Conformity Magazine - March 2009 - (Page 21) The Classification clause defines the overcurrent protector equipment internal microclimate temperature range and maximum rated AC fault voltage (see Tables 1 and 2). The Characteristics clause defines that all the overcurrent protectors must be capable of being used in systems with maximum continuous operating voltages of 150 V rms and 200 V DC. The overcurrent protector maximum continuous current rating (holding current in some technologies) must be below the lowest current of its current-time template curve, but a specific current value is not given. Table 3 provides the relationships between the current limiter templates, A through I, and the specified power line fault and lightning impulse tests, 1 through 17. Table 4 of the Test clause lists the nine power line fault tests, the source equipment standards and the applicable templates. Table 5 lists the nine power lightning impulse tests, the source equipment standards and the applicable templates. Annex A importantly specifies the current-time templates (A through I) graphically and as plot point tables. Testing Approach Figure 1a shows a typical set up for testing an equipment port and the port immediate circuitry. The test generator might feed the port via a known or assumed primary overvoltage protector. From the port terminal, the overcurrent protector usually feeds a secondary overvoltage protector and then the protected electronic circuit. If the secondary overvoltage protector is a switching type component, when switching occurs the port circuit effectively becomes just the overcurrent protector, as shown in Figure 1b. The Subject 2564 test approach is to apply the equipment port test directly to the overcurrent protector component. High-level port safety tests are done to verify that the component doesn’t create hazards such as fire, electrical bridging or fragmentation. Many of the tests will check for current overload of the wiring feeding the port. Wiring overload checks normally involve the use of wiring simulation, test failure being if the simulator is damaged or its time-current limit values are exceeded. Subject 2564 wiring simulation approach is unified and precise – current-time templates. Templates are given graphically and as a table of plot points. A digital oscilloscope is used to record the component current-time characteristic during a power fault test. The current-time record can then be evaluated against the template current-time limit. Depending on the equipment port type, the maximum template current may be 25 A rms, 40 A rms, or 60 A rms (termed the maximum limited duration fault current). To understand how these were devised, we need to look at the origin of existing wiring simulation methods. The melting temperature of copper is over 1000 °C, and it is the wire insulation that is normally the first to be damaged by too much current. In 1992, ANSI/TIA/EIA-5716, Telecommunications Telephone Terminal Equipment Electrical, Thermal, Mechanical Environmental Performance Requirements, stated that traditional 32 AWG telephone tinsel cord insulation softened at 2.2 A continuous, 7 A for 5 seconds, and an i2t=400 A2s for short durations. The standard assumed that the cord fed three pieces of terminal equipment, and that these Microclimate Controlled Uncontrolled Special Temperature range (°C) +5 to +70 -40 to +85 Manufacturer/user defined Classification Class I Class II Class III Table 1: Microclimates Maximum AC fault voltage (V rms) 600 425 285 120 Specific voltage/√2 Maximum primary protector limiting voltage category (V) 1000 (default voltage) 600 (medium-voltage) 400 (low-voltage) NA (specific-voltage) Classification Group I Group II Group III Group IV Group V Table 2: Maximum rated AC fault voltages marCh 2009 Conformity 21 http://www.citel.us http://www.citel.us http://www.citel.us
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