 
|
  
   |
| About
CE-Mag Free Subscriptions Current Issue Article Archives ESD Help Mr. Static Web Gallery Staff Info Contact us
|
|
|
||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
|
|
||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
The New GR-1089-CORE: AC Power Fault TestsMick
J. Maytum Part 1 of this article gave an overview of the GR-1089-CORE section changes and detailed the lightning tests. This second part covers the ac power fault testing. The degree of change in the standard merits a whole article focused on the topic. Harmonization with other U.S. and international standards was a large factor in the changes, but the standard accounts for legacy equipment performance. General As with the lightning tests, the test sample size is three telecommunications port samples. The new four-wire interface type-A test requires an ac generator with four separate outputs.
Power Fault Testing First-Level AC Power Fault Tests (Telecommunications Port). Three changes were made to the tests: objectives becoming requirements, harmonization to the ITU-T recommendations, and limiter threshold testing. Table I shows the test conditions. Tests 6 through 9 have been changed from objectives to requirements, which means that the equipment must work after these tests. This change represents an increased requirement for interrupting current limiters such as fuses and thick-film line feed resistors (LFRs.) Figures 1 and 2 show the previous and new requirements for current-limiter resistance values of 50 W, 10 W, and 0.
The current values are maximum values, calculated assuming that the current-limiter resistance is the equipment resistance. Previously, the current interrupter could not operate at about 1 A, 1 second (1 A2s). Now the most stressful condition is about 5 A, 0.4 second (10 A2s). Many existing LFRs may not be able to meet the new requirement, necessitating the inclusion of a resettable current limiter, such as a positive-temperature-coefficient (PTC) thermistor. A PTC thermistor reduces the current, avoiding LFR fracture and permanent current interruption. Harmonization with ITU-T recommendations is shown in Table I by the figures in parentheses. Three harmonization methods are used: voltage change (test 7), source resistance change (test 7), and time change (tests 8 and 9). The changes to tests 7, 8, and 9 match the enhanced level conditions of the ITU-T power induction test 2.2.2 with an i2t value of 10 A2s, with a source resistance of 200 W. Compared with the previous i2t conditions of tests 7 through 9, test 7 is the same, test 8 is increased by 11%, and test 9 is reduced by 20%. Tests 3 and 4 already correspond to the 1-A2s test condition of the basic level conditions of the ITU-T power induction test 2.2.2.
In Figures 1 and 2, test 3 generator levels are 600 V rms and 1 A rms. Note 6 says that test 4 generator levels are set to 1000 V rms and 1 A rms to operate the added primary protector. Note 6 is unchanged from the previous issue and continues to neglect the equipment resistance loading on the generator. The default 600-V rms carbon-block primary protector operates only when the equipment load impedance develops at least 600 V rms. For the test 4 generator source resistance of 1000 W, the equipment resistance must be more than 1000 X 600/(1000 – 600) = 1500 W. However, many pieces of equipment will have impedances below 1500 W, so the test objective of primary operation is not fulfilled. First-level threshold testing of the equipment current and voltage limiters is now done with the resettable current limiter bypassed and the overvoltage limiter removed. The advantage to this new approach is that the circuits following the limiters are now stressed to the worst-case-condition voltage and current rather than to the values of the fitted limiters. As explained in the first part of this article, threshold means the limiter maximum let through, which in this case is under ac conditions. For the voltage limiter, the maximum limited voltage value needs to be identified. The generator open-circuit voltage is then set to this level and applied to the port with the limiter removed. For example, a TISP4350H3BJ thyristor voltage limiter has a typical ac protection voltage of ±310 V and a maximum guaranteed value of ±350 V. Under test, this protector would be removed and a 350-V peak, 248-V rms ac generator applied to the port. This test may apply up to 40 V more than the typical protector would allow, and this increases the confidence in the equipment withstand, in spite of the small number of samples used.
