pots low-ratio piston pumps and air-operated diaphragm pumps all meet these criteria. High shear pumps that run at a high constant speed (rotary vane, gear) are usually not appropriate for field lubrication. The continuous shear from the blades causes deterioration in lubricant performance and builds high pressure in the feed lines. The diameter and length of the outlet hose on a pump are also important. The hose and any other line restrictions should be large enough to support the desired flow rates. The usual question is whether a pump has enough draw to pull the liquid into the pump. One of the pump types we recommend, a "pressure pot", has all the liquid "within" the pump, so priming, draw, and cavitation are not concerns. [Top of page ] "Power Sum" Compliance Notes
Historically, the rationale behind specifying power sum NEXT requirements for connecting hardware was based upon the thinking that, "if power sum channels are going to be specified, then power sum components should be specified as well". Although there is some logic behind the statement, significant supporting data exists to demonstrate that a worst case power sum measurement is difficult (if not impossible) to consistently and accurately apply to connecting hardware qualification. To make matters worse, because a test algorithm for worst case power sum data collection does not exist, different manufacturer's "power sum" claims may mean different things. For example, a company can easily claim power sum category 5 performance for a product based upon data collected at one frequency point (100 MHz), measured in one test orientation, with a high end test plug (i.e. not to the full test plug range specified in '568-A), or testing in one termination mode (i.e. differential mode only). There are literally hundreds of test configurations that need to be analyzed before measured power sum performance can be reasonably claimed. Even then , there is no assurance that the worst case test configuration has been evaluated. The TIA PN-2948 Connecting Hardware Task Group has recently addressed these concerns by specifying worst pair NEXT characterization for connecting hardware. The good news is that an equivalent worst case pair-to-pair NEXT margin that correlates to category 5 power sum performance can be easily calculated. By assessing the statistical relationship between pair-to-pair NEXT values in connecting hardware, the TIA PN-3727 UTP Systems Task Group has determined that a common and differertial mode pair-to-pair NEXT margin of 3 dB over category 5 requirements is sufficient to ensure category 5 power sum performance. Based upon this criterion, make sure manufacturers have verified power sum performance for all of the following:
In specifying connecting hardware, be sure to note worst case guaranteed pair-to-pair NEXT performance and verify that measurement margins are based upon testing with and without common mode terminations. Although power sum claims may sound appealing, it is important to note that the power sum algorithm has a tendency to mask worst case pair-to-pair performance for components that may prove cumulative (i.e. two worst case pair-to-pair combinations that are in series and interact adversely) in a channel configuration. Claims to power sum performance are not sufficient to ensure proper channel NEXT performance.
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