Conformity Magazine - March 2009 - (Page 22) drew equal currents during the i2t period. This meant that each piece of terminal equipment had an i2t limit of 400/32 = 45 A2s (see Figure 2a). The wiring simulator is actually a wiring network simulator. Things went downhill at this point because 571 stated, “a fuse that meets these parameters is the Bussmann MDQ 1-6/10 (formally MDX-type).” Fuses are not precision components and should not be used as gauges. Even through the published MDQ 1-6/10 fusing characteristic is a reasonable match to the wiring simulator parameters, an individual fuse can be very different. In 1999, ANSI/TIA/EIA-571-A7 changed the wiring simulator i2t parameter. 571-A assumed that the cord fed two, not three, pieces of terminal equipment. This meant that each piece of terminal equipment now had an i2t limit of 400/22 = 100 A2s (see Figure 2b). To meet the increased i2t value, the fuse was changed from the (dual element) Bussmann MDQ 1-6/10 to the (single element) Bussmann MDL-2. But the published MDL-2 fusing characteristic is a poor match to the wiring simulator parameters, being some 20% to 30% different, even before considering the individual fuse variability. Figure 59.2 of UL 1459, UL Standard for Safety for Telephone Equipment, expressed the wiring simulator requirement as a current-time limit curve. The three main parameters were 2.3 A continuous, 7 A for 7 seconds, and an i2t = 50 A2s for short durations. A Bussmann MDQ 1-6/10 fuse was considered a reasonable emulation of the limit curve. When the wiring is of heaver gauge, such as 26 AWG, higher current limits and alternative simulations are proposed. Typically a length of the chosen wire gauge is used, draped with a cheesecloth indicator. The equipment fails if cheesecloth ignition or charring results from AC testing. But, unless the installed cable has hightemperature insulation, the use of a cheesecloth indicator is questionable. It takes several hundreds of degrees to char cheesecloth, a temperature at which any normal PVC wire insulation would have melted. GR-1089-CORE, Issue 4 uses a current-time template to define a wiring simulator. The limiting curve is based on the published MDL-2 fusing characteristic, with the long term fusing value increased from 2.6 A to 3.0 A. Figure 1a: Typical port test circuit Figure 1b: Simplified port test circuit for overcurrent protector components Figure 2a: Wiring and equipment limit i2t values for the 571 three-branch circuit Figure 2b: The 571-A two-branch circuit Figure 3: Power fault test circuit 22 Conformity marCh 2009
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