Certain applications have asymmetrical limiting voltages, and the positive and negative peak voltages will be different. In Part 1 of this article, a TISP61089B gated limiter had example values of +2.5 V and –56.5 V. Obviously, generating a 60-Hz ac sine wave of +2.5 V peak and –56.5 V peak is not straightforward. Separate test runs of half sine waves of +2.5 V peak and –56.5 V peak is the easiest solution, but this approach results in less heating than simultaneous application of a +2.5-V-peak and a –56.5-V-peak waveform. More-complex solutions could entail a 59-V peak-to-peak (2.5 V + 56.5 V) sine wave biased by a –27-V dc (–59 V/2 + 2.5) source or by using electronic switches to switch in an appropriate ac source voltage in each polarity (see Figure 3). Self-resettable current limiters will operate in some tests and reduce the test current. In such cases, the current limiter is bypassed, and the limiter maximum operating threshold current is applied to the port. PTC thermistors are the most common form of self-resettable current limiters, and these respond to rms current. Testing should apply an rms current equal to the maximum trip current of the PTC thermistor. Asymmetrical voltage limiters complicate testing because the resulting circuit current could be asymmetrical. However, modern digital oscilloscopes can automatically calculate the rms value of a waveform, allowing the correct rms current value to be set. Telecommunications Port
Current-Limiting Protector Tests for Non-Customer-Premises Equipment. Digital services equipment, operating at bit rates higher than 1.544 Mb/second or 1.1 MHz, have two new conditional requirements. Digital services equipment should not require an external current limiter. If the equipment does need an external current limiter, then the limiter must comply with the GR-974-CORE digital requirements. Two new current-limit indicators have been added. To harmonize with UL 60950, the use of an MDL 2.0-A fuse is now allowed as well as the previously allowed MDQ 16/10-A fuse. In recognition of modern digital oscilloscope waveform capture capabilities, noninvasive current measurement using a current probe is now acceptable to determine the performance of the equipment current limiter. Under test, the equipment shall not create a safety hazard or operate any current-limit indicator fuse. The maximum equipment current-time limits are indicated by the green line in Figure 4. For comparison, the previous limit values are shown by the red line in Figure 4. The equipment must now reduce or interrupt the current earlier in the 3–50-second time range. However, the current is allowed to pass nearly 3 A long term compared with the previous 2 A.
This long-term increase has occurred due to legacy equipment considerations, adoption of the UL 60950 MDL 2.0-A fuse, and a blind spot created by the previous ac test generator arrangement. Figure 5 shows the published curves for the MDL 2.0-A and MDQ 16/10-A fuses. The MDQ is a double-element fuse, and this gives the step in the 0.5–2-second region. Being a single-element fuse, the MDL has a smooth curve. Comparing these curves to those in Figure 4 shows the equipment limit curve previously used was based on the MDQ 16/10-A fuse characteristic, and the new limit curve is based on the MDL 2.0-A fuse characteristic. Figure 5 also shows the MDL 2.0-A fuse maximum limit of 120 seconds at 4 A. Typically, the fuse will operate at 120 seconds with only 2 A flowing. This 4-A maximum to 2-A typical current-indicator spread shows how inaccurate, and hence inappropriate, the use of commercial fuses can be. Fortunately, modern digital measurements can preclude the use of fuse indicators.
The 30-A ac test generator has been changed from a 20-W, variable-voltage source to a fixed 600-V rms source, variable resistance. This change was implemented to avoid the blind spot created by the original generator. In many cases, the equipment input circuit can be simplified to a current and voltage limiter. In Figure 6, these limiters are shown as a fuse current limiter and a thyristor voltage limiter. The prospective short-circuit current can only flow once the thyristor has switched into a low-voltage state.
To cause this switch, the generator voltage must exceed the voltage-limiting level of the thyristor. Below this voltage level, current flow is impossible. Use of the original 20-W variable voltage source generator prevented equipment testing down to 2 A rms for voltage limiters of 80 V and above. For example, a 200-V limiter would prevent testing below 5 A rms (assumes switching at generator voltage peak, so that current flows for only 50% of the half-cycle). Changing to a 600-V rms source, variable-resistance generator fixes this current-range-truncation problem. The default primary let-through voltage is 600 V rms, so it is puzzling that a 600-V rms source, variable-resistance generator was not used in the first place (see Figure 7). The specific generator-current levels that the equipment must be tested to are shown in Table II. Two test-current levels (2.2 A rms and 2.6 A rms) are in the acceptable region and, therefore, do not require the wiring simulator to be fitted. These current levels are 15-minute hazard tests, and it is not important whether the equipment current limiter operates.
The 2.2-A rms test level is the lowest unacceptable current retained from the previous issue for legacy purposes. Testing at the 2.2-A rms level duplicates the 2.2-A rms second-level ac power fault test 4, and so the results from test 4 can be used. The 2.6-A rms test level is midway between the 2.2-A rms level and the new lowest unacceptable current level of 3.0 A rms. Its purpose is to test for any hazard that might be missed by just testing at the 2.2- and 3.0-A rms current levels (blind spot avoidance).
Current-Limiting Protector Tests for Customer-Premises Equipment. Tests are performed using a wiring simulator, which differs depending on the means of connection to the outside plant. Equipment connected with premises-type wiring must be tested as in the previous section. For equipment that is directly connected with cable, the wiring simulator must be a 30-cm length of 26 AWG copper wire draped in cheesecloth. Figure 8 shows some laboratory current-time measurements of a 26-AWG-wire simulator. Compared with the customer-premises wiring simulator, the cable simulator allows about three to four times the current flow, enabling use of higher-current-rating fusible limiters. The hazard tests now extend to 7 A rms, and whether the equipment current limiter operates is not important.
Second-Level AC Power Fault Tests. Non-customer-premises equipment and customer-premises equipment have the same tests. Table III shows the test conditions, which haven't changed from the previous issue of the standard. Again, equipment-limiter threshold testing is done by removing the voltage limiter and bypassing the current limiter. One difference in the bypassed-current-limiter test is that the limiters can be resettable or nonresettable (e.g., fuses). Only first-level tests consider resettable current limiters because the equipment must work after testing. For first-level testing, the current is set to the maximum current that would not be interrupted by the current limiter within a 15-minute period. Comprehending both resettable and nonresettable limiters, second-level testing sets the available current value to the maximum operating threshold of the current limiting device. This test current is the maximum current that would not be interrupted or reduced by the specified current limiter within the test duration (5 or 900 seconds). Unfortunately, the test note goes on to say, "The maximum operating threshold for a current-limiting device is 135% of the rated current," without qualifying, as in the previous issue, that this threshold applied only to fuses.
Equipment using resettable current limiters should be tested with a current equal to the maximum trip current for the test duration. However, people could misinterpret the maximum trip current as rated current. Using a current 1.35 times the highest trip current will overstress the equipment compared with the actual worst-case condition. The 1.35 factor comes from UL 248, which, for standard fuses up to 30 A, specifies that the fuse must operate within 60 minutes at 1.35 times the rated current. However, the fuses used in telecommunications port interfaces must withstand lightning tests and are usually surge-withstand fuses. Microfuse (10-mm) versions of these special fuses do not comply with the 1.35 factor, and a 1.25-A-rated fuse is often designed not to interrupt the current in 2.2-A rms testing. This fuse has a 2.2/1.25 = 1.6 factor. Testing with 1.35 times the rated current (1.7 A rms) will understress the equipment compared with the actual worst-case condition. Conclusion The changes to the previous ac tests are subtle rather than major. However, these changes will drive different testing procedures, component improvement, and more-rigorous circuit design. Mick J. Maytum is applications manager for Bourns Ltd. (Bedford, UK). He can be reached via e-mail at mick.maytum@bourns.com.
|
||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